Tadayuki Kodama

Galaxy protocluster candidates around z∼ 2.4 radio galaxies

We study the environments of six radio galaxies at 2.2 < z < 2.6 using wide-field near-infrared images. We use colour cuts to identify galaxies in this redshift range, and find that three of the radio galaxies are surrounded by significant surface overdensities of such galaxies. The excess galaxies that comprise these overdensities are strongly clustered, suggesting they are physically associated. The colour distribution of the galaxies responsible for the overdensity is consistent with those of galaxies that lie within a narrow redshift range at z ∼ 2.4. Thus the excess galaxies are consistent with being companions of the radio galaxies. The overdensities have estimated masses in excess of 10 14 M � , and are dense enough to collapse into virialized structures by the present day; these structures may evolve into groups or clusters of galaxies. A flux-limited sample of protocluster galaxies with K < 20.6 mag is derived by statistically subtracting the foreground and background galaxies. The colour distribution of the protocluster galaxies is bimodal, consisting of a dominant blue sequence, comprising 77 ± 10 per cent of the galaxies and a poorly populated red sequence. The blue protocluster galaxies have similar colours to local star-forming irregular galaxies (U − V AB ∼ 0.6), suggesting that most protocluster galaxies are still forming stars at the observed epoch. The blue colours and lack of a dominant protocluster red sequence imply that these cluster galaxies form the bulk of their stars at z 3.

A Strong Lyα Emitter at z = 6.33 in the Subaru Deep Field Selected as an i'-Dropout*

We report on the discovery of a star-forming galaxy at z = 6.33 in the Subaru Deep Field. This object is selected as a candidate of an i-dropout high-redshift galaxy around z = 6 because of its red i - z color in our deep optical imaging survey in the Subaru Deep Field. Our follow-up optical spectroscopy reveals that this object is a strong Ly? emitter with only very faint ultraviolet continuum. The rest-frame equivalent width of the detected Ly? emission is as much as 130 ?. Thus the light detected in our z image is largely attributed to the Ly? emission, i.e., ~40% of the z-band flux is the strong Ly? emission, giving a very red i - z color. This is consistent with the photometric property of this object because the narrowband data obtained with the NB921 filter show a significant depression, z - NB921 = -0.54 mag. By using the photometric data, we show that some other objects among the 48 i-dropout high-redshift galaxy candidates found in the Subaru Deep Field also show a significant NB921 depression. We briefly discuss the nature of these NB921-depressed objects.

Lyα blobs like company: the discovery of a candidate 100 kpc Lyα blob near to a radio galaxy with a giant Lyα halo B3 J2330+3927 at z= 3.1★

We present the discovery of a candidate of giant radio-quiet Lyα blob (RQLAB) in a large-scale structure around a high-redshift radio galaxy (HzRG) lying in a giant Lyα halo B3xa0J2330+3927 at redshift z= 3.087. We obtained narrow- and broad-band imaging around B3xa0J2330+3927 with Subaru/Suprime-Cam to search for Lyα emitters (LAEs) and absorbers (LAAs) at redshift z= 3.09 ± 0.03. We detected candidate 127 LAEs and 26 LAAs in the field of view of 31 × 24xa0arcmin2 (58 × 44 comoving Mpc). We found that B3xa0J2330+3927 is surrounded by a 130xa0kpc Lyα halo and a large-scale (∼60 × 20 comoving Mpc) filamentary structure. The large-scale structure contains one prominent local density peak with an overdensity of greater than 5, which is 8xa0arcmin (15 comoving Mpc) away from B3xa0J2330+3927. In this peak, we discovered a candidate 100xa0kpc RQLAB. The existence of both types of Lyα nebulae in the same large-scale structure suggests that giant Lyα nebulae need special large-scale environments to form. On smaller scales, however, the location of B3xa0J2330+3927 is not a significant local density peak in this structure, in contrast to the RQLAB. There are two possible interpretations of the difference of the local environments of these two Lyα nebulae. First, RQLAB may need a prominent (δ∼ 5) density peak of galaxies to form through intense starbursts due to frequent galaxy interactions/mergers and/or continuous gas accretion in an overdense environment. On the other hand, Lyα halo around HzRG may not always need a prominent density peak to form if the surrounding Lyα halo is mainly powered by its radio and active galactic nucleus activities. Alternatively, both RQLAB and Lyα halo around HzRG may need prominent density peaks to form but we could not completely trace the density of galaxies because we missed evolved and dusty galaxies in this survey.

Early-Type galaxies at z ~ 1.3. III. On the dependence of Formation Epochs and Star Formation Histories on Stellar Mass and Environment

We study the environmental dependence of stellar population properties at z ~ 1.3. We derive galaxy properties (stellar masses, ages, and star formation histories) for samples of massive, red, passive early-type galaxies (ETGs) in two high-redshift clusters, RXJ0849+4452 and RXJ0848+4453 (with redshifts of z = 1.26 and 1.27, respectively), and compare them with those measured for the RDCS1252.9–2927 cluster at z = 1.24 and with those measured for a similarly mass-selected sample of field contemporaries drawn from the GOODS-South field. Robust estimates of the aforementioned parameters have been obtained by comparing a large grid of composite stellar population models with extensive 8- to 10-band photometric coverage, from the rest-frame far-ultraviolet to the infrared. We find no variations of the overall stellar population properties among the different samples of cluster ETGs. However, when comparing cluster versus field stellar population properties we find that, even if the ages are similar and depend only on galaxy mass, the ones in the field do employ longer timescales to assemble their final mass. We find that, approximately 1 Gyr after the onset of star formation, the majority (75%) of cluster galaxies have already assembled most (>80%) of their final mass, while, by the same time, fewer (35%) field ETGs have. Thus, we conclude that while galaxy mass regulates the timing of galaxy formation, the environment regulates the timescale of their star formation histories.

Galaxy protocluster candidates at 1.6 < z ≲ 2

We present a study of protoclusters associated with high redshift radio galaxies. We imaged MRC 1017-220 (z = 1.77) and MRC 0156252 (z = 2.02) using the near-infrared wide-field (7.5 � × 7.5 � )i mager VLT/HAWK-I in the Y, H ,a ndKs bands. We present the first deep Y-band galaxy number counts within a large area (∼200 arcmin 2 ). We develop a purely near-infrared colour selection technique to isolate galaxies at 1.6 < z < 3 that may be associated with the two targets, dividing them into (i) red passively evolving or dusty star forming galaxies or (ii) blue/star formation dominated galaxies with little or no dust. Both targeted fields show an excess of star forming galaxies with respect to control fields. No clear overdensity of red galaxies is detected in the surroundings of MRC 1017-220 although the spatial distribution of the red galaxies resembles a filament-like structure within which the radio galaxy is embedded. In contrast, a significant overdensity of red galaxies is detected in the field of MRC 0156-252, ranging from a factor of ∼2−3 times the field density at large scales (2.5 Mpc, angular distance) up to a factor of ∼3−4 times the field density within a 1 Mpc radius of the radio galaxy. Half of these red galaxies have colours consistent with red sequence models at z ∼ 2, with a large fraction being bright (Ks < 21.5, i.e. massive). In addition, we also find three galaxies within 5 �� of MRC 0156-252 suggesting that the radio galaxy has multiple companions within ∼50 kpc. We conclude that the field of MRC0156-252 shows many remarkable similarities with the wellstudied protocluster surrounding PKS1138-262 (z = 2.16) suggesting that MRC 0156-252 is associated with a galaxy protocluster at z ∼ 2.

A galaxy populations study of a radio-selected protocluster at z∼ 3.1

We present a population study of several types of galaxies within the protocluster surrounding the radio galaxy MRC 0316-257 at z ~3.1. In addition to the known population of Lyα emitters and [O III] emitters, we use colour-selection techniques to identify protocluster candidates that are Lyman break galaxies (LBG) and Balmer break galaxies (BBGs). The radio galaxy field contains an excess of LBG candidates, with a surface density 1.6 ± 0.3 times larger than found for comparable blank fields. This surface overdensity corresponds to an LBG volume overdensity of ~8 ± 4. The BBG photometric redshift distribution peaks at the protoclusters redshift, but we detect no significant surface overdensity of BBG. This is not surprising because a volume overdensity similar to the LBGs would have resulted in a surface density of ~1.2 that found in the blank field. This could not have been detected in our sample. Masses and star formation rates of the candidate protocluster galaxies are determined using spectral energy distribution fitting. These properties are not significantly different from those of field galaxies. The galaxies with the highest masses and star formation rates are located near the radio galaxy, indicating that the protocluster environment influences galaxy evolution at z ~ 3. We conclude that the protocluster around MRC 0316-257 is still in the early stages of formation.

Star formation activities of galaxies in the large-scale structures at z = 1.2

Recent wide-field imaging observations of the X-ray luminous cluster RDCS J1252.9-2927 at z = 1.24 uncovered several galaxy groups that appear to be embedded in filamentary structure extending from the cluster core. We make a spectroscopic study of the galaxies in these groups using GMOS on Gemini-South and FORS2 on VLT with the aim of determining if these galaxies are physically associated to the cluster. We find that three groups contain galaxies at the cluster redshift and that they are probably bound to the cluster. This is the first confirmation of filamentary structure as traced by galaxy groups at z > 1. We then use several spectral features in the FORS2 spectra to determine the star formation histories of group galaxies. We find a population of relatively red star-forming galaxies in the groups that are absent from the cluster core. While similarly red star forming galaxies can also be found in the field, the average strength of the Hδ line is systematically weaker in group galaxies. Interestingly, these groups at z = 1.2 are in an environment in which the on-going build-up of red sequence is happening. The unusual line strengths can be explained by star formation that is heavily obscured by dust. We hypothesize that galaxy-galaxy interactions, which is more efficient in the group environment, is the mechanism that drives these dust obscured star formation. The hypothesis can be tested by obtaining spectral observations in the near-IR, high resolution imaging observations and observations in the mid-IR.

An AzTEC 1.1‐mm survey for ULIRGs in the field of the Galaxy Cluster MS 0451.6−0305

We have undertaken a deep (σ∼ 1.1 mJy) 1.1-mm survey of the z= 0.54 cluster MS 0451.6−0305 using the AzTEC camera on the James Clerk Maxwell Telescope. We detect 36 sources with signal-to-noise ratio (S/N) ≥ 3.5 in the central 0.10 deg2 and present the AzTEC map, catalogue and number counts. We identify counterparts to 18 sources (50 per cent) using radio, mid-infrared, Spitzer InfraRed Array Camera (IRAC) and Submillimetre Array data. Optical, near- and mid-infrared spectral energy distributions are compiled for the 14 of these galaxies with detectable counterparts, which are expected to contain all likely cluster members. We then use photometric redshifts and colour selection to separate background galaxies from potential cluster members and test the reliability of this technique using archival observations of submillimetre galaxies. We find two potential MS 0451−03 members, which, if they are both cluster galaxies, have a total star formation rate (SFR) of ∼100 M⊙ yr−1– a significant fraction of the combined SFR of all the other galaxies in MS 0451−03. We also examine the stacked rest-frame mid-infrared, millimetre and radio emission of cluster members below our AzTEC detection limit, and find that the SFRs of mid-IR-selected galaxies in the cluster and redshift-matched field populations are comparable. In contrast, the average SFR of the morphologically classified late-type cluster population is nearly three times less than the corresponding redshift-matched field galaxies. This suggests that these galaxies may be in the process of being transformed on the red sequence by the cluster environment. Our survey demonstrates that although the environment of MS 0451−03 appears to suppress star formation in late-type galaxies, it can support active, dust-obscured mid-IR galaxies and potentially millimetre-detected LIRGs.

ALMA 26 arcmin2 Survey of GOODS-S at One-millimeter (ASAGAO): X-Ray AGN Properties of Millimeter-selected Galaxies

We investigate the X-ray active galactic nucleus (AGN) properties of millimeter galaxies in the Great Observatories Origins Deep Survey South (GOODS-S) field detected with the Atacama Large Millimeter/submillimeter Array (ALMA), by utilizing the Chandra 7-Ms data, the deepest X-ray survey to date. Our millimeter galaxy sample comes from the ASAGAO survey covering 26 arcmin(2) (12 sources at a 1.2. mm flux-density limit of approximate to 0.6 mJy), supplemented by the deeper but narrower 1.3. mm survey of a part of the ASAGAO field by Dunlop et al. Ofthe 25 total millimeter galaxies, 14 have Chandra counterparts. The observed AGN fractions at z = 1.5-3 are found to be 90(-19)(+8)% and 57(-25)(+23)% for the ultra-luminous and luminous infrared galaxies with log L-IR/L-circle dot = 12-12.8 and log L-IR/L-circle dot = 11.5-12, respectively. The majority (similar to 2/3) of the ALMA and/or Herschel detected X-ray AGNs at z = 1.5-3 appear to be star-formation-dominant populations, having L-X/L-IR ratios smaller than the simultaneous evolution value expected from the local black-hole-mass-to-stellar-mass (M-BH-M*) relation. On the basis of the L-X and stellar mass relation, we infer that a large fraction of star-forming galaxies at z = 1.5-3 have black hole masses that are smaller than those expected from the local M-BH-M* relation. This contrasts previous reports on luminous AGNs at the same redshifts detected in wider and shallower surveys, which are subject to selection biases against lower luminosity AGNs. Our results are consistent with an evolutionary scenario in which star formation occurs first, and an AGN-dominant phase follows later, in objects that finally evolve into galaxies with classical bulges.

First Results on the Cluster Galaxy Population from the Subaru Hyper Suprime-Cam Survey. III. Brightest Cluster Galaxies, Stellar Mass Distribution, and Active Galaxies

The unprecedented depth and area surveyed by the Subaru Strategic Program with the Hyper Suprime-Cam (HSC-SSP) have enabled us to construct and publish the largest distant cluster sample out to