Ichi Tanaka

The Subaru/XMM-Newton Deep Survey (SXDS) - II. Optical Imaging and Photometric Catalogs

We present multiYwave band optical imaging data obtained from observations of the Subaru/XMM-Newton Deep Survey (SXDS). The survey field, centered at R:A: ¼ 02 h 18 m 00 s , decl: ¼� 05 � 00 0 00 00 , has been the focus of a wide range of multiwavelength observing programs spanning from X-ray to radio wavelengths. A large part of the optical imaging observations are carried out with Suprime-Cam on Subaru Telescope at Mauna Kea in the course of Subaru Telescope ‘‘Observatory Projects.’’ This paper describes our optical observations, data reduction and analysis procedures employed, and the characteristics of the data products. A total area of 1.22 deg 2 is covered in five contiguous subfields,eachof whichcorrespondstoasingleSuprime-Camfieldof view(� 34 0 ; 27 0 ),infivebroadbandfilters,B, V,Rc,i 0 ,andz 0 ,tothedepthsof B ¼ 28:4,V ¼ 27:8,Rc ¼ 27:7,i 0 ¼ 27:7,andz 0 ¼ 26:6,respectively(AB,3 � , � ¼ 2 00 ). The data are reduced and compiled into five multiYwave band photometric catalogs, separately for each SuprimeCampointing.Thei 0 -bandcatalogscontainabout900,000objects,makingtheSXDScatalogsoneof thelargestmultiY wavebandcatalogsincorrespondingdepthandareacoverage.TheSXDScatalogscanbeusedforanextensiverangeof astronomicalapplicationssuchasthenumberdensityof theGalactichalostarstothelarge-scalestructuresatthedistant universe. The number counts of galaxies are derived and compared with those of existing deep extragalactic surveys. The optical data, the source catalogs, and configuration files used to create the catalogs are publicly available via the SXDS Web page (http://www.naoj.org/Science/SubaruProject/SXDS/index.html). Subject headingg cosmology: observations — galaxies: evolution — galaxies: formation — galaxies: photometry — large-scale structure of universe

Down‐sizing in galaxy formation at z∼ 1 in the Subaru/XMM–Newton Deep Survey (SXDS)

We use the deep wide-field optical imaging data of the Subaru/XMM‐Newton Deep Survey to discuss the luminosity- (mass-)dependent galaxy colours down to z � = 25.0 (5 × 10 9 h −2 70 M� ) for z ∼ 1 galaxies in colour-selected high-density regions. We find an apparent absence of galaxies on the red colour‐magnitude sequence below z � ∼ 24.2, corresponding to ∼M ∗ + 2( ∼ 10 10 M� ) with respect to passively evolving galaxies at z ∼ 1. Galaxies brighter than M ∗ − 0.5 (8 × 10 10 M� ), however, are predominantly red passively evolving systems, with few blue star-forming galaxies at these magnitudes. This apparent age gradient, where massive galaxies are dominated by old stellar populations while less massive galaxies have more extended star formation histories, supports the ‘downsizing’ idea where the mass of galaxies hosting star formation decreases as the Universe ages. Combined with the lack of evolution in the shape of the stellar mass function for massive galaxies since at least z ∼ 1, it appears that galaxy formation processes (both star formation and mass assembly) should have occurred in an accelerated way in massive systems in highdensity regions, while these processes should have been slower in smaller systems. This result provides an interesting challenge for modern cold dark matter based galaxy formation theories which predict later formation epochs of massive systems, commonly referred to as ‘bottom-up’.

The first appearance of the red sequence of galaxies in proto‐clusters at 2 ≲z≲ 3

We explore the evolved galaxy population in the proto-clusters around four high-z radio galaxies at 2 ≤ z ≤ 3 based on wide-field near-infrared (NIR) imaging. Three of the four fields are known proto-clusters as demonstrated by overdensities of line-emitting galaxies at the same redshifts as the radio galaxies found by narrow-band surveys and spectroscopic follow-up observations. We imaged the fields of three targets (PKS 1138-262, USS 0943-242 and MRC 0316-257) to a depth of K s ∼ 22 (Vega magnitude, 5σ) over a 4 x 7 arcmin 2 area centred on the radio galaxies with a new wide-field NIR camera, Multi-Object Infra-Red Camera and Spectrograph (MOIRCS), on the Subaru Telescope. Another target (USS 1558-003) was observed with Son of ISAAC on the New technology Telescope (NTT) to a depth of K s = 20.5 (5σ) over a 5 x 5 arcmin 2 area. We apply colour cuts in J-K s and/or JHK s in order to exclusively search for galaxies located at high redshifts: z > 2. To the 5σ limiting magnitudes, we see a significant excess of NIR-selected galaxies by a factor of 2 to 3 compared to those found in the field of GOODS-South. The spatial distribution of these NIR-selected galaxies is not uniform and traces structures similar to those of emission-line galaxies, although the samples of NIR-selected galaxies and emitters show little overlap, from which we conclude that the former tend to be an evolved population with much higher stellar mass than the latter, young and active emitters. We focus on the NIR colour-magnitude sequence of the evolved population and find that the bright-end (M stars > 10 11 M ⊙ ) of the red sequence is well populated by z ∼ 2 but much less so in the z ∼ 3 proto-clusters. This may imply that the bright-end of the colour-magnitude sequence first appeared between z = 3 and 2, an era coinciding with the appearance of sub-mm galaxies and the peak of the cosmic star formation rate. Our observations show that during the same epoch, massive galaxies are forming in high-density environments by vigorous star formation and assembly.

MOIRCS Deep Survey. IV. Evolution of Galaxy Stellar Mass Function Back to z ~ 3

We use very deep near-infrared (NIR) imaging data obtained in MOIRCS Deep Survey (MODS) to investigate the evolution of the galaxy stellar mass function back to z ~ 3. The MODS data reach J = 24.2, H = 23.1, and K = 23.1 (5σ, Vega magnitude) over 103 arcmin2 (wide) and J = 25.1, H = 23.7, and K = 24.1 over 28 arcmin2 (deep) in the GOODS-North region. The wide and very deep NIR data allow us to measure the number density of galaxies down to low stellar mass (109-1010 M ☉) even at high redshift with high statistical accuracy. The normalization of the mass function decreases with redshift, and the integrated stellar mass density becomes ~8%-18% of the local value at z ~ 2 and ~4%-9% at z ~ 3, which are consistent with results of previous studies in general fields. Furthermore, we found that the low-mass slope becomes steeper with redshift from α ~ –1.3 at z ~ 1 to α ~ –1.6 at z ~ 3 and that the evolution of the number density of low-mass (109-1010 M ☉) galaxies is weaker than that of M* (~1011 M ☉) galaxies. This indicates that the contribution of low-mass galaxies to the total stellar mass density has been significant at high redshift. The steepening of the low-mass slope with redshift is an opposite trend expected from the stellar mass dependence of the specific star formation rate reported in previous studies. The present result suggests that the hierarchical merging process overwhelmed the effect of the stellar mass growth by star formation and was very important for the stellar mass assembly of these galaxies at 1 z 3.

Panoramic Hα and mid-infrared mapping of star formation in a z = 0.8 cluster

We present the first wide-field Hα imaging survey around the distant cluster RXJ 1716.4+6708 at z = 0.81 with a narrow-band filter on MOIRCS/Subaru, which reveals the star formation activities down to a star formation rate (SFR) of ∼1 M ⊙ yr -1 without extinction correction. Combining with a wide-field mid-infrared (MIR) imaging survey with the AKARI satellite, we compare in detail the unobscured and obscured star formation activities in the cluster. We find that both Hα emitters and MIR galaxies avoid the cluster central region and their spatial distribution is quite similar. Most of the Hα emitters show blue colours, but we find some Hα emitters on the red sequence. The MIR galaxies tend to be systematically redder than the Hα emitters probably due to heavy dust extinction. Interestingly, the red Hα emitters and the red MIR galaxies (i.e. dusty red galaxies) are most commonly seen in the medium-density environment such as cluster outskirts, groups and filaments, where optical colours of galaxies change. We investigate the amount of hidden star formation by calculating a ratio, SFR(IR)/SFR(Hα), and find that A Hα exceeds ∼3 in extreme cases for actively star-forming galaxies with SFR(IR) ≳20 M ⊙ yr -1 . It is notable that most of such very dusty galaxies with A Hα ≳ 3 are also located in the medium-density environment. These findings suggest that dusty star formation is triggered in the infall region of the cluster, implying a probable link between galaxy transition and dusty star formation. We finally calculate the cluster total SFR and find that the cluster total SFR based on Hα alone can be underestimated more than a factor of ∼2 even after 1-mag extinction correction. We suggest that the mass-normalized cluster SFR rapidly declines since z ∼ 1 following ∝ (1 + z) 6 , although the uncertainty is still large.

MOIRCS DEEP SURVEY. VI. NEAR-INFRARED SPECTROSCOPY OF K-SELECTED STAR-FORMING GALAXIES AT z ∼ 2*

We present the results of near-infrared multi-object spectroscopic observations for 37 BzK-color-selected star-forming galaxies conducted with MOIRCS on the Subaru Telescope. The sample is drawn from the Ks -band-selected catalog of the MOIRCS Deep Survey in the GOODS-N region. About half of our samples are selected from the publicly available 24 ?m-source catalog of the Multiband Imaging Photometer for Spitzer on board the Spitzer Space Telescope. H? emission lines are detected from 23 galaxies, of which the median redshift is 2.12. We derived the star formation rates (SFRs) from extinction-corrected H? luminosities. The extinction correction is estimated from the spectral energy distribution (SED) fitting of multiband photometric data covering UV to near-infrared wavelengths. The Balmer decrement of the stacked emission lines shows that the amount of extinction for the ionized gas is larger than that for the stellar continuum. From a comparison of the extinction-corrected H? luminosity and other SFR indicators, we found that the relation between the dust properties of stellar continuum and ionized gas is different depending on the intrinsic SFR (differential extinction). We compared SFRs estimated from extinction-corrected H? luminosities with stellar masses estimated from SED fitting. The comparison shows no correlation between SFR and stellar mass. Some galaxies with stellar mass smaller than ~1010 M ? show SFRs higher than ~100 M ? yr?1. The specific SFRs (SSFRs) of these galaxies are remarkably high; galaxies which have SSFR higher than ~10?8 yr?1 are found in eight of the present sample. From the best-fit parameters of SED fitting for these high-SSFR galaxies, we find that the average age of the stellar population is younger than 100?Myr, which is consistent with the implied high SSFR. The large SFR implies the possibility that the high-SSFR galaxies significantly contribute to the cosmic SFR density of the universe at z ~ 2. When we apply the larger extinction correction for the ionized gas or the differential extinction correction, the total SFR density estimated from the H?-emission-line galaxies is 0.089-0.136 M ? yr?1 Mpc?3, which is consistent with the total SFR densities in the literature. The metallicity of the high-SSFR galaxies, which is estimated from the N2 index, is larger than that expected from the mass-metallicity relation of UV-selected galaxies at z ~ 2 by Erb et al.

Optical Spectropolarimetry of SN 2002ap: A High-Velocity Asymmetric Explosion*

We present spectropolarimetry of the Type Ic supernova SN 2002ap and give a preliminary analysis: the data were taken at two epochs, close to and 1 month later than the visual maximum (2002 February 8). In addition, we present June 9 spectropolarimetry without analysis. The data show the development of linear polarization. Distinct polarization profiles were seen only in the O I λ7773 multiplet/Ca II IR triplet absorption trough at maximum light and in the O I λ7773 multiplet and Ca II IR triplet absorption troughs a month later, with the latter showing a peak polarization as high as ~2%. The intrinsic polarization shows three clear position angles: 80° for the February continuum, 120° for the February line feature, and 150° for the March data. We conclude that there are multiple asymmetric components in the ejecta. We suggest that the supernova has a bulk asymmetry with an axial ratio projected on the sky that is different from 1 by an order of 10%. Furthermore, we suggest very speculatively that a high-velocity ejecta component moving faster than ~0.115c (e.g., a jet) contributes to polarization in the February epoch.

Massive starburst galaxies in a z = 2.16 proto-cluster unveiled by panoramic Hα mapping

We present a panoramic narrow-band study of Hα emitters in the field of the z = 2.16 proto-cluster around PKS 1138−262 using MOIRCS on the Subaru Telescope. We find 83 Hα emitters down to a star formation rate of SFR (Hα) ∼ 10 M⊙ yr−1 across a ∼ 7 × 7 arcmin2 region centred on the radio galaxy, and identify ∼10-Mpc scale filaments of emitters running across this region. By examining the properties of Hα emitters within the large-scale structure, we find that galaxies in the higher density environments at z = 2.16 tend to have redder colours and higher stellar masses compared to galaxies in more underdense regions. We also find a population of Hα emitters with red colours [(J − Ks) ≳ 1], which are much more frequent in the denser environments and which have apparently very high stellar masses with M* ≳ 1011 M⊙, implying that these cluster galaxies have already formed a large part of their stellar mass before z ∼ 2. Spitzer Space Telescope 24-μm data suggest that many of these red Hα emitters are bright, dusty starbursts (rather than quiescent sources). We also find that the proto-cluster galaxies follow the same correlation between SFR and M* (the ‘main sequence’) of z ∼ 2 field star-forming galaxies, but with an excess of massive galaxies. These very massive star-forming galaxies are not seen in our similar, previous study of z ∼ 1 clusters, suggesting that their star formation activity has been shut off at 1 ≲ z ≲ 2. We infer that the massive red (but active) galaxies in this rich proto-cluster are likely to be the products of environmental effects, and they represent the accelerated galaxy formation and evolution in a biased high-density region in the early Universe.

A STARBURSTING PROTO-CLUSTER IN MAKING ASSOCIATED WITH A RADIO GALAXY AT z = 2.53 DISCOVERED BY Hα IMAGING

We report a discovery of a proto-cluster in vigorous assembly and hosting strong star-forming activities, associated with a radio galaxy USS 1558-003 at z = 2.53, as traced by wide-field narrow-band H? imaging with MOIRCS on the Subaru Telescope. We find 68 H? emitters with dust-uncorrected star formation rates (SFRs) down to 8.6 M ??yr?1. Their spatial distribution indicates that there are three prominent clumps of H? emitters: one surrounding the radio galaxy, the second located at ~1.5?Mpc away to the southwest, and the third located between the two. These contiguous three systems are very likely to merge together in the near future and may grow to a single more massive cluster at a later time. While most H? emitters reside in the blue cloud on the color-magnitude diagram, some emitters have very red colors with J ? Ks > 1.38(AB). Interestingly, such red H? emitters are located toward the faint end of the red sequence, and they tend to be located in high density clumps. We do not see any statistically significant difference in the distributions of individual SFRs or stellar masses of the H? emitters between the dense clumps and the other regions, suggesting that this is one of the notable sites where the progenitors of massive galaxies in the present-day clusters were in their vigorous formation phase. Finally, we find that H? emission of the radio galaxy is fairly extended spatially over ~45. However, it is not as widespread as its Ly? halo, meaning that the Ly? emission is indeed severely extended by resonant scattering.

Protoclusters with evolved populations around radio galaxies at z∼ 2.5

We report the discovery of protocluster candidates around high-redshift radio galaxies at z ∼ 2.5 on the basis of clear statistical excess of colour-selected galaxies around them seen in the deep near-infrared imaging data obtained with CISCO on the Subaru Telescope. We have observed six targets, all at similar redshifts at z ∼ 2.5, and our data reach J = 23.5, H = 22.6 and K = 21.8 (5σ) and cover a 1.6 x 1.6 arcmin 2 field centred on each radio galaxy. We apply colour cuts in JHK bands in order to exclusively search for galaxies located at high redshifts, z > 2. Over the magnitude range of 19.5 2.3 by a factor of 2 around the combined radio galaxies fields compared to those found in the general field of the Great Observatories Origins Deep Survey-South (GOODS-S). The excess of galaxies around the radio galaxies fields becomes more than a factor of 3 around 19.5 < K < 20.5 when the two-colour cuts are applied with JHK bands. Such overdensity of the colour-selected galaxies suggests that those fields tend to host high-density regions at high redshifts, although there seems to be the variety of the density of the colour-selected galaxies in each field. In particular, two radio galaxies fields out of the six observed fields show very strong density excess and these are likely to be protoclusters associated with the radio galaxies which would evolve into rich clusters of galaxies dominated by old passively evolving galaxies.