Chihiro Tokoku

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.

MOIRCS: Multi-object Infrared Camera and Spectrograph for SUBARU

MOIRCS is a new Cassegrain instrument of Subaru telescope, dedicated for wide field imaging and multi-object spectroscopy in near-infrared. MOIRCS has been constructed jointly by Tohoku University and the Subaru Telescope and saw the first light in Sept., 2004. The commissioning observations to study both imaging and spectroscopic performance were conducted for about one year. MOIRCS mounts two 2048 × 2048 HAWAII2 arrays and provides a field of view of 4 x 7 with a pixel scale of 0.117. All-lens optical design is optimized for 0.8 to 2.5 μm with no practical chromatic aberration. Observations confirm the high image quality over the field of view without any perceptible degradation even at the field edge. The best seeing we have obtained so far is FWHM=0.18. A novel design of MOIRCS enables us to perform multi-object spectroscopy with aluminum slit masks, which are housed in a carrousel dewar and cooled to ~ 110 K. When choosing MOS mode, a manipulator pulls out a slit mask from the carrousel into the MOIRCS main dewar and sets it properly at the Cassegrain focus. The carrousel is shuttered by a gate valve, so that it can be warmed and cooled independently to exchange slit-mask sets during daytime. We have tested various configurations of 30 or more multi-slit positions in various sky fields and found that targets are dropped at the centers of slits or guide holes within a dispersion of about 0.3 pixels (0.03). MOIRCS has been open to common use specifically for imaging observations since Feb. 2006. The MOS function will be available in next August.

The Lifetime of Protoplanetary Disks in a Low-metallicity Environment

The extreme outer Galaxy (EOG), the region with a Galactic radius of more than 18 kpc, is known to have a very low metallicity, about one-tenth that of the solar neighborhood. We obtained the deep near-infrared (NIR) images of two very young (~0.5xa0Myr) star-forming clusters that are two of the most distant embedded clusters in the EOG. We find that in both clusters the fraction of stars with NIR excess, which originates from the circumstellar dust disk at radii of ≤0.1 AU, is significantly lower than those in the solar neighborhood. Our results suggest that most of the stars forming in the low-metallicity environment experience disk dispersal at an earlier stage (<1xa0Myr) than those forming in the solar metallicity environment (as much as ~5-6xa0Myr). Such a rapid disk dispersal may make the formation of planets difficult, and the shorter disk lifetime with a lower metallicity, could contribute to the strong metallicity dependence of the well-known planet-metallicity correlation, which states that the probability of a star hosting a planet increases steeply with stellar metallicity. The reason for the rapid disk dispersal could be the increase of the mass accretion rate and/or the effective far-ultraviolet photoevaporation due to the low extinction; however, another unknown mechanism for the EOG environment could be contributing significantly.

MOIRCS Deep Survey. I : DRG Number Counts

We use very deep near-infrared imaging data taken with Multi-Object InfraRed Camera and Spectrograph (MOIRCS) on the Subaru Telescope to investigate the number counts of Distant Red Galaxies (DRGs). We have observed a 4x7 arcmin^2 field in the Great Observatories Origins Deep Survey North (GOODS-N), and our data reach J=24.6 and K=23.2 (5sigma, Vega magnitude). The surface density of DRGs selected by J-K>2.3 is 2.35+-0.31 arcmin^-2 at K 22 is smaller than that expected from the number counts at the brighter magnitude. The result indicates that while there are many bright galaxies at 2 22 suggest that the mass-dependent color distribution, where most of low-mass galaxies are blue while more massive galaxies tend to have redder colors, had already been established at that epoch.

MOIRCS Deep Survey. IX. Deep Near-Infrared Imaging Data and Source Catalog

We present deep J-, H-, and Ks-band imaging data of the MOIRCS Deep Survey (MODS), which was carried out with Multi-Object Infrared Camera and Spectrograph (MOIRCS) mounted on the Subaru telescope in the GOODS-North region. The data reach 5sigma total limiting magnitudes for point sources of J=23.9, H=22.8, and Ks=22.8 (Vega magnitude) over 103 arcmin^2 (wide field). In 28 arcmin^2 of the survey area, which is ultra deep field of the MODS (deep field), the data reach the 5sigma depths of J=24.8, H=23.4, and Ks=23.8. The spatial resolutions of the combined images are FWHM ~ 0.6 arcsec and ~ 0.5 arcsec for the wide and deep fields in all bands, respectively. Combining the MODS data with the multi-wavelength public data taken with the HST, Spitzer, and other ground-based telescopes in the GOODS field, we construct a multi-wavelength photometric catalog of Ks-selected sources. Using the catalog, we present Ks-band number counts and near-infrared color distribution of the detected objects, and demonstrate some selection techniques with the NIR colors for high redshift galaxies. These data and catalog are publicly available via internet.

Moircs Deep Survey. III. Active Galactic Nuclei in Massive Galaxies at z = 2-4

We investigate the X-ray properties of the K-band-selected galaxies at redshift 2 < z < 4 by using our deep near-infrared images obtained in the Multi-Object Infrared Camera and Spectrograph Deep Survey project and the published Chandra X-ray source catalog. Sixty-one X-ray sources with the 2-10 keV luminosity L{sub X} = 10{sup 42}-10{sup 44} erg s{sup -1} are identified with the K-selected galaxies and we found that they are exclusively (90%) associated with the massive objects with a stellar mass larger than 10{sup 10.5} M{sub sun}. Our results are consistent with the idea that the M {sub BH}/M{sub str} ratio of the galaxies at z = 2-4 is similar to the present-day value. On the other hand, the active galactic nucleus (AGN) detection rate among the very massive galaxies with a stellar mass larger than 10{sup 11} M{sub sun} is high, 33% (26/78). They are active objects in the sense that the black hole mass accretion rate is {approx}1%-50% of the Eddington limit if they indeed have similar M {sub BH}/M {sub str} ratio with those observed in the local universe. The active duration in the AGN duty cycle of the high-redshift massive galaxies seems large.

MOIRCS Deep Survey. II. Clustering Properties of K-Band Selected Galaxies in GOODS-North Region

We present the first measurement of clustering properties of low-mass galaxies with a stellar mass down to M� � 10 9 Mat 1 1.3 show a large bias of b � 7.2 ˙ 1.3 on scales of up to � � 100 00 or 3.1 comoving Mpc, while blue galaxies with 0.5 <JK< 1.3 have a weak clustering signal on large scales, but a possible strong small-scale excess at �< 10 00 .F or massive galaxies with M� & 10 10 Mˇ ,w e estimate the correlation length and bias to be r0 � 4.5 h � 1 Mpc and b = 1.9 - 3.5, which are much larger than those of low-mass (M� � 10 9 -10 10 Mˇ) galaxies. The comparison of our measurements with analytic CDM models constrains the properties of hosting dark halos, and indicates that the low-mass galaxies would be progenitors of galaxies with a typical luminosity of L. L� in th el ocal Universe. The blue galaxies in low-mass samples are more strongly clustered in more massive halos with higher occupation numbers than low-mass red galaxies. This fact suggests an environment effect due to the halo mass on the star-formation activity at high-z.

Subaru/MOIRCS Near-Infrared Imaging in the Proto-Cluster Region at z = 3.1

We present the results of deep near-infrared imaging observations of the z = 3:1 proto-cluster region in the SSA 22a field taken by MOIRCS mounted on the Subaru Telescope. We observed a 21.7 arcmin field to depths of J = 24.5, H = 24.3, and K = 23.9 (5 ). We examined the distribution of the K-selected galaxies at z 3 by using a simple color cut for distant red galaxies (DRGs) as well as a photometric-redshift selection technique. The marginal density excess of DRGs and the photo-z selected objects were found around the two most luminous Ly blobs (LABs). We investigated the correlation between the K-selected objects and the LABs, and found that several galaxies with stellar mass, M = 10–10Mˇ, exist in the vicinity of LABs, especially around the two most luminous ones. We also found that 7 of the 8 LABs in the field have plausible Ks-band counterparts, and the sum of the stellar mass possibly associated with LABs correlates with their luminosity and surface brightness, which implies that the origin of Ly emission may be closely correlated with their previous star-formation phenomena.