Yuka Katsuno Uchimoto

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.

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 Deep Survey. VIII. Evolution of Star Formation Activity as a Function of Stellar Mass in Galaxies since z~3

We study the evolution of star formation activity of galaxies at 0.5 1 and becomes significant at z ~ 3, especially in the case with the SFR based on MIPS 24 μm. In galaxies with M star = 1010–11.5 M ☉, those with a relatively narrow range of SSFR (1 dex) dominates the cosmic SFRD at 0.5 < z < 3.5. The SSFR of galaxies that dominate the SFRD systematically increases with redshift. At 2.5 < z < 3.5, the high-SSFR population, which is relatively small in number, dominates the SFRD. Major star formation in the universe at higher redshift seems to be associated with a more rapid growth of stellar mass of galaxies.

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.

LONG-TERM OPTICAL CONTINUUM COLOR VARIABILITY OF NEARBY ACTIVE GALACTIC NUCLEI

We examine whether the spectral energy distribution of optical continuum emission of active galactic nuclei (AGNs) changes during flux variation, based on accurate and frequent monitoring observations of 11 nearby Seyfert galaxies and QSOs carried out in the B, V, and I bands for seven years by the MAGNUM telescope. The multi-epoch flux data in any two different bands obtained on the same night show a very tight linear flux-to-flux relationship for all target AGNs. The flux of the host galaxy within the photometric aperture is carefully estimated by surface brightness fitting to available high-resolution Hubble Space Telescope images and MAGNUM images. The flux of narrow emission lines in the photometric bands is also estimated from available spectroscopic data. We find that the non-variable component of the host galaxy plus narrow emission lines for all target AGNs is located on the fainter extension of the linear regression line of multi-epoch flux data in the flux-to-flux diagram. This result strongly indicates that the spectral shape of AGN continuum emission in the optical region (~4400-7900 A) does not systematically change during flux variation. The trend of spectral hardening that optical continuum emission becomes bluer as it becomes brighter, which has been reported by many studies, is therefore interpreted as the domination of the variable component of the nearly constant spectral shape of an AGN as it brightens over the non-variable component of the host galaxy plus narrow lines, which is usually redder than AGN continuum emission.

The formation of the massive galaxies in the SSA22 z = 3.1 protocluster

We study the properties of K-band-selected galaxies (K AB 1011 M ☉ at z phot ~ 3.1 have colors consistent with those of quiescent galaxies with ages >0.5 Gyr. This fraction increases to ≈50% after correcting for unrelated foreground/background objects. We also find that 30% of the massive galaxies are heavily reddened, dusty, star-forming galaxies. Few such quiescent galaxies at similar redshifts are seen in typical survey fields. An excess surface density of 24 μm sources at z phot ~ 3.1 is also observed, implying the presence of dusty star-formation activity in the protocluster. Cross-correlation with the X-ray data indicates that the fraction of K-band-selected protocluster galaxies hosting active galactic nuclei (AGNs) is also high compared with the field. The sky distribution of the quiescent galaxies, the 24 μm sources, and the X-ray AGNs show clustering around a density peak of z = 3.1 Lyα emitters. A significant fraction of the massive galaxies have already become quiescent, while dusty star-formation is still active in the SSA22 protocluster. These findings indicate that we are witnessing the formation epoch of massive early-type galaxies in the centers of the predecessors to present-day rich galaxy clusters.

Assembly of Massive Galaxies in a High-z Protocluster

We present the results of wide-field deep JHK imaging of the SSA22 field using the MOIRCS instrument equipped with the Subaru telescope. The observed field is 112 arcmin^2 in area, which covers the z = 3.1 protocluster characterized by the overdensities of Lyα emitters (LAEs) and Lyα blobs (LABs). The 5σ limiting magnitude is K_AB = 24.3. We extract the potential protocluster members from the K-selected sample by using the multi-band photometric-redshift selection as well as the simple color cut for distant red galaxies (DRGs; J – K_AB > 1.4). The surface number density of DRGs in our observed fields shows clear excess compared with those in the blank fields, and the location of the densest area whose projected overdensity is twice the average coincides with the large-scale density peak of LAEs. We also found that K-band counterparts with z_phot ≃ 3.1 are detected for 75% (15/20) of the LABs within their Lyα halo, and the 40% (8/20) of LABs have multiple components, which gives a direct evidence of the hierarchical multiple merging in galaxy formation. The stellar mass of LABs correlates with their luminosity, isophotal area, and the Lyα velocity widths, implying that the physical scale and the dynamical motion of Lyα emission are closely related to their previous star formation activities. Highly dust-obscured galaxies such as hyper extremely red objects (J – K_AB > 2.1) and plausible K-band counterparts of submillimeter sources are also populated in the high-density region.

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.