Yusei Koyama

National Astronomical Observatory of Japan

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Tadayuki Kodama

Tohoku University

130

Rhythm Shimakawa

Waseda University

Ichi Tanaka

Subaru (Japan)

Masao Hayashi

Kanazawa University Hospital

Ken-ichi Tadaki

Hokkai Gakuen University

Tomoko L. Suzuki

Kavli Institute for the Physics and Mathematics of the Universe

Jose Manuel Pérez-Martínez

Instituto de Astrofísica de Canarias

Kentaro Motohara

The University of Tokyo

Yutaka Komiyama

National Astronomical Observatory of Japan

Masayuki Tanaka

The Graduate University for Advanced Studies, SOKENDAI

Cooperative Institutions

National Astronomical Observatory of Japan

211

The University of Tokyo

210

National Institutes of Natural Sciences

Kavli Institute for the Physics and Mathematics of the Universe

The Graduate University for Advanced Studies, SOKENDAI

Tohoku University

Max Planck Society

Centre National de la Recherche Scientifique

Tokyo University of Science

Kyoto University

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Large-scale environment of z ∼ 5.7 C iv absorption systems –II. Spectroscopy of Lyman α emitters★

Monthly Notices of the Royal Astronomical Society (2015)

C. Díaz E. V. Ryan‐Weber Jeff Cooke Yusei Koyama Masami Ouchi

The flow of baryons to and from a galaxy, which is fundamental for galaxy formation and evolution, can be studied with galaxy-metal absorption system pairs. Our search for galaxies around CIV absorption systems at $z\sim5.7$ showed an excess of photometric Lyman-$\alpha$ emitter (LAE) candidates in the fields J1030+0524 and J1137+3549. Here we present spectroscopic follow-up of 33 LAEs in both fields. In the first field, three out of the five LAEs within 10$h^{{-}1}$ projected comoving Mpc from the CIV system are within $\pm500$ km s$^{{-}1}$ from the absorption at $z_{\text{CIV}}=5.7242\pm0.0001$. The closest candidate (LAE 103027+052419) is robustly confirmed at $212.8^{+14}_{-0.4}h^{-1}$ physical kpc from the CIV system. In the second field, the LAE sample is selected at a lower redshift ($\Delta z\sim0.04$) than the CIV absorption system as a result of the filter transmission and, thus, do not trace its environment. The observed properties of LAE 103027+052419 indicate that it is near the high-mass end of the LAE distribution, probably having a large HI column density and large-scale outflows. Therefore, our results suggest that the CIV system is likely produced by a star-forming galaxy which has been injecting metals into the intergalactic medium since $z>6$. Thus, the CIV system is either produced by LAE 103027+052419, implying that outflows can enrich larger volumes at $z>6$ than at $z\sim3.5$, or an undetected dwarf galaxy. In either case, CIV systems like this one trace the ionized intergalactic medium at the end of cosmic hydrogen reionization and may trace the sources of the ionizing flux density.

10.1093/mnras/stu2738

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Identification of variability in recent star formation histories of local galaxies based on H$\alpha$/UV ratio

arXiv (Cornell University) (2017)

Rhythm Shimakawa Yusei Koyama J. X. Prochaska Yicheng Guo Ken-ichi Tadaki

Because the timescale of H$\alpha$ emission (several tens of Myr) following star formation is significantly shorter than that of UV radiation (a few hundred Myr), the H$\alpha$/UV flux ratio provides insight on the star formation histories (SFHs) of galaxies on timescales shorter than $\sim100$ Myr. We present H$\alpha$/UV ratios for galaxies at $z=$ 0.02--0.1 on the familiar star-forming main sequence based on the AKARI-GALEX-SDSS archive dataset. The data provide us with robust measurements of dust-corrected SFRs in both H$\alpha$ and UV for 1,050 galaxies. The results show a correlation between the H$\alpha$/UV ratio and the deviation from the main sequence in the sense that galaxies above/below the main sequence tend to have higher/lower H$\alpha$/UV ratios. This trend increases the dispersion of the main sequence by 0.04 dex (a small fraction of the total scatter of 0.36 dex), suggesting that diversity of recent SFHs of galaxies has a direct impact on the observed main sequence scatter. We caution that the results suffer from incompleteness and a selection bias which may lead us to miss many sources with high H$\alpha$/UV ratios, this could further increase the scatter from SFHs in the star-forming main sequence.

Star (game theory)

Sequence (biology)

Source

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

Publications of the Astronomical Society of Japan (2019)

Masao Hayashi Yusei Koyama Tadayuki Kodama Yutaka Komiyama Yen‐Ting Lin

We present the large-scale structure over 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 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 by the follow-up spectroscopy with Subaru/FOCAS and Gemini-N/GMOS. This suggests that the structure ever known as the CL1604 supercluster is the tip of the iceberg. We investigate stellar population of the red-sequence galaxies using 4000 A break and Balmer H$\delta$ absorption line. Almost all of the red-sequence galaxies brighter than 21.5 mag in $z$-band show an old stellar population with $\gtrsim2$ Gyr. The comparison of composite spectra of the red-sequence galaxies in the individual clusters show that the galaxies at a similar redshift have 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 at 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.

Supercluster (genetic)

Stellar population

10.1093/pasj/psz097

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A Phase-space View of Cold-gas Properties of Virgo Cluster Galaxies: Multiple Quenching Processes at Work?

The Astrophysical Journal (2021)

Kana Morokuma-Matsui Tadayuki Kodama Tomoki Morokuma Kouichiro Nakanishi Yusei Koyama

Abstract We investigate the cold-gas properties of massive Virgo galaxies (>10 9 M ⊙ ) at <3 R 200 ( R 200 is the radius where the mean interior density is 200 times the critical density) on the projected phase-space diagram with the largest archival data set to date to understand the environmental effects on galaxy evolution in the Virgo cluster. We find lower H i and H 2 mass fractions and higher star formation efficiencies (SFEs) from H i and H 2 in the Virgo galaxies than in the field galaxies for matched stellar masses; the Virgo galaxies generally follow the field relationships between the offset from the main sequence of the star-forming galaxies [Δ(MS)] and the gas fractions and SFEs, to the slight offset to lower gas fractions or higher SFEs compared to field galaxies at Δ(MS) < 0; lower gas fractions in galaxies with smaller clustocentric distance and velocity; and lower gas fractions in the galaxies in the W cloud, a substructure of the Virgo cluster. Our results suggest the cold-gas properties of some Virgo galaxies are affected by their environment at least at 3 R 200 maybe via strangulation and/or preprocesses, and H i and H 2 in some galaxies are removed by ram pressure at <1.5 R 200 . Our data cannot rule out the possibility of other processes such as strangulation and galaxy harassment accounting for gas reduction in some galaxies at <1.5 R 200 . Future dedicated observations of a mass-limited complete sample are required for definitive conclusions.

Virgo Cluster

10.3847/1538-4357/abedb6

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Wide-Field Study of Distant Clusters of Galaxies with SPICA

Yusei Koyama Tadayuki Kodama

We propose an intensive, systematic study of distant clusters of galaxies at z ~ 0.5–5 and their surrounding infall regions, taking advantage of the significant wide fieldof-view of MIRACLE (6' × 6') and the high sensitivity at λ ≳ 20 μm of MIRACLE and SAFARI. Galaxy clusters and groups are mainly composed of red quiescent galaxies in the local Universe, but the star formation activity becomes much higher in distant clusters. Our coordinated AKARI/Subaru observations have revealed many dusty starburst galaxies in particular in intermediate-density environments such as groups at z ~ 1. Interestingly, this is also the same environment in which we see a sharp transition from blue galaxies to red ones. This suggests that dusty starbursts are strongly linked with the physical process that alters the properties of galaxies as they infall and assemble to higher density regions. We also expect that the site of galaxy formation with high star formation activity shifts to higher-density environments as we go back in time, probably due to spatial bias in galaxy formation. By observing many distant clusters at various redshifts between z ~ 0.5–5 with SPICA, and identifying dusty, star-forming galaxies across the wide area in and around the clusters/proto-clusters, we will trace back the star formation history of galaxies as a function of environment. We will thus be able to identify directly the epoch and the site of formation of present-day early-type galaxies.

Spica

10.1051/spica/200904013

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Environmental dependence of polycyclic aromatic hydrocarbon emission atz~ 0.8

Astronomy and Astrophysics (2015)

K. L. Murata Yusei Koyama Masayuki Tanaka Hideo Matsuhara Tadayuki Kodama

We study the environmental dependence of the strength of polycyclic aromatic hydrocarbon (PAH) emission using AKARI observations of RX J0152.7-1357, a galaxy cluster at z = 0.84. PAH emission reflects the physical conditions of galaxies and dominates 8 μm luminosity (L8), which can be directly measured with the L15 band of AKARI. The ratio of L8 to infrared luminosity (LIR) is used as a tracer of the PAH strength. Both photometric and spectroscopic redshifts are applied to identify the cluster members. The galaxies detected with the L15 band tend to reside in the outskirts of the cluster and have an optically green colour, R − z′ ~ 1.2. We find no clear difference between the L8/LIR behaviour of galaxies in field and cluster environments. The L8/LIR of cluster galaxies decreases with specific star formation rate divided by that of main-sequence galaxies, and with LIR, consistent with the results for field galaxies. The relation between L8/LIR and LIR is between those at z = 0 and z = 2 in the literature. Our data also shows that starburst galaxies, which have lower L8/LIR than main-sequence galaxies, are located only in the outskirts of the cluster. All these findings extend previous studies, indicating that environment affects only the fraction of galaxy types and does not affect the L8/LIR behaviour of star-forming galaxies.

10.1051/0004-6361/201526276

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Strange Filamentary Structures (“Fireballs”) around a Merger Galaxy in the Coma Cluster of Galaxies

The Astrophysical Journal (2008)

M. Yoshida Masafumi Yagi Yutaka Komiyama Hisanori Furusawa Nobunari Kashikawa

We found an unusual complex of narrow blue filaments, bright blue knots, and Hα-emitting filaments and clouds, which morphologically resembled a complex of "fireballs," extending up to 80 kpc south from an E+A galaxy RB 199 in the Coma Cluster. The galaxy has a highly disturbed morphology indicative of a galaxy-galaxy merger remnant. The narrow blue filaments extend in straight shapes toward the south from the galaxy, and several bright blue knots are located at the southern ends of the filaments. The RC band absolute magnitudes, half-light radii, and estimated masses of the bright knots are ~–12 to –13 mag, ~200-300 pc, and ~106-107 M☉, respectively. Long, narrow Hα-emitting filaments are connected at the south edge of the knots. The average color of the fireballs is B − RC ≈ 0.5, which is bluer than RB 199 (B − R = 0.99), suggesting that most of the stars in the fireballs were formed within several times 108 yr. The narrow blue filaments exhibit almost no Hα emission. Strong Hα and UV emission appear in the bright knots. These characteristics indicate that star formation recently ceased in the blue filaments and now continues in the bright knots. The gas stripped by some mechanism from the disk of RB 199 may be traveling in intergalactic space, forming stars left along its trajectory. The most plausible fireball formation mechanism is ram pressure stripping by high-speed collision between the galaxy and the hot intracluster medium. The fireballs may be a snapshot of diffuse intracluster population formation, or halo star population formation in a cluster galaxy.

Coma Cluster

Ram pressure

10.1086/592430

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Citations (103)

Dependence of the build-up of the colour-magnitude relation on cluster richness at z 0.8

Monthly Notices of the Royal Astronomical Society (2007)

Yusei Koyama Tadayuki Kodama Masaomi Tanaka Kazuhiro Shimasaku S. Okamura

We present environmental dependence of the build-up of the colour–magnitude relation (CMR) at z∼ 0.8. It is well established that massive early-type galaxies exhibit a tight CMR in clusters up to at least z∼ 1. The faint end of the relation, however, has been much less explored especially at high redshifts primarily due to limited depths of the data. Some recent papers have reported a deficit of the faint red galaxies on the CMR at 0.8 ≲z≲ 1, but this has not been well confirmed yet and is still controversial. Using a deep, multi-colour, panoramic imaging data set of the distant cluster RXJ1716.4+6708 at z= 0.81, newly taken with the Prime Focus Camera (Suprime-Cam) on the Subaru Telescope, we carry out an analysis of faint red galaxies, taking care not to underestimate the number of these galaxies. We find that there is a sharp decline in the number of red galaxies toward the faint end of the CMR below M*+ 2. We compare our result with those for other clusters at z∼ 0.8 taken from the literature, which show or do not show the deficit. We suggest that the 'deficit' of faint red galaxies is dependent on the richness or mass of the clusters, in the sense that poorer systems show stronger deficits. This indicates that the evolutionary stage of less massive galaxies depends critically on environment.

Coma Cluster

10.1111/j.1365-2966.2007.12445.x

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Citations (31)

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

Monthly Notices of the Royal Astronomical Society (2015)

Yusei Koyama Tadayuki Kodama Masao Hayashi Rhythm Shimakawa I. Yamamura

Using star-forming galaxies sample in the nearby Universe (0.02

Balmer series

Extinction (optical mineralogy)

Mass ratio

Stellar mass

10.1093/mnras/stv1599

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Citations (33)

The Hyper Suprime-Cam SSP Survey: Overview and survey design

Publications of the Astronomical Society of Japan (2017)

H. Aihara N. Arimoto R. Armstrong S. Arnouts Neta A. Bahcall

Hyper Suprime-Cam (HSC) is a wide-field imaging camera on the prime focus of the 8.2m Subaru telescope on the summit of Maunakea in Hawaii. A team of scientists from Japan, Taiwan and Princeton University is using HSC to carry out a 300-night multi-band imaging survey of the high-latitude sky. The survey includes three layers: the Wide layer will cover 1400 deg$^2$ in five broad bands ($grizy$), with a $5\,\sigma$ point-source depth of $r \approx 26$. The Deep layer covers a total of 26~deg$^2$ in four fields, going roughly a magnitude fainter, while the UltraDeep layer goes almost a magnitude fainter still in two pointings of HSC (a total of 3.5 deg$^2$). Here we describe the instrument, the science goals of the survey, and the survey strategy and data processing. This paper serves as an introduction to a special issue of the Publications of the Astronomical Society of Japan, which includes a large number of technical and scientific papers describing results from the early phases of this survey.

Summit

Subaru Telescope

Survey research

10.1093/pasj/psx066

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Citations (877)