Updated on 2024/07/19

写真a

 
SHIMAKAWA, Rhythm
 
Affiliation
Affiliated organization, Waseda Institute for Advanced Study
Job title
Associate Professor(non-tenure-track)
Degree
Doctor of Philosophy ( 2017.03 The Graduate University for Advanced Studies )

Research Experience

  • 2023.07
    -
    Now

    Waseda University   Center for Data Science   Associate Professor (fixed-term)

    concurrently

  • 2023.04
    -
    Now

    Waseda University   Institue for Advanced Study   Associate Professor (Non-tenured)

  • 2018.04
    -
    2023.03

    National Astronomical Observatory of Japan   Subaru Telescope   NAOJ Fellow

  • 2017.04
    -
    2018.03

    University of California   Santa Cruz   JSPS Overseas Fellow

  • 2015.04
    -
    2017.03

    The Graduate University for Advanced Studies   JSPS Research Fellow for Young Scientists

  • 2013.04
    -
    2015.03

    Subaru Telescope   Subaru Research Intern

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Education Background

  • 2012.04
    -
    2017.03

    The Graduate University for Advanced Studies   School of Physical Sciences   Department of Astronomical Science  

  • 2008.04
    -
    2012.03

    Osaka University   School of Science  

Professional Memberships

  • 2020.12
    -
    Now

    The Astronomical Society of Japan

Research Areas

  • Statistical science   データサイエンス / Astronomy   光学赤外線天文学

Research Interests

  • Galaxies

  • Data Astronomy

  • High-redshift Galaxies

  • Void

  • Galaxy Clusters

  • Machine Learning

  • Citizen Astronomy

  • Intergalactic Medium

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Awards

  • The SOKENDAI Research Award

    2017.09   The Graduate University for Advanced Studies  

  • SOKENDAI Future Scientist Award

    2015.04   The Graduate University for Advanced Studies  

Media Coverage

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Papers

  • GALAXY CRUISE: Spiral and ring classifications for bright galaxies at z = 0.01–0.3

    Rhythm Shimakawa, Masayuki Tanaka, Kei Ito, Makoto Ando

    Publications of the Astronomical Society of Japan    2024.04  [Refereed]

    Authorship:Lead author

    DOI

    Scopus

  • New insights into the role of AGNs in forming the cluster red sequence

    Rhythm Shimakawa, Jose Manuel Perez-Martínez, Yusei Koyama, Masayuki Tanaka, Ichi Tanaka, Tadayuki Kodama, Nina A. Hatch, Huub J.A. Röttgering, Helmut Dannerbauer, Jaron D. Kurk

    Monthly Notices of the Royal Astronomical Society   528 ( 2 ) 3679 - 3695  2024.02  [Refereed]

    Authorship:Lead author

     View Summary

    As a considerable investment of time from various telescope facilities was dedicated toward studying the Spiderweb protocluster at z = 2.2, it so far remains one of the most extensively studied protocluster. We report here the latest results in this field, adding a new dimension to previous research on cluster formation at high redshift. Previous studies have reported a significant overdensity (δ∼10) of massive Hα (+ [N ii])-emitting galaxies in 3700 comoving Mpc3. Many of these were previously considered to be dusty, actively star-forming galaxies, given their rest-frame optical and infrared features. However, this study argues that a third of them are more likely to be 'passively evolving' galaxies with low-luminosity active galactic nuclei (AGNs) rather than star-forming galaxies, given the multiwavelength spectral energy distribution (SED) fitting including an AGN component. For their SED-based star formation rates to be valid, bulk of their Hα + [N ii] emission should come from the central AGNs. This difference in interpretation between this work and past studies, including ours, is particularly supported by the recent deep Chandra/X-ray observation. Furthermore, we have spectroscopically confirmed a quiescent nature for one of these AGNs, with its multiple stellar absorption lines but also low-ionization emission lines. This important update provides new insights into the role of AGNs in forming the cluster red sequence observed in the present-day universe.

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    3
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  • Inspiraling streams of enriched gas observed around a massive galaxy 11 billion years ago

    Shiwu Zhang, Zheng Cai, Dandan Xu, Rhythm Shimakawa, Fabrizio Arrigoni Battaia, Jason Xavier Prochaska, Renyue Cen, Zheng Zheng, Yunjing Wu, Qiong Li, Liming Dou, Jianfeng Wu, Ann Zabludoff, Xiaohui Fan, Yanli Ai, Emmet Gabriel Golden-Marx, Miao Li, Youjun Lu, Xiangcheng Ma, Sen Wang, Ran Wang, Feng Yuan

    Science   380 ( 6644 ) 494 - 498  2023.05  [Refereed]

     View Summary

    Stars form in galaxies, from gas that has been accreted from the intergalactic medium. Simulations have shown that recycling of gas—the reaccretion of gas that was previously ejected from a galaxy—could sustain star formation in the early Universe. We observe the gas surrounding a massive galaxy at redshift 2.3 and detect emission lines from neutral hydrogen, helium, and ionized carbon that extend 100 kiloparsecs from the galaxy. The kinematics of this circumgalactic gas is consistent with an inspiraling stream. The carbon abundance indicates that the gas had already been enriched with elements heavier than helium, previously ejected from a galaxy. We interpret the results as evidence of gas recycling during high-redshift galaxy assembly.

    DOI

  • King Ghidorah Supercluster: Mapping the light and dark matter in a new supercluster at z = 0.55 using the subaru hyper suprime-cam

    Rhythm Shimakawa, Nobuhiro Okabe, Masato Shirasaki, Masayuki Tanaka

    Monthly Notices of the Royal Astronomical Society: Letters   519 ( 1 ) L45 - L50  2022.11  [Refereed]

    Authorship:Lead author

     View Summary

    ABSTRACT

    This paper reports our discovery of the most massive supercluster, termed the King Ghidorah Supercluster (KGSc), at z = 0.50–0.64 in the Third Public Data Release of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP PDR3) over 690 deg2, as well as an initial result for a galaxy and dark matter mapping. The primary structure of the KGSc comprises triple broad weak-lensing (WL) peaks over 70 comoving Mpc. Such extensive WL detection at z > 0.5 can only currently be achieved using the wide-field high-quality images produced by the HSC-SSP. The structure is also contiguous with multiple large-scale structures across a ∼400 comoving Mpc scale. The entire field has a notable overdensity (δ = 14.7 ± 4.5) of red-sequence clusters. Additionally, large-scale underdensities can be found in the foreground along the line of sight. We confirmed the overdensities in stellar mass and dark matter distributions to be tightly coupled and estimated the total mass of the main structure to be 1 × 1016 solar masses, according to the mock data analyses based on large-volume cosmological simulations. Further, upcoming wide-field multi-object spectrographs such as the Subaru Prime Focus Spectrograph may aid in providing additional insights into distant superclusters beyond the 100 Mpc scale.

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    2
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  • Search for extended Lyman-α emission around 9k quasars at z = 2–3

    Rhythm Shimakawa

    Monthly Notices of the Royal Astronomical Society   514 ( 3 ) 3910 - 3924  2022.06  [Refereed]

    Authorship:Lead author

     View Summary

    ABSTRACT

    Enormous Lyα nebulae (ELANe) around quasars have provided unique insights into the formation of massive galaxies and their associations with super-massive black holes since their discovery. However, their detection remains highly limited. This paper introduces a systematic search for extended Lyα emission around 8683 quasars at z = 2.34–3.00 using a simple but very effective broad-band gri selection based on the Third Public Data Release of the Hyper Suprime-Cam Subaru Strategic Program. Although the broad-band selection detects only bright Lyα emission (≳ 1 × 10−17 erg s−1cm−2 arcsec−2) compared with narrow-band imaging and integral field spectroscopy, we can apply this method to far more sources than such common approaches. We first generated continuum g-band images without contributions from Lyα emission for host and satellite galaxies using r- and i-bands. Then, we established Lyα maps by subtracting them from observed g-band images with Lyα emissions. Consequently, we discovered extended Lyα emission (with masked area >40 arcsec2) for 7 and 32 out of 366 and 8317 quasars in the Deep and Ultra-deep (35 deg2) and Wide (890 deg2) layers, parts of which may be potential candidates of ELANe. However, none of them seem to be equivalent to the largest ELANe ever found. We detected higher fractions of quasars with large nebulae around more luminous or radio-loud quasars, supporting previous results. Future applications to the forthcoming big data from the Vera C. Rubin Observatory will help us detect more promising candidates. The source catalogue and obtained Lyα properties for all the quasar targets are accessible as online material.

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  • Where's Swimmy?: Mining unique color features buried in galaxies by deep anomaly detection using Subaru Hyper Suprime-Cam data

    Takumi S. Tanaka, Rhythm Shimakawa, Kazuhiro Shimasaku, Yoshiki Toba, Nobunari Kashikawa, Masayuki Tanaka, Akio K. Inoue

    Publications of the Astronomical Society of Japan   74 ( 1 ) 1 - 23  2022.02  [Refereed]

     View Summary

    We present the Swimmy (Subaru WIde-field Machine-learning anoMalY) survey program, a deep-learning-based search for unique sources using multicolored (grizy) imaging data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). This program aims to detect unexpected, novel, and rare populations and phenomena, by utilizing the deep imaging data acquired from the wide-field coverage of the HSC-SSP. This article, as the first paper in the Swimmy series, describes an anomaly detection technique to select unique populations as "outliers"from the data-set. The model was tested with known extreme emission-line galaxies (XELGs) and quasars, which consequently confirmed that the proposed method successfully selected ∼60%-70% of the quasars and 60% of the XELGs without labeled training data. In reference to the spectral information of local galaxies at z = 0.05-0.2 obtained from the Sloan Digital Sky Survey, we investigated the physical properties of the selected anomalies and compared them based on the significance of their outlier values. The results revealed that XELGs constitute notable fractions of the most anomalous galaxies, and certain galaxies manifest unique morphological features. In summary, deep anomaly detection is an effective tool that can search rare objects, and, ultimately, unknown unknowns with large data-sets. Further development of the proposed model and selection process can promote the practical applications required to achieve specific scientific goals.

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  • Subaru Hyper Suprime-Cam excavates colossal over- and underdense structures over 360 deg2 out to z = 1

    Rhythm Shimakawa, Yuichi Higuchi, Masato Shirasaki, Masayuki Tanaka, Yen-Ting Lin, Masao Hayashi, Rieko Momose, Chien-Hsiu Lee, Haruka Kusakabe, Tadayuki Kodama, Naoaki Yamamoto

    Monthly Notices of the Royal Astronomical Society   503 ( 3 ) 3896 - 3912  2021.04  [Refereed]

    Authorship:Lead author

     View Summary

    <title>ABSTRACT</title>
    Subaru Strategic Program with the Hyper-Suprime Cam (HSC-SSP) has proven to be successful with its extremely wide area coverage in past years. Taking advantages of this feature, we report initial results from exploration and research of expansive over- and underdense structures at z = 0.3–1 based on the second Public Data Release where optical 5-band photometric data for ∼ eight million sources with i &amp;lt; 23 mag are available over ∼360 deg2. We not only confirm known superclusters but also find candidates of titanic over- and underdense regions out to z = 1. The mock data analysis suggests that the density peaks would involve one or more massive dark matter haloes (&amp;gt;1014 M⊙) of the redshift, and the density troughs tend to be empty of massive haloes over &amp;gt;10 comoving Mpc. Besides, the density peaks and troughs at z ≲ 0.6 are in part identified as positive and negative weak lensing signals respectively, in mean tangential shear profiles, showing a good agreement with those inferred from the full-sky weak lensing simulation. The coming extensive spectroscopic surveys will be able to resolve these colossal structures in 3D space. The number density information over the entire survey field is available as grid-point data on the website of the HSC-SSP data release (https://hsc.mtk.nao.ac.jp/ssp/data-release/).

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    8
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  • MAHALO Deep Cluster Survey II. Characterizing massive forming galaxies in the Spiderweb protocluster at z = 2.2

    Rhythm Shimakawa, Yusei Koyama, Huub J A Röttgering, Tadayuki Kodama, Masao Hayashi, Nina A Hatch, Helmut Dannerbauer, Ichi Tanaka, Ken-ichi Tadaki, Tomoko L Suzuki, Nao Fukagawa, Zheng Cai, Jaron D Kurk

    Monthly Notices of the Royal Astronomical Society   481 ( 4 ) 5630 - 5650  2018.12  [Refereed]

    Authorship:Lead author

    DOI

  • MAHALO Deep Cluster Survey I. Accelerated and enhanced galaxy formation in the densest regions of a protocluster at z = 2.5

    Rhythm Shimakawa, Tadayuki Kodama, Masao Hayashi, J. Xavier Prochaska, Ichi Tanaka, Zheng Cai, Tomoko L. Suzuki, Ken-ichi Tadaki, Yusei Koyama

    Monthly Notices of the Royal Astronomical Society   473 ( 2 ) 1977 - 1999  2018.01  [Refereed]

    Authorship:Lead author

    DOI

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    49
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  • Direct evidence for Lyα depletion in the protocluster core

    RHYTHM SHIMAKAWA

    \mnras   468 ( 1 ) L21 - L25  2017.06  [Refereed]

    Authorship:Lead author

    DOI

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    26
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  • An early phase of environmental effects on galaxy properties unveiled by near-infrared spectroscopy of protocluster galaxies at z > 2

    RHYTHM SHIMAKAWA

    \mnras   448 ( 1 ) 666 - 680  2015.03  [Refereed]

    Authorship:Lead author

    DOI

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    57
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  • A Massive Quiescent Galaxy in a Group Environment at z = 4.53

    Takumi Kakimoto, Masayuki Tanaka, Masato Onodera, Rhythm Shimakawa, Po-Feng Wu, Katriona M. L. Gould, Kei Ito, Shuowen Jin, Mariko Kubo, Tomoko L. Suzuki, Sune Toft, Francesco Valentino, Kiyoto Yabe

    The Astrophysical Journal    2024.03  [Refereed]

    DOI

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  • ERGO-ML: comparing IllustrisTNG and HSC galaxy images via contrastive learning

    Lukas Eisert, Connor Bottrell, Annalisa Pillepich, Rhythm Shimakawa, Vicente Rodriguez-Gomez, Dylan Nelson, Eirini Angeloudi, Marc Huertas-Company

    Monthly Notices of the Royal Astronomical Society    2024.02  [Refereed]

    DOI

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  • Enhanced star formation and metallicity deficit in the USS 1558−003 forming protocluster at z = 2.53

    J. M. Pérez-Martínez, T. Kodama, Y. Koyama, R. Shimakawa, T. L. Suzuki, K. Daikuhara, K. Adachi, M. Onodera, I. Tanaka

    Monthly Notices of the Royal Astronomical Society   527 ( 4 ) 10221 - 10238  2024.02  [Refereed]

     View Summary

    We use K-band multi-object near-infrared spectroscopy with Keck/MOSFIRE to search for environmental imprints on the gas properties of 27 narrow-band selected H α emitters (HAEs) across the three major clumps of the assembling USS1558−003 protocluster at z = 2.53. We target the H α and [N II]λ6584 emission lines to obtain star formation rates (SFR) and gas-phase oxygen abundances for our sources, confirming the membership of 23 objects. HAEs belonging to this protocluster display enhanced SFRs with respect to the main sequence of star formation at the same cosmic epoch. This effect is more prominent for low-mass galaxies (log M∗/M◦ < 10.0), which may be experiencing a vigorous phase of mass assembly shortly after they were formed. We compute the individual and stacked gas-phase metallicities for our sources finding a metallicity deficit for low-mass objects when compared against the field mass–metallicity relation and the massive Spiderweb protocluster at z = 2.16. These results suggest that HAEs within USS1558−003 may be less evolved than those in the Spiderweb protocluster. Finally, we explore the gas metallicity–gas fraction relation for a small sample of five galaxies with CO(3–2) molecular gas information. Assuming our objects are in equilibrium, we obtain a relatively wide range of mass loading factors (λ = 0.5–2) matching field samples at the cosmic noon but in contrast with our previous results in the Spiderweb protocluster. We speculate that these discrepancies between protoclusters may be (partly) driven by differences in their current dynamical and mass assembly stages, hinting at the co-evolution of protoclusters and their galaxy populations at 2 < z < 3.

    DOI

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    2
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  • Line-of-sight structure of troughs identified in Subaru Hyper Suprime-Cam Year 3 weak lensing mass maps

    Takumi Shimasue, Ken Osato, Masamune Oguri, Rhythm Shimakawa, Atsushi J. Nishizawa

    Monthly Notices of the Royal Astronomical Society   527 ( 3 ) 5974 - 5987  2024.01  [Refereed]

     View Summary

    We perform the weak lensing mass mapping analysis to identify troughs, which are defined as local minima in the mass map. Since weak lensing probes the projected matter distribution along the line of sight, these troughs can be produced by single or multiple voids projected along the line of sight. To scrutinize the origins of the weak lensing troughs, we systematically investigate the line-of-sight structure of troughs selected from the latest Subaru Hyper Suprime-Cam (HSC) Year 3 weak lensing data covering 433.48 deg2. From a curved sky mass map constructed with the HSC data, we identify 15 troughs with the signal-to-noise ratio higher than 5.7 and address their line-of-sight density structure utilizing redshift distributions of two galaxy samples, photometric luminous red galaxies observed by HSC and spectroscopic galaxies detected by Baryon Oscillation Spectroscopic Survey. While most weak lensing signals due to the troughs are explained by multiple voids aligned along the line of sight, we find that two of the 15 troughs potentially originate from single voids at redshift ∼0.3. The single void interpretation appears to be consistent with our three-dimensional mass mapping analysis. We argue that single voids can indeed reproduce observed weak lensing signals at the troughs if these voids are not spherical but are highly elongated along the line-of-sight direction.

    DOI

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    1
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  • COALAS II. Extended molecular gas reservoirs are common in a distant, forming galaxy cluster

    Z. Chen, H. Dannerbauer, M. D. Lehnert, B. H.C. Emonts, Q. Gu, J. R. Allison, J. B. Champagne, N. Hatch, B. Indermüehle, R. P. Norris, J. M. Pérez-Martínez, H. J.A. Röttgering, P. Serra, N. Seymour, R. Shimakawa, A. P. Thomson, C. M. Casey, C. De Breuck, G. Drouart, T. Kodama, Y. Koyama, C. D.P. Lagos, P. Macgregor, G. Miley, J. M. Rodríguez-Espinosa, M. Sánchez-Portal, B. Ziegler

    Monthly Notices of the Royal Astronomical Society   527 ( 3 ) 8950 - 8972  2024.01  [Refereed]

     View Summary

    This paper presents the results of 475h of interferometric observations with the Australia Telescope Compact Array towards the Spiderweb protocluster at z=2.16. We search for large, extended molecular gas reservoirs among 46 previously detected CO(1−0) emitters, employing a customised method we developed. Based on the CO emission images and position–velocity diagrams, as well as the ranking of sources using a binary weighting of six different criteria, we have identified 14 robust and 7 tentative candidates that exhibit large extended molecular gas reservoirs. These extended reservoirs are defined as having sizes greater than 40 kpc or supergalactic scale. This result suggests a high frequency of extended gas reservoirs, comprising at least 30 percent of our CO-selected sample. An environmental study of the candidates is carried out based on Nth nearest neighbour and we find that the large molecular gas reservoirs tend to exist in denser regions. The spatial distribution of our candidates is mainly centred on the core region of the Spiderweb protocluster. The performance and adaptability of our method are discussed. We found 13 (potentially) extended gas reservoirs located in eight galaxy (proto)clusters from the literature. We noticed that large extended molecular gas reservoirs surrounding (normal) star-forming galaxies in protoclusters are rare. This may be attributable to the lack of observations low-J CO transitions and the lack of quantitative analyses of molecular gas morphologies. The large gas reservoirs in the Spiderweb protocluster are potential sources of the intracluster medium seen in low redshift Virgo- or Coma-like galaxy clusters.

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    2
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  • Environmental Impacts on the Rest-frame UV Size and Morphology of Star-forming Galaxies at z ∼ 2

    Abdurrahman Naufal, Yusei Koyama, Rhythm Shimakawa, Tadayuki Kodama

    Astrophysical Journal   2 ( 170 )  2023.12  [Refereed]

     View Summary

    We report the measurement of rest-frame UV size and morphology of Hα-emission-selected star-forming galaxies (HAEs) in four protoclusters at z ∼ 2 (PKS 1138-262, USS 1558-003, PHz G237.0+42.5, and CC 2.2) using archival Hubble Space Telescope Advanced Camera Survey (HST/ACS) F814W data. We compare the measurement of 122 HAEs in protoclusters detected by HST/ACS to a coeval comparison field sample of 436 HAEs. We find the size distributions of protocluster and field HAEs are similar with a typical half-light radius of ∼2.5 kpc. At fixed stellar mass, there is no significant difference between HAEs in the protocluster and in the field, which is also supported by stacking analyses. This result suggests that the environment does not significantly affect the size of galaxies during the star-forming phase at this epoch. Based on the Sérsic index and nonparametric morphologies, HAE morphologies in both environments at z ∼ 2 in rest-frame UV are consistent with disk-like star-forming galaxies, although we also find 29% ± 4% HAEs showing disturbed morphologies. The fraction of disturbed galaxies is higher in the protocluster environment, with 39% ± 8% protocluster HAEs showing disturbed morphologies, compared to 26% ± 4% in the comparison field. The apparent disturbed morphologies are correlated with higher star formation activity and may be caused by either in situ giant clumps or mergers.

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  • GALAXY CRUISE: Deep insights into interacting galaxies in the local Universe†

    Masayuki Tanaka, Michitaro Koike, Seiichiro Naito, Junko Shibata, Kumiko Usuda-Sato, Hitoshi Yamaoka, Makoto Ando, Kei Ito, Umi Kobayashi, Yutaro Kofuji, Atsuki Kuwata, Suzuka Nakano, Rhythm Shimakawa, Ken Ichi Tadaki, Suguru Takebayashi, Chie Tsuchiya, Tomofumi Umemoto, Connor Bottrell

    Publications of the Astronomical Society of Japan   75 ( 5 ) 986 - 1010  2023.10  [Refereed]

     View Summary

    We present the first results from GALAXY CRUISE, a community (or citizen) science project based on data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). The current paradigm of galaxy evolution suggests that galaxies grow hierarchically via mergers, but our observational understanding of the role of mergers is still limited. The data from HSC-SSP are ideally suited to improve our understanding with improved identifications of interacting galaxies thanks to the superb depth and image quality of HSC-SSP. We launched a community science project, GALAXY CRUISE, in 2019 and have collected over two million independent classifications of 20686 galaxies at z < 0.2. We first characterize the accuracy of the participants’ classifications and demonstrate that it surpasses previous studies based on shallower imaging data. We then investigate various aspects of interacting galaxies in detail. We show that there is a clear sign of enhanced activities of super-massive black holes and star formation in interacting galaxies compared to those in isolated galaxies. The enhancement seems particularly strong for galaxies undergoing violent mergers. We also show that the mass growth rate inferred from our results is roughly consistent with the observed evolution of the stellar mass function. The second season of GALAXY CRUISE is currently underway and we conclude with future prospects. We make the morphological classification catalog used in this paper publicly available at the GALAXY CRUISE website, which will be particularly useful for machine-learning applications.

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    6
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  • Signs of environmental effects on star-forming galaxies in the Spiderweb protocluster at z = 2.16

    J M Pérez-Martínez, H Dannerbauer, T Kodama, Y Koyama, R Shimakawa, T L Suzuki, R Calvi, Z Chen, K Daikuhara, N A Hatch, A Laza-Ramos, D Sobral, J P Stott, I Tanaka

    Monthly Notices of the Royal Astronomical Society   518 ( 2 ) 1707 - 1734  2022.09  [Refereed]

     View Summary

    ABSTRACT

    We use multi-object near-infrared spectroscopy with VLT/KMOS to investigate the role of the environment in the evolution of the ionized gas properties of narrow-band-selected H α emitters (HAEs) in the Spiderweb protocluster at z = 2.16. Based on rest-frame optical emission lines, H α and [N ii]λ6584, we confirm the cluster membership of 39 of our targets (i.e. 93 per cent success rate), and measure their star formation rates (SFR), gas-phase oxygen abundances, and effective radius. We parametrize the environment where our targets reside using local and global density indicators based on previous samples of spectroscopic and narrow-band cluster members. We find that star-forming galaxies embedded in the Spiderweb protocluster display SFRs compatible with those of the main sequence and morphologies comparable to those of late-type galaxies at z = 2.2 in the field. We also report a mild gas-phase metallicity enhancement (0.06 ± 0.03 dex) at intermediate stellar masses. Furthermore, we identify two UVJ-selected quiescent galaxies with residual H α-based star formation and find signs of extreme dust obscuration in a small sample of starbursty submillimetre galaxies based on their FIR and H α emission. Interestingly, the spatial distribution of these objects differs from the rest of HAEs, avoiding the protocluster core. Finally, we explore the gas fraction–gas metallicity diagram for seven galaxies with molecular gas masses measured by ATCA using CO(1−0). In the context of the gas-regulator model, our objects are consistent with relatively low mass-loading factors, suggesting lower outflow activity than field samples at the cosmic noon and thus, hinting at the onset of environmental effects in this massive protocluster.

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  • High-resolution ALMA Study of CO J = 2–1 Line and Dust Continuum Emissions in Cluster Galaxies at z = 1.46

    Ryota Ikeda, Ken-ichi Tadaki, Daisuke Iono, Tadayuki Kodama, Jeffrey C. C. Chan, Bunyo Hatsukade, Masao Hayashi, Takuma Izumi, Kotaro Kohno, Yusei Koyama, Rhythm Shimakawa, Tomoko L. Suzuki, Yoichi Tamura, Ichi Tanaka

    The Astrophysical Journal   933 ( 1 ) 11 - 11  2022.07  [Refereed]

     View Summary

    Abstract

    We present new Atacama Large Millimeter/submillimeter Array (ALMA) results obtained from spatially resolved CO J = 2–1 line (0.″4 resolution) and 870 μm continuum (0.″2 resolution) observations of cluster galaxies in XMMXCS J2215.9-1738 at z = 1.46. Our sample comprises 17 galaxies within ∼0.5 Mpc (0.6R200) of the cluster center, all of which have previously been detected in the CO J = 2–1 line at a lower resolution. The effective radii of both the CO J = 2–1 line and 870 μm dust continuum emissions are robustly measured for nine galaxies by modeling the visibilities. We find that the CO J = 2–1 line emission in all of the nine galaxies is more extended than the dust continuum emission by a factor of 2.8 ± 1.4. We investigate the spatially resolved Kennicutt–Schmidt (KS) relation in two regions within the interstellar medium of the galaxies. The relation for our sample reveals that the central region (0 &lt; r &lt; Re,870μm) of galaxies tends to have a shorter gas depletion timescale, i.e., a higher star formation efficiency, compared to the extended region (Re,870μm &lt; r &lt; Re,CO). Overall, our result suggests that star formation activities are concentrated inside the extended gas reservoir, possibly resulting in the formation of a bulge structure. We find consistency between the ALMA 870 μm radii of star-forming members and the Hubble Space Telescope/1.6 μm radii of passive members in a mass–size distribution, which suggests a transition from star-forming to passive members within ∼0.5 Gyr. In addition, no clear differences in the KS relation nor in the sizes are found between galaxies with and without a close companion.

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  • Passive spiral galaxies deeply captured by Subaru Hyper Suprime-Cam

    Rhythm Shimakawa, Masayuki Tanaka, Connor Bottrell, Po-Feng Wu, Yu-Yen Chang, Yoshiki Toba, Sadman Ali

    Publications of the Astronomical Society of Japan   74 ( 3 ) 612 - 624  2022.06  [Refereed]

    Authorship:Lead author

     View Summary

    Abstract

    This paper presents a thousand passive spiral galaxy samples at z = 0.01–0.3 based on a combined analysis of the Third Public Data Release of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP PDR3) and the GALEX–SDSS–WISE Legacy Catalog (GSWLC-2). Among 54871 gri galaxy cutouts taken from the HSC-SSP PDR3 over 1072 deg2, we conducted a search with deep-learning morphological classification for candidates of passive spirals below the star-forming main sequence derived by ultraviolet to mid-infrared spectral energy distribution fitting in the GSWLC-2. We then classified the candidates into 1100 passive spirals and 1141 secondary samples based on visual inspections. Most of the latter cases are considered to be passive ringed S0 or pseudo-ringed galaxies. The remaining secondary samples have ambiguous morphologies, including two peculiar objects with diamond-shaped stellar wings. The selected passive spirals have a similar distribution to the general quiescent galaxies on the EWHδ–Dn4000 diagram and concentration indices. Moreover, we detected an enhanced passive fraction of spiral galaxies in X-ray clusters. Passive spirals in galaxy clusters are preferentially located in the midterm or late infall phase on the phase–space diagram, supporting the ram pressure scenario, which has been widely advocated in previous studies. The source catalog and gri-composite images are available on the HSC-SSP PDR3 website 〈https://hsc.mtk.nao.ac.jp/ssp/data-release/〉. Future updates, including integration with a citizen science project dedicated to the HSC data, will achieve more effective and comprehensive classifications.

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  • A systematic search for galaxy protocluster cores at the transition epoch of their star formation activity

    Makoto Ando, Kazuhiro Shimasaku, Rieko Momose, Kei Ito, Marcin Sawicki, Rhythm Shimakawa

    Monthly Notices of the Royal Astronomical Society   513 ( 3 ) 3252 - 3272  2022.05  [Refereed]

     View Summary

    ABSTRACT

    The redshift of z ∼ 1.5 is the transition epoch of protoclusters (PCs) from the star-forming phase into the quenching phase, and hence an appropriate era to investigate the build up of the quenched population. We define a ‘core’ as the most massive halo in a given PC, where environmental effects are likely to work most effectively, and search for cores at 1 &amp;lt; z &amp;lt; 1.5. We use a photometric redshift catalogue of a wide (effective area of ${\sim}22.2\, \mathrm{deg}^{2}$) and deep ($i\sim 26.8\, \mathrm{mag}$) optical survey with Subaru Hyper-Suprime Cam. Regarding galaxies with log (M*/M⊙) &amp;gt; 11.3 as the central galaxies of PC cores, we estimate their average halo mass by clustering analysis and find it to be log (Mh/M⊙) ∼ 13.7. An expected mass growth by the IllustrisTNG simulation and the observed overdensities around them suggest that the PC cores we find are progenitors of present-day clusters. Classifying our galaxy sample into red and blue galaxies, we calculate the stellar mass function (SMF) and the red galaxy fraction. The SMFs in the PC cores are more-top heavy than field, implying early high-mass galaxy formation and disruption of low-mass galaxies. We also find that the red fraction increases with stellar mass, consistent with stellar mass dependent environmental quenching recently found at z &amp;gt; 1. Interestingly, although the cores with red and blue centrals have similar halo masses, only those with red centrals show a significant red fraction excess compared to the field, suggesting a conformity effect. Some observational features of PC cores may imply that the conformity is caused by assembly bias.

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  • Hα emission in the outskirts of galaxies at z = 0.4

    Rhythm Shimakawa, Masayuki Tanaka, Satoshi Kikuta, Masao Hayashi

    Publications of the Astronomical Society of Japan   74 ( 2 ) 318 - 325  2022.04  [Refereed]

    Authorship:Lead author

     View Summary

    This paper reports detections of Hα emission and stellar continuum out to approximately 30 physical kiloparsecs, and Hα directionality in the outskirts of Hα-emitting galaxies (Hα emitters) at $z$ = 0.4. This research adopts narrow-band selected Hα emitters at $z$ = 0.4 from the emission-line object catalog by Hayashi et al. (2020, PASJ, 72, 86), which is based on data in the Deep and Ultradeep layers of the Hyper Suprime-Cam Subaru Strategic Program. Deep narrow- and broad-band images of 8625 Hα emitters across 16.8 deg2 enable us to construct deep composite emission-line and continuum images. The stacked images show diffuse Hα emission (down to ∼5 × 10-20 erg s-1 cm-2 arcsec-2) and stellar continuum (down to ∼5 × 10-22 erg s-1 cm-2 Å-1 arcsec-2), extending beyond 10 kpc at stellar masses >109 $M⊙$, parts of which may originate from stellar halos. Those radial profiles are broadly consistent with each other. In addition, we obtain a dependence of the Hα emission on the position angle because relatively higher Hα equivalent width has been detected along the minor-axis towards galaxy disks. While the Hα directionality could be attributed to biconical outflows, further research with hydrodynamic simulations is highly demanded to pin down the exact cause.

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  • The Environmental Dependence of Gas Properties in Dense Cores of a Protocluster at z ∼ 2.5 Revealed with ALMA

    Kohei Aoyama, Tadayuki Kodama, Tomoko L. Suzuki, Ken Ichi Tadaki, Rhythm Shimakawa, Masao Hayashi, Yusei Koyama, Jose Manuel Pérez-Martínez

    Astrophysical Journal   924 ( 2 )  2022.01  [Refereed]

     View Summary

    In a protocluster USS1558-003 at z = 2.53, galaxies in the dense cores show systematically elevated star-forming activity compared to those in less dense regions. To understand its origin, we look into the gas properties of the galaxies in the dense cores by conducting deep 1.1 mm observations with the Atacama Large Millimeter/submillimeter Array. We detect interstellar dust continuum emission from 12 member galaxies and estimate their molecular gas masses. Comparing these gas masses with our previous measurements from the CO(3-2) line, we infer that the latter might be overestimated. We find that the gas to stellar mass ratios of the galaxies in the dense cores tend to be higher (at M ∗ ∼ 1010 M o˙ where we see the enhanced star-forming activity), suggesting that such large gas masses can sustain their high star-forming activity. However, if we compare the gas properties of these protocluster galaxies with the gas scaling relations constructed for field galaxies at a similar cosmic epoch, we find no significant environmental difference at the same stellar mass and star formation rate. Although both gas mass ratios and star-forming activity are enhanced in the majority of member galaxies, they appear to follow the same scaling relation as field galaxies. Our results are consistent with the scenario in which the cold gas is efficiently supplied to protocluster cores and to galaxies therein along surrounding filamentary structures, which leads to the high gas mass fractions and thus the elevated star formation activity, but without changing the star formation law.

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  • Subaru Hyper Suprime-Cam revisits the large-scale environmental dependence on galaxy morphology over 360 deg2at z = 0.3-0.6

    Rhythm Shimakawa, Takumi S. Tanaka, Seiji Toshikage, Masayuki Tanaka

    Publications of the Astronomical Society of Japan   73 ( 6 ) 1575 - 1588  2021.12  [Refereed]

    Authorship:Lead author

     View Summary

    This study investigates the role of large-scale environments on the fraction of spiral galaxies at z = 0.3-0.6 sliced to three redshift bins of Δz = 0.1. Here, we sample 276220 massive galaxies in a limited stellar mass of 5 × 1010 solar mass (∼M∗) over 360 deg2, as obtained from the Second Public Data Release of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). By combining projected two-dimensional density information (Shimakawa et al. 2021, MNRAS, 503, 3896) and the CAMIRA cluster catalog (Oguri et al. 2018, PASJ, 70, S20), we investigate the spiral fraction across large-scale overdensities and in the vicinity of red sequence clusters. We adopt transfer learning to reduce the cost of labeling spiral galaxies significantly and then perform stacking analysis across the entire field to overcome the limitations of sample size. Here we employ a morphological classification catalog by the Galaxy Zoo Hubble (Willett et al., 2017, MNRAS, 464, 4176) to train the deep learning model. Based on 74103 sources classified as spirals, we find moderate morphology-density relations on a 10 comoving Mpc scale, thanks to the wide-field coverage of HSC-SSP. Clear deficits of spiral galaxies have also been confirmed, in and around 1136 red sequence clusters. Furthermore, we verify whether there is a large-scale environmental dependence on rest-frame u - r colors of spiral galaxies; such a tendency was not observed in our sample.

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  • What Determines the H i Gas Content in Galaxies? Morphological Dependence of the H i Gas Fraction across the M *–SFR Plane

    Shgieru V. Namiki, Yusei Koyama, Shuhei Koyama, Takuji Yamashita, Masao Hayashi, Martha P. Haynes, Rhythm Shimakawa, Masato Onodera

    The Astrophysical Journal   918 ( 2 ) 68 - 68  2021.09  [Refereed]

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  • COALAS. I. ATCA CO(1-0) survey and luminosity function in the Spiderweb protocluster at z = 2.16

    S. Jin, H. Dannerbauer, B. Emonts, P. Serra, C. D. P. Lagos, A. P. Thomson, L. Bassini, M. Lehnert, J. R. Allison, J. B. Champagne, B. Indermühle, R. P. Norris, N. Seymour, R. Shimakawa, C. M. Casey, C. De Breuck, G. Drouart, N. Hatch, T. Kodama, Y. Koyama, P. Macgregor, G. Miley, R. Overzier, J. M. Pérez-Martínez, J. M. Rodríguez-Espinosa, H. Röttgering, M. Sánchez Portal, B. Ziegler

    Astronomy & Astrophysics   652   A11 - A11  2021.08  [Refereed]

     View Summary

    We report a detailed CO(1−0) survey of a galaxy protocluster field at <italic>z</italic> = 2.16, based on 475 h of observations with the Australia Telescope Compact Array. We constructed a large mosaic of 13 individual pointings, covering an area of 21 arcmin2 and ±6500 km s−1 range in velocity. We obtained a robust sample of 46 CO(1−0) detections spanning <italic>z</italic> = 2.09 − 2.22, constituting the largest sample of molecular gas measurements in protoclusters to date. The CO emitters show an overdensity at <italic>z</italic> = 2.12 − 2.21, suggesting a galaxy super-protocluster or a protocluster connected to large-scale filaments of ∼120 cMpc in size. We find that 90% of CO emitters have distances &gt;0.′5−4′ to the center galaxy, indicating that small area surveys would miss the majority of gas reservoirs in similar structures. Half of the CO emitters have velocities larger than escape velocities, which appears gravitationally unbound to the cluster core. These unbound sources are barely found within the <italic>R</italic>200 radius around the center, which is consistent with a picture in which the cluster core is collapsed while outer regions are still in formation. Compared to other protoclusters, this structure contains a relatively higher number of CO emitters with relatively narrow line widths and high luminosities, indicating galaxy mergers. We used these CO emitters to place the first constraint on the CO luminosity function and molecular gas density in an overdense environment. The amplitude of the CO luminosity function is 1.6 ± 0.5 orders of magnitude higher than that observed for field galaxy samples at <italic>z</italic> ∼ 2, and one order of magnitude higher than predictions for galaxy protoclusters from semi-analytical SHARK models. We derive a high molecular gas density of 0.6 − 1.3 × 109 <italic>M</italic> cMpc−3 for this structure, which is consistent with predictions for cold gas density of massive structures from hydro-dynamical DIANOGA simulations.

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  • Erratum: Extended star-forming region within galaxies in a dense proto-cluster core at z = 2.53 (Publications of the Astronomical Society of Japan (2019) 71 (69) DOI: 10.1093/pasj/psz047)

    Tomoko L. Suzuki, Yosuke Minowa, Yusei Koyama, Tadayuki Kodama, Masao Hayashi, Rhythm Shimakawa, Ichi Tanaka, Ken-Ichi Tadaki

    Publications of the Astronomical Society of Japan   73 ( 3 ) 773  2021.06

     View Summary

    In the original article (2019, PASJ, 71, 69
    doi: 10.1093/pasj/psz047), one grant number was omitted from the acknowledgments. The following is the corrected and complete version of the acknowledgments.

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  • Dust, Gas, and Metal Content in Star-forming Galaxies at z ∼ 3.3 Revealed with ALMA and Near-IR Spectroscopy

    Tomoko L. Suzuki, Masato Onodera, Tadayuki Kodama, Emanuele Daddi, Masao Hayashi, Yusei Koyama, Rhythm Shimakawa, Ian Smail, David Sobral, Sandro Tacchella, Ichi Tanaka

    The Astrophysical Journal   908 ( 1 ) 15 - 15  2021.02  [Refereed]

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  • Statistical Correlation between the Distribution of Lyα Emitters and Intergalactic Medium H i at z ∼ 2.2 Mapped by the Subaru/Hyper Suprime-Cam

    Yongming Liang, Nobunari Kashikawa, Zheng Cai, Xiaohui Fan, J. Xavier Prochaska, Kazuhiro Shimasaku, Masayuki Tanaka, Hisakazu Uchiyama, Kei Ito, Rhythm Shimakawa, Kentaro Nagamine, Ikkoh Shimizu, Masafusa Onoue, Jun Toshikawa

    The Astrophysical Journal   907 ( 1 ) 3 - 3  2021.01  [Refereed]

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  • Broadband Selection, Spectroscopic Identification, and Physical Properties of a Population of Extreme Emission-line Galaxies at 3 < z < 3.7

    Masato Onodera, Rhythm Shimakawa, Tomoko L. Suzuki, Ichi Tanaka, Yuichi Harikane, Masao Hayashi, Tadayuki Kodama, Yusei Koyama, Kimihiko Nakajima, Takatoshi Shibuya

    The Astrophysical Journal   904 ( 2 ) 180 - 180  2020.12  [Refereed]

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  • A 16 deg2 survey of emission-line galaxies at z &lt; 1.6 from HSC-SSP PDR2 and CHORUS

    Masao Hayashi, Rhythm Shimakawa, Masayuki Tanaka, Masato Onodera, Yusei Koyama, Akio K Inoue, Yutaka Komiyama, Chien-Hsiu Lee, Yen-Ting Lin, Kiyoto Yabe

    Publications of the Astronomical Society of Japan   72 ( 5 )  2020.10  [Refereed]

     View Summary

    <title>Abstract</title>
    We have conducted a comprehensive survey of emission-line galaxies at z ≲ 1.6 based on narrow-band (NB) imaging data taken with Hyper Suprime-Cam (HSC) on the Subaru telescope. In this paper, we update the catalogs of Hα, [O iii], and [O ii] emission-line galaxies using the data from the second Public Data Release (PDR2) of the Subaru Strategic Program (SSP) of the HSC and Cosmic HydrOgen Reionization Unveiled with Subaru (CHORUS) survey along with the spectroscopic redshifts for 2019 emission-line galaxies selected with the PDR1 data. The wider effective coverage of NB816 and NB921, 16.3 deg2 and 16.9 deg2, respectively, are available in the Deep and UltraDeep layers of HSC-SSP from the PDR2. The CHORUS survey provides us with data with additional three NBs (NB527, NB718, and NB973) in the COSMOS field in the UltraDeep layer (1.37 deg2). The five NB datasets allow us to investigate the star-forming galaxies presenting emission-lines at 14 specific redshifts ranging from z ∼ 1.6 down to z ∼ 0.05. We revisit the distribution of large-scale structures and luminosity functions (LFs) for the emission-line galaxies with the large samples of 75377 emission-line galaxies selected. The redshift revolution of LFs shows that the star formation rate densities (SFRDs) decreases monotonically from z ∼ 1.6, which is consistent with the cosmic SFRD known to-date. Our samples of emission-line galaxies covering a sufficiently large survey volume are useful to investigate the evolution of star-forming galaxies since the cosmic noon in a wide range of environments including galaxy clusters, filaments, and voids.

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  • Spin parity of spiral galaxies II: a catalogue of 80 k spiral galaxies using big data from the Subaru Hyper Suprime-Cam survey and deep learning

    Ken-ichi Tadaki, Masanori Iye, Hideya Fukumoto, Masao Hayashi, Cristian E Rusu, Rhythm Shimakawa, Tomoka Tosaki

    Monthly Notices of the Royal Astronomical Society   496 ( 4 ) 4276 - 4286  2020.08  [Refereed]

     View Summary

    <title>ABSTRACT</title>
    We report an automated morphological classification of galaxies into S-wise spirals, Z-wise spirals, and non-spirals using big image data taken from Subaru/Hyper Suprime-Cam (HSC) Survey and a convolutional neural network (CNN)-based deep learning technique. The HSC i-band images are about 36 times deeper than those from the Sloan Digital Sky Survey (SDSS) and have a two times higher spatial resolution, allowing us to identify substructures such as spiral arms and bars in galaxies at z &amp;gt; 0.1. We train CNN classifiers by using HSC images of 1447 S-spirals, 1382 Z-spirals, and 51 650 non-spirals. As the number of images in each class is unbalanced, we augment the data of spiral galaxies by horizontal flipping, rotation, and rescaling of images to make the numbers of three classes similar. The trained CNN models correctly classify 97.5 per cent of the validation data, which is not used for training. We apply the CNNs to HSC images of a half million galaxies with an i-band magnitude of i &amp;lt; 20 over an area of 320 deg2. 37 917 S-spirals and 38 718 Z-spirals are identified, indicating no significant difference between the numbers of two classes. Among a total of 76 635 spiral galaxies, 48 576 are located at z &amp;gt; 0.2, where we are hardly able to identify spiral arms in the SDSS images. Our attempt demonstrates that a combination of the HSC big data and CNNs has a large potential to classify various types of morphology such as bars, mergers, and strongly lensed objects.

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  • Quantifying the Effect of Field Variance on the Hα Luminosity Function with the New Numerical Galaxy Catalog (ν2GC)

    Kazuyuki Ogura, Masahiro Nagashima, Rhythm Shimakawa, Masao Hayashi, Masakazu A. R. Kobayashi, Taira Oogi, Tomoaki Ishiyama, Yusei Koyama, Ryu Makiya, Katsuya Okoshi, Masato Onodera, Hikari Shirakata

    The Astrophysical Journal    2020.05  [Refereed]

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  • Erratum: “Stellar Velocity Dispersion of a Massive Quenching Galaxy at z = 4.01” (2019, ApJL, 885, L34)

    Masayuki Tanaka, Francesco Valentino, Sune Toft, Masato Onodera, Rhythm Shimakawa, Daniel Ceverino, Andreas L. Faisst, Anna Gallazzi, Carlos Gómez-Guijarro, Mariko Kubo, Georgios E. Magdis, Charles L. Steinhardt, Mikkel Stockmann, Kiyoto Yabe, Johannes Zabl

    The Astrophysical Journal Letters    2020.05  [Refereed]

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  • Quiescent Galaxies 1.5 Billion Years after the Big Bang and Their Progenitors

    Francesco Valentino, Masayuki Tanaka, Iary Davidzon, Sune Toft, Carlos Gómez-Guijarro, Mikkel Stockmann, Masato Onodera, Gabriel Brammer, Daniel Ceverino, Andreas L. Faisst, Anna Gallazzi, Christopher C. Hayward, Olivier Ilbert, Mariko Kubo, Georgios E. Magdis, Jonatan Selsing, Rhythm Shimakawa, Martin Sparre, Charles Steinhardt, Kiyoto Yabe, Johannes Zabl

    The Astrophysical Journal   889 ( 2 ) 93 - 93  2020.01  [Refereed]

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  • The whole picture of the large-scale structure of the CL1604 supercluster at z ∼ 0.9

    Masao Hayashi, Yusei Koyama, Tadayuki Kodama, Yutaka Komiyama, Yen-Ting Lin, Satoshi Miyazaki, Rhythm Shimakawa, Tomoko L Suzuki, Ichi Tanaka, Moegi Yamamoto, Naoaki Yamamoto

    Publications of the Astronomical Society of Japan   71 ( 6 )  2019.12  [Refereed]

     View Summary

    <title>Abstract</title>
    We present the large-scale structure over a more than 50 comoving Mpc scale at $z \sim 0.9$ where the CL1604 supercluster, which is one of the largest structures ever known at high redshifts, is embedded. The wide-field deep imaging survey by the Subaru Strategic Program with the Hyper Suprime-Cam reveals that the already-known CL1604 supercluster is a mere part of larger-scale structure extending to both the north and the south. We confirm that there are galaxy clusters at three slightly different redshifts in the northern and southern sides of the supercluster by determining the redshifts of 55 red-sequence galaxies and 82 star-forming galaxies in total via follow-up spectroscopy with Subaru/FOCAS and Gemini-N/GMOS. This suggests that the structure known as the CL1604 supercluster is the tip of the iceberg. We investigate the stellar population of the red-sequence galaxies using 4000 Å break and Balmer H$\delta$ absorption lines. Almost all of the red-sequence galaxies brighter than $21.5\:$mag in the z band show an old stellar population of $\gtrsim\! 2\:$Gyr. The comparison of composite spectra of the red-sequence galaxies in the individual clusters show that the galaxies at a similar redshift have a similar stellar population age, even if they are located $\sim\! 50\:$Mpc apart from each other. However, there could be a large variation in the star formation history. Therefore, it is likely that galaxies associated with the large-scale structure on a 50 Mpc scale formed at almost the same time, have assembled into the denser regions, and then have evolved with different star formation history along the hierarchical growth of the cosmic web.

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  • Extended star-forming regions within galaxies in a dense proto-cluster core at z = 2.53†

    Tomoko L Suzuki, Yosuke Minowa, Yusei Koyama, Tadayuki Kodama, Masao Hayashi, Rhythm Shimakawa, Ichi Tanaka, Ken-ichi Tadaki

    Publications of the Astronomical Society of Japan   71 ( 4 )  2019.08  [Refereed]

     View Summary

    <title>Abstract</title>
    At z ∼ 2, star formation activity is thought to be high even in high-density environments such as galaxy clusters and proto-clusters. One of the critical but outstanding issues is if the structural growth of star-forming galaxies can differ depending on their surrounding environments. In order to investigate how galaxies grow their structures and what physical processes are involved in the evolution of galaxies, one requires spatially resolved images of not only stellar components but also star-forming regions within galaxies. We conducted (Adaptive Optics) AO-assisted imaging observations for star-forming galaxies in a dense proto-cluster core at z = 2.53 with IRCS and AO188 mounted on the Subaru Telescope. A combination of AO and narrow-band filters allows us to obtain resolved maps of Hα-emitting regions with an angular resolution of ${0{^{\prime\prime}_{. } }1}$–${0{^{\prime\prime}_{. } }2}$, which corresponds to ∼1 kpc at z ∼ 2.5. Based on stacking analyses, we compare radial profiles of star-forming regions and stellar components and find that the star-forming region of a sub-sample with log (M*/$M_\odot$) ∼ 10–11 is more extended than the stellar component, indicating the inside-out growth of the structure. This trend is similar to the one for star-forming galaxies in general fields at z = 2–2.5 obtained with the same observational technique. Our results suggest that the structural evolution of star-forming galaxies at z = 2–2.5 is mainly driven by internal secular processes irrespective of surrounding environments.

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  • A Spectroscopic Study of a Rich Cluster at z = 1.52 with Subaru and LBT: The Environmental Impacts on the Mass–Metallicity Relation

    Shigeru V. Namiki, Yusei Koyama, Masao Hayashi, Ken-ichi Tadaki, Nobunari Kashikawa, Masato Onodera, Rhythm Shimakawa, Tadayuki Kodama, Ichi Tanaka, N. M. Förster Schreiber, Jaron Kurk, R. Genzel

    The Astrophysical Journal   877 ( 2 ) 118 - 118  2019.06  [Refereed]

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  • Do Galaxy Morphologies Really Affect the Efficiency of Star Formation During the Phase of Galaxy Transition?

    Shuhei Koyama, Yusei Koyama, Takuji Yamashita, Masao Hayashi, Hideo Matsuhara, Takao Nakagawa, Shigeru V. Namiki, Tomoko L. Suzuki, Nao Fukagawa, Tadayuki Kodama, Lihwai Lin, Kana Morokuma-Matsui, Rhythm Shimakawa, Ichi Tanaka

    The Astrophysical Journal   874 ( 2 ) 142 - 142  2019.04  [Refereed]

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  • Environmental impacts on molecular gas in protocluster galaxies at z ∼ 2

    Ken-ichi Tadaki, Tadayuki Kodama, Masao Hayashi, Rhythm Shimakawa, Yusei Koyama, Minju Lee, Ichi Tanaka, Bunyo Hatsukade, Daisuke Iono, Kotaro Kohno, Yuichi Matsuda, Tomoko L Suzuki, Yoichi Tamura, Jun Toshikawa, Hideki Umehata

    Publications of the Astronomical Society of Japan   71 ( 2 )  2019.04  [Refereed]

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  • The Fraction of Active Galactic Nuclei in the USS 1558–003 Protocluster at z = 2.53

    Michael Macuga, Paul Martini, Eric D. Miller, Mark Brodwin, Masao Hayashi, Tadayuki Kodama, Yusei Koyama, Roderik A. Overzier, Rhythm Shimakawa, Ken-ichi Tadaki, Ichi Tanaka

    The Astrophysical Journal    2019.03  [Refereed]

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  • On the different levels of dust attenuation to nebular and stellar light in star-forming galaxies

    Yusei Koyama, Rhythm Shimakawa, Issei Yamamura, Tadayuki Kodama, Masao Hayashi

    Publications of the Astronomical Society of Japan   71 ( 1 )  2019.01  [Refereed]

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  • Development status of the simultaneous two-color near-infrared multi-object spectrograph SWIMS for the TAO 6.5m telescope

    Masahiro Konishi, Kentaro Motohara, Hidenori Takahashi, Natsuko Kato, Yasunori Terao, Hirofumi Ohashi, Yukihiro Kono, Tsutomu Aoki, Mamoru Doi, Takafumi Kamizuka, Kotaro Kohno, Takeo Minezaki, Takashi Miyata, Tomoki Morokuma, Shigeyuki Sako, Takao Soyano, Yuzuru Yoshii, Kosuke Kushibiki, Ken Tateuchi, Yutaro Kitagawa, Soya Todo, Tomoko Suzuki, Masao Hayashi, Ichi Tanaka, Ken-ichi Tadaki, Jun Toshikawa, Yusei Koyama, Yoichi Tamura, Toshihiko Tanabe, Masuo Tanaka, Ken'ichi Tarusawa, Shintaro Koshida, Masahito S. Uchiyama, Tomohiro Mori, Jumpei Yamaguchi, Yutaka Yoshida, Ryou Ohsawa, Bunyo Hatsukade, Tadayuki Kodama, Yutaka Kobayakawa, Rhythm Shimakawa

    Ground-based and Airborne Instrumentation for Astronomy VII    2018.07

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  • Molecular Gas Reservoirs in Cluster Galaxies at z = 1.46

    Masao Hayashi, Ken-ichi Tadaki, Tadayuki Kodama, Kotaro Kohno, Yuki Yamaguchi, Bunyo Hatsukade, Yusei Koyama, Rhythm Shimakawa, Yoichi Tamura, Tomoko L. Suzuki

    The Astrophysical Journal   856 ( 2 ) 118 - 118  2018.03  [Refereed]

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  • The nature of H α-selected galaxies along the large-scale structure at z = 0.4 revealed by Subaru Hyper Suprime-Cam survey

    Koyama, Y., Hayashi, M., Tanaka, M., Kodama, T., Shimakawa, R., Yamamoto, M., Nakata, F., Tanaka, I., Suzuki, T.L., Tadaki, K.-I., Nishizawa, A.J., Yabe, K., Toba, Y., Lin, L., Jian, H.-Y., Komiyama, Y.

    Publications of the Astronomical Society of Japan   70  2018

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  • The Interstellar Medium in [O iii]-selected Star-forming Galaxies at z ∼ 3.2

    Tomoko L. Suzuki, Tadayuki Kodama, Masato Onodera, Rhythm Shimakawa, Masao Hayashi, Ken-ichi Tadaki, Yusei Koyama, Ichi Tanaka, David Sobral, Ian Smail, Philip N. Best, Ali A. Khostovan, Yosuke Minowa, Moegi Yamamoto

    The Astrophysical Journal   849 ( 1 ) 39 - 39  2017.11  [Refereed]

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    (Scopus)
  • A 16 deg2 survey of emission-line galaxies at z < 1.5 in HSC-SSP Public Data Release 1

    Masao Hayashi, Masayuki Tanaka, Rhythm Shimakawa, Hisanori Furusawa, Rieko Momose, Yusei Koyama, John D. Silverman, Tadayuki Kodama, Yutaka Komiyama, Alexie Leauthaud, Yen-Ting Lin, Satoshi Miyazaki, Tohru Nagao, Atsushi J. Nishizawa, Masami Ouchi, Takatoshi Shibuya, Ken-ichi Tadaki, Kiyoto Yabe

    Publications of the Astronomical Society of Japan    2017.10  [Refereed]

    DOI

    Scopus

    27
    Citation
    (Scopus)
  • A Universal Correlation between Star Formation Activity and Molecular Gas Properties Across Environments

    Shuhei Koyama, Yusei Koyama, Takuji Yamashita, Kana Morokuma-Matsui, Hideo Matsuhara, Takao Nakagawa, Masao Hayashi, Tadayuki Kodama, Rhythm Shimakawa, Tomoko L. Suzuki, Ken-ichi Tadaki, Ichi Tanaka, Moegi Yamamoto

    The Astrophysical Journal   847 ( 2 )  2017.10  [Refereed]

     View Summary

    We present the molecular gas mass fraction (f(H2)) and star formation efficiency (SFE) of local galaxies on the basis of our new CO(J = 1 - 0) observations with the Nobeyama 45 m radio telescope, combined with the COLDGASS galaxy catalog, as a function of galaxy environment defined as the local number density of galaxies measured with SDSS DR7 spectroscopic data. Our sample covers a wide range in the stellar mass and star formation rate (SFR), and also covers a wide environmental range over two orders of magnitude. This allows us to conduct the first systematic study of environmental dependence of molecular gas properties in galaxies from the lowest-to the highest-density environments in the local universe. We confirm that both f(H2) and SFE have strong positive correlations with the SFR offset from the star-forming main sequence (Delta MS) and, most importantly, we find that these correlations are universal across all environments. Our result demonstrates that star formation activity within individual galaxies is primarily controlled by their molecular gas content, regardless of their global environment. Therefore, we claim that one always needs to be careful about the Delta MS distribution of the sample when investigating the environmental effects on the H-2 gas content in galaxies.

    DOI

    Scopus

    20
    Citation
    (Scopus)
  • Direct evidence for Ly alpha depletion in the protocluster core

    Shimakawa Rhythm, Kodama Tadayuki, Hayashi Masao, Tanaka Ichi, Matsuda Yuichi, Kashikawa Nobunari, Shibuya Takatoshi, Tadaki Ken-ichi, Koyama Yusei, Suzuki Tomoko L, Yamamoto Moegi

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY   468 ( 1 ) L21 - L25  2017.06  [Refereed]

    DOI

    Scopus

    26
    Citation
    (Scopus)
  • Similarities and uniqueness of Lyα emitters among star-forming galaxies at z = 2.5

    RHYTHM SHIMAKAWA

    \mnras     stx091 - stx091  2017.06  [Refereed]

    Authorship:Lead author

    DOI

    Scopus

    16
    Citation
    (Scopus)
  • Rotating Starburst Cores in Massive Galaxies at z = 2.5

    Ken-ichi Tadaki, Tadayuki Kodama, Erica J. Nelson, Sirio Belli, Natascha M. Förster Schreiber, Reinhard Genzel, Masao Hayashi, Rodrigo Herrera-Camus, Yusei Koyama, Philipp Lang, Dieter Lutz, Rhythm Shimakawa, Linda J. Tacconi, Hannah Übler, Emily Wisnioski, Stijn Wuyts, Bunyo Hatsukade, Magdalena Lippa, Kouichiro Nakanishi, Soh Ikarashi, Kotaro Kohno, Tomoko L. Suzuki, Yoichi Tamura, Ichi Tanaka

    The Astrophysical Journal   841 ( 2 ) L25 - L25  2017.05  [Refereed]

    DOI

    Scopus

    66
    Citation
    (Scopus)
  • Evolutionary Phases of Gas-rich Galaxies in a Galaxy Cluster at z = 1.46

    Masao Hayashi, Tadayuki Kodama, Kotaro Kohno, Yuki Yamaguchi, Ken-ichi Tadaki, Bunyo Hatsukade, Yusei Koyama, Rhythm Shimakawa, Yoichi Tamura, Tomoko L. Suzuki

    The Astrophysical Journal   841 ( 2 ) L21 - L21  2017.05  [Refereed]

    DOI

    Scopus

    40
    Citation
    (Scopus)
  • BULGE-FORMING GALAXIES WITH AN EXTENDED ROTATING DISK ATz∼ 2

    Ken-ichi Tadaki, Reinhard Genzel, Tadayuki Kodama, Stijn Wuyts, Emily Wisnioski, Natascha M. Förster Schreiber, Andreas Burkert, Philipp Lang, Linda J. Tacconi, Dieter Lutz, Sirio Belli, Richard I. Davies, Bunyo Hatsukade, Masao Hayashi, Rodrigo Herrera-Camus, Soh Ikarashi, Shigeki Inoue, Kotaro Kohno, Yusei Koyama, J. Trevor Mendel, Kouichiro Nakanishi, Rhythm Shimakawa, Tomoko L. Suzuki, Yoichi Tamura, Ichi Tanaka, Hannah Übler, Dave J. Wilman

    The Astrophysical Journal   834 ( 2 ) 135 - 135  2017.01  [Refereed]

    DOI

    Scopus

    103
    Citation
    (Scopus)
  • [O iii] emission line as a tracer of star-forming galaxies at high redshifts: comparison between Hα and [O iii] emitters at z=2.23 in HiZELS

    T. L. Suzuki, T. Kodama, D. Sobral, A. A. Khostovan, M. Hayashi, R. Shimakawa, Y. Koyama, K.-i. Tadaki, I. Tanaka, Y. Minowa, M. Yamamoto, I. Smail, P. N. Best

    Monthly Notices of the Royal Astronomical Society   462 ( 1 ) 181 - 189  2016.10  [Refereed]

    DOI

    Scopus

    20
    Citation
    (Scopus)
  • ENHANCED STAR FORMATION OF LESS MASSIVE GALAXIES IN A PROTOCLUSTER AT z = 2.5

    Masao Hayashi, Tadayuki Kodama, Ichi Tanaka, Rhythm Shimakawa, Yusei Koyama, Ken-ichi Tadaki, Tomoko L. Suzuki, Moegi Yamamoto

    The Astrophysical Journal Letters   826 ( 2 ) L28 - L28  2016.08  [Refereed]

    DOI

    Scopus

    28
    Citation
    (Scopus)
  • Toward unveiling internal properties of HII regions and their connections at the cosmic noon era

    Shimakawa Rhythm, Kodama Tadayuki, Hayashi Masao, Tadaki Ken-ichi, Suzuki Tomoko L, Koyama Yusei, Tanaka Ichi, Yamamoto Moegi

    GALAXIES AT HIGH REDSHIFT AND THEIR EVOLUTION OVER COSMIC TIME   11 ( S319 ) 53  2016

    DOI

  • Predicting dust extinction properties of star-forming galaxies from Hα/UV ratio

    RHYTHM SHIMAKAWA

    \mnras   453 ( 1 ) 879 - 892  2015.10  [Refereed]

    DOI

    Scopus

    32
    Citation
    (Scopus)
  • Correlation between star formation activity and electron density of ionized gas at z = 2.5

    RHYTHM SHIMAKAWA

    \mnras   451 ( 2 ) 1284 - 1289  2015.08  [Refereed]

    Authorship:Lead author

    DOI

    Scopus

    46
    Citation
    (Scopus)
  • Galaxy Formation at z > 3 Revealed by Narrowband-selected [O III] Emission Line Galaxies

    RHYTHM SHIMAKAWA

    \apj   806 ( 2 ) 208 - 208  2015.06  [Refereed]

    DOI

    Scopus

    15
    Citation
    (Scopus)
  • Identification of the progenitors of rich clusters and member galaxies in rapid formation at z > 2

    RHYTHM SHIMAKAWA

    \mnras   441 ( 1 ) L1 - L5  2014.06  [Refereed]

    Authorship:Lead author

    DOI

    Scopus

    55
    Citation
    (Scopus)
  • EVIDENCE FOR A GAS-RICH MAJOR MERGER IN A PROTO-CLUSTER AT z = 2.5

    Ken-ichi Tadaki, Tadayuki Kodama, Yoichi Tamura, Masao Hayashi, Yusei Koyama, Rhythm Shimakawa, Ichi Tanaka, Kotaro Kohno, Bunyo Hatsukade, Kenta Suzuki

    The Astrophysical Journal   788 ( 2 ) L23 - L23  2014.05  [Refereed]

     View Summary

    Gas-rich major mergers in high-redshift proto-clusters are important events, perhaps leading to the creation of the slowly rotating remnants seen in the cores of clusters in the present day. Here, we present a deep Jansky Very Large Array observation of CO J = 1-0 emission line in a proto-cluster at z = 2.5, USS1558-003. The target field is an extremely dense region, where 20 H alpha emitters (HAEs) are clustering. We have successfully detected the CO emission line from three HAEs and discovered a close pair of red and blue CO-emitting HAEs. Given their close proximity (similar to 30 kpc), small velocity offset (similar to 300 km s (1)), and similar stellar masses, they could be in the early phase of a gas-rich major merger. For the red HAE, we derive a total infrared luminosity of L-IR = 5.1 x 10(12) L-circle dot using MIPS 24 mu m and radio continuum images. The LIR/L'(CO) ratio is significantly enhanced compared to local spirals and high-redshift disks with a similar CO luminosity, which is indicative of a starburst mode. We find the gas depletion timescale is shorter than that of normal star-forming galaxies regardless of adopted CO-H-2 conversion factors. The identification of such a rare event suggests that gas-rich major mergers frequently take place in proto-clusters at z > 2 and may involve the formation processes of slow rotators seen in local massive clusters.

    DOI

  • 22
    Citation
    (Scopus)
  • Mapping and resolving galaxy formation at its peak epoch with Mahalo-Subaru and Gracias-ALMA

    Tadayuki Kodama, Masao Hayashi, Yusei Koyama, Ken-Ichi Tadaki, Ichi Tanaka, Rhythm Shimakawa, Tomoko Suzuki, Moegi Yamamoto

    Proceedings of the International Astronomical Union   10 ( 309 ) 255 - 258  2014

     View Summary

    MAHALO-Subaru (MApping HAlpha and Lines of Oxygen with Subaru) project aims to investigate how the star forming activities in galaxies are propagated as a function of time, mass, and environment. It employs a unique set of narrow-band filters on MOIRCS/Subaru to search for Ha emitters associated to the proto-clusters or in narrow redshift slices in the general field. We have shown not only filamentary/clumpy structures of all the proto-clusters but also very high star formation activities therein especially at z &gt
    2. HST images from the CANDELS survey have revealed that nearly half of the Hα emitters in the field at z ̃ 2 have clumpy structures. Among them, "red dusty clumps" are preferentially found at or near the mass center of galaxies. Therefore, they are probably linked to the formation of bulge component. To explore physical states and the mode of star formation of those forming galaxies, we have started Gracias-ALMA project in full coordination with the Mahalo-Subaru project. We will resolve molecular gas contents and dusty star formation within these galaxies, and tell whether clumps are formed by gravitational instability of gas rich disks, and whether bulges are formed by clump migration or through galaxy-galaxy mergers.

    DOI

    Scopus

  • Environmental dependence of galaxy formation explored by near-infrared spectroscopy of two protoclusters at z > 2

    Shimakawa Rhythm, Kodama Tadayuki, Tadaki Ken-ichi, Hayashi Masao, Koyama Yusei, Tanaka Ichi

    GALAXIES IN 3D ACROSS THE UNIVERSE   10 ( 309 ) 345  2014

    Authorship:Lead author

    DOI

  • 40
    Citation
    (Scopus)
  • Nature of Hα Selected Galaxies at z > 2. I. Main-sequence and Dusty Star-forming Galaxies

    RHYTHM SHIMAKAWA

    \apj   778 ( 2 ) 114 - 114  2013.12  [Refereed]

    DOI

    Scopus

    32
    Citation
    (Scopus)
  • Mahalo-subaru: Mapping star formation at the peak epoch of massive galaxy formation

    Tadayuki Kodama, Masao Hayashi, Yusei Koyama, Ken-Ichi Tadaki, Ichi Tanaka, Rhythm Shimakawa

    Proceedings of the International Astronomical Union   8 ( 295 ) 74 - 77  2012.08

     View Summary

    MAHALO-Subaru (MApping HAlpha and Lines of Oxygen with Subaru) is our on-going large programme which aims to investigate how the star forming activities in galaxies are propagated as a function of time, mass, and environment. We are targeting 10 clusters and proto-clusters at 0.4&lt
    z&lt
    2.6, and two general fields (GOODS-N and SXDF-CANDELS) with Suprime-Cam and MOIRCS by utilizing our unique sets of narrow-band filters. The narrow-band imaging can map out star forming galaxies with the redshifted Halpha and/or [OII] emission lines from our targets, and thus providing relatively unbiased views of star forming activities across time and environment. We have almost completed narrow-band imaging of our targets, and found that star forming activity is very high even in the proto-cluster cores (zâ‰1.5), and that the peak of star formation is shifted outwards with time, indicating the inside-out formation of clusters. Moreover, we have identified many red emitters especially in high density regions at z&gt
    2, which suggests that the mode of star formation and/or the activation of AGN are dependent on environment, and thus holding the key to the environmental effects at the early stage of cluster galaxies formation and evolution. © 2013 International Astronomical Union.

    DOI

    Scopus

    12
    Citation
    (Scopus)

▼display all

Books and Other Publications

  • 早稲田大学高等研究所紀要

    ( Part: Joint editor)

    早稲田大学高等研究所  2024.03

  • すばる望遠鏡とTMTが結ぶ新たな宇宙像(サイエンスブック2020)

    ( Part: Contributor, 3.2.3 銀河形成とバリオン物質の循環)

    自然科学研究機構国立天文台  2020.03

Presentations

  • すばるPFSで探る宇宙最大の低密度環境における銀河の形成進化

    嶋川里澄

    日本天文学会・2024年春季年会 

    Presentation date: 2024.03

  • New insight into the role of AGNs in forming the cluster red sequence

    Rhythm Shimakawa

    Presentation date: 2023.09

  • GALAXY CRUISE: Morphological classification with deep learning

    Rhythm Shimakawa

    Presentation date: 2023.03

  • Extragaractic Data Astronomy and Virtual Reality

    Rhythm Shimakawa  [Invited]

    Sundai Astonomy Course 

    Presentation date: 2022.12

  • The New Age of Data-Driven Astronomy

    Rhythm Shimakawa  [Invited]

    HSC-AGN Collaboration Meeting 

    Presentation date: 2022.11

  • Search for extended Lyman-α nebulae around >10k quasars at z > 2

    Rhythm Shimakawa

    Presentation date: 2022.09

  • Passive spiral galaxies deeply captured by Subaru/HSC

    Rhythm Shimakawa

    Presentation date: 2022.03

  • Subaru Telescope and AI open the way to understanding galaxy evolution

    Rhythm Shimakawa  [Invited]

    Subaru Telescope data analysis workshop 

    Presentation date: 2021.11

  • Battle Royale in Astronomy

    Rhythm Shimakawa  [Invited]

    Astro-Wakate workshop 2020 

    Presentation date: 2019.08

  • Galaxy formation in protoclusters at the cosmic high noon

    Rhythm Shimakawa  [Invited]

    EAO Subaru Science Workshop 2019 

    Presentation date: 2019.01

  • Towards the perfect consensus on redshift evolution of the ISM conditions

    Rhythm Shimakawa  [Invited]

    PFS Galaxy Workshop 

    Presentation date: 2015.11

▼display all

Research Projects

  • JWSTと究極の多波長・高解像観測による銀河団銀河の形態発現プロセスの解明

    日本学術振興会  科学研究費助成事業

    Project Year :

    2023.04
    -
    2026.03
     

    小山 佑世, 嶋川 里澄, 但木 謙一, 児玉 忠恭

  • A new picture of the local Universe unveiled by the Subaru Telescope

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)

    Project Year :

    2022.04
    -
    2025.03
     

  • Finding rare objects with artificial intelligence and big data from Subaru Telescope

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2022.04
    -
    2025.03
     

  • Identification of ionized gaseous halos in distant galaxies with extremely deep data

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Grant-in-Aid for Early-Career Scientists

    Project Year :

    2019.04
    -
    2021.03
     

    Shimakawa Rhythm

     View Summary

    This work is motivated to detect faint ionizing photons in outer regions of distant galaxies, by using the large data-set taken from the Subaru Hyper Suprime-Cam since 2014. It is expected that stacking analysis of deep narrow-band imaging data for H-Alpha emission at the certain redshifts will allow us to detect faint-end of line emission associated with galaxies. As a result, we successfully stack about 7,000 galaxies, and find a H-Alpha emission-line envelope out to a few ten kilo parsec in four billion light-years away.

  • 近赤外分光診断で明らかにする銀河形成最盛期の物理過程と環境効果

    日本学術振興会  科学研究費助成事業 特別研究員奨励費

    Project Year :

    2015.04
    -
    2017.03
     

    嶋川 里澄

     View Summary

    現在の宇宙の大規模構造を織り成す銀河団では、星形成を終了した古い大質量銀河が大半を占めている。しかしこのような大質量銀河がいつどこでどのようにして形成されたかについては良く分かっておらず、我々天文学者が長年抱える大問題の1つとなっている。この問題を明らかにするため、銀河団の過去、つまり遠方の原始銀河団を観測し、銀河団銀河形成の姿を直接調べることが極めて重要である。
    理論的に一番の課題は、銀河団は自身のもつ高い重力エネルギーの開放によって超高温のガスで満たされているため、内部のガスは十分に冷えることができず、大質量銀河を形成するような活発な星形成活動を再現できないことにある。そこで近年では原始銀河団では外から大量の冷たいガスが直接供給されるのではないかと考えられている。このような背景の元、本研究では110億光年前の原始銀河団に着目し、すばる望遠鏡の近赤外と可視撮像装置を駆使してH-ALphaとLy-Alpha輝線の観測を行った。この二つの輝線は共に星形成銀河から放出されるものであるが、後者は前者と違い、周囲の中性水素によって散乱・吸収を受けるため、 両者の輝線強度を比較することで、原始銀河団コアに冷たい水素ガスがどれほどあるかを知ることができる。
    結果、銀河団コアではそれ以外の場所に比べて有意に強い中性水素吸収があることが観測的に初めてわかった。これはつまり原始銀河団コアには外から冷たいガス供給が豊富にあることを示唆しており、宇宙遠方の原始銀河団では今日みられる大質量銀河を加速的に形成できる環境にあることを示している。本結果は申請者の博士論文としてまとめられている。

  • The peak epoch of galaxy formation as revealed by Subaru NIR imaging with 18 filters

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A)

    Project Year :

    2012.04
    -
    2017.03
     

    Kodama Tadayuki, MOTOHARA Kentaro, KONISHI Masahiro, KOYAMA Yusei, TADAKI Ken-ichi, TOSHIKAWA Jun, SHIMAKAWA Rizumu, SUZUKI Tomoko, YAMAMOTO Moegi

     View Summary

    The galaxy formation is peaked at around 10 Billion years ago in the Universe, and thus it is the critical epoch for studies of galaxy formation and evolution. We make a series of narrow-band and medium-band filters on a new NIR camera SWIMS in order to conduct a large super multi-band imaging survey by capturing nebular emission lines from star forming galaxies and spectral break feature from older galaxies. This enables us to explore the histories of star formation and mass assembly and their environmental dependence. We have optimized the survey design based on the sensitivities of each filter. In preparation for the above mentioned large survey, we have conducted various observational projects with Subaru and ALMA telescopes targeting the galaxies in this critical peak epoch of their formation. These results are published in 26 papers in major refereed journals.

▼display all

Misc

  • The Nature and Environment of Dusty Star-Forming Galaxies Near and Far Revealed by AKARI

    Koyama Yusei, Matsuki Yasuhiro, Koyama Shuhei, Nakagawa Takao, Matsuhara Hideo, Takita Satoshi, Yamashita Takuji, MorokumaMatsui Kana, Kodama Tadayuki, Shimakawa Rhythm, Namiki Shigeru, Hayashi Masao, Tadaki Kenichi

      ( 17 ) 205 - 208  2018.03

     View Summary

    The 4th AKARI International Conference: The Cosmic Wheel and the Legacy of the AKARI archive: from galaxies and stars to planets and life (October 17-20, 2017. The University of Tokyo), Bunkyo-ku, Tokyo, JapanWe present our continuous efforts over the last decade - since the launch of AKARI - to unveil the environmental impacts on dust properties of galaxies in the local and distant universe by making full use of the AKARI 'all-sky' and 'pointed' observations. We first introduce our new result on the environmental effects on the dust temperature (T(sub dust)) of local star-forming galaxies. By performing stacking analysis of the AKARI FIS all-sky map (at the positions of SDSS star-forming galaxies in different environments), we find that T(sub dust) of galaxies increases with increasing environmental density, supporting a cold dust stripping scenario in high-density environments. We also present the results from our systematic, wide-field MIR 'pointed' observations of distant clusters with AKARI/IRC, in combination with our Subaru H alpha observing campaign. Taking advantage of the wide-field coverage of AKARI (and Subaru), we revealed that dust-obscured galaxies are most frequently triggered at the periphery of distant clusters. The coincidence of the environment of dusty galaxies and that of galaxy color transition suggests a strong link between dusty galaxies and the process of environmental quenching during the course of cluster-scale assembly.Physical characteristics: Original contains color illustrations

    CiNii

 

Syllabus

Teaching Experience

  • Fundamental Physics B

    Waseda University  

    2024.04
    -
    Now
     

  • Fundamental Physics A

    Waseda University  

    2024.04
    -
    Now
     

 

Social Activities

Academic Activities

Sub-affiliation

  • Faculty of Science and Engineering   School of Fundamental Science and Engineering

Research Institute

  • 2023
    -
    2024

    Center for Data Science   Concurrent Researcher

Internal Special Research Projects

  • ジェイムズ・ウェッブ宇宙望遠鏡が拓く銀河団とブラックホールの共進化研究の新展開

    2023   田中壱, 児玉忠恭, 小山佑世, 田中賢幸, Perez-Martinez, J. M., Hatch, N. A., Rottgering, H. J. A., Dannerbauer, H.

     View Summary

    当該課題は2021年末にNASAによって打ち上げられた次世代宇宙望遠鏡であるジェイムズ・ウェッブ宇宙望遠鏡にて、第一期国際共同利用提案として採択された、2023年6月に予定する近赤外観測に向けた多波長データ解析と観測準備、および実際に取得された観測データの解析を主としたものである。観測対象はスパイダーウェッブ原始銀河団と呼ばれる、過去20年に渡って精力的に調査がなされた銀河団の祖先で、ジェイムズ・ウェッブ宇宙望遠鏡の第一期観測として最適と呼べる有名領域である。近年、代表者を含めた研究チームが同天体の観測を国際的に牽引しており、本課題では、代表者はこの原始的銀河団でまさに成長を終えようとしている多数の巨大銀河と、内包する大質量ブラックホールに焦点を当てて、研究を行った。本研究では、過去20年の研究遺産を総括するように、これまでの可視・近赤外観測データの解析に加えて、チャンドラX線宇宙望遠鏡やALMA望遠鏡など、X線からサブミリ波に渡る多波長データを全て集めて、包括的な研究を行なった。結果、非常に興味深いことに、これまで成長途上だと思われていた巨大銀河の半数が、すでに星形成活動を終えつつあり、これら全てにおいて活動的ブラックホールの兆候が捉えられた。これは銀河の活動の終焉と巨大ブラックホール活動が密接にリンクしていることを意味している。本結果は2023年9月に名古屋大学で開催された日本天文学会で報告後、国際査読論文として投稿し、2024年初頭に受理、および発行された。2023年6月にはジェイムズ・ウェッブ宇宙望遠鏡データも無事取得され、現在精力的に解析にあたっている最中であり、科学的インパクトの高い成果が得られることが大いに期待される。