Evolution of the Dependence of Rest-frame Color and Morphology Distribution on Stellar Mass for Galaxies in the Hubble Deep Field North

Using the Subaru very deep K'-band imaging and HST WFPC2/NICMOS archival data of the Hubble Deep Field North, we investigate the evolution of the stellar mass, color, morphology of galaxies to z ~ 3. We mainly examine the rest-frame U-V color distribution of galaxies as a function of stellar mass. At 0.3<z<2, galaxies seem to be divided into the two populations at around the stellar mass of ~5x10^9 M_solar. The low-mass galaxies have relatively bluer rest U-V color and their color does not show clear correlation with stellar mass over the range of 10^8-5x10^9 M_solar. On the other hand, at higher mass, the more massive galaxies tend to have the redder U-V color. The average U-V color of the low-mass galaxies becomes bluer gradually with redshift, from U-V ~ 0.2 at z ~ 0.5 to U-V ~ -0.2 at z ~ 2. On the contrary, the correlation between the stellar mass and rest U-V color of the high-mass population does not seem to change significantly between z ~ 0.3 and z ~ 2. At z>2, it is seen that more massive galaxies tend to have redder U-V color over the range of 10^9-10^10 M_solar. These results suggests that the star formation history of galaxies depends on their stellar mass very much. The low-mass population is likely to have relatively long star formation timescale. At the stellar mass larger than ~5x10^9 M_solar, there must be some mechanisms which suppress the star formation in galaxies at 0<z<2.Comment: 45 pages, 31 figures, accepted for publication in Ap

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<z<3.5 as a function of stellar mass, using very deep NIR data taken with Multi-Object Infrared Camera and Spectrograph (MOIRCS) on the Subaru telescope in the GOODS-North region. The NIR imaging data reach K ~ 23-24 Vega magnitude and they allow us to construct a nearly stellar mass-limited sample down to ~ 10^{9.5-10} Msun even at z~3. We estimated star formation rates (SFRs) of the sample with two indicators, namely, the Spitzer/MIPS 24um flux and the rest-frame 2800A luminosity. The SFR distribution at a fixed Mstar shifts to higher values with increasing redshift at 0.5<z<3.5. More massive galaxies show stronger evolution of SFR at z>~1. We found galaxies at 2.5<z<3.5 show a bimodality in their SSFR distribution, which can be divided into two populations by a constant SSFR of ~2 Gyr^{-1}. Galaxies in the low-SSFR group have SSFRs of ~ 0.5-1.0 Gyr^{-1}, while the high-SSFR population shows ~10 Gyr^{-1}. The cosmic SFRD is dominated by galaxies with Mstar = 10^{10-11} Msun at 0.5<z<3.5, while the contribution of massive galaxies with Mstar = 10^{11-11.5} Msun shows a strong evolution at z>1 and becomes significant at z~3, especially in the case with the SFR based on MIPS 24um. In galaxies with Mstar = 10^{10-11.5} Msun, those with a relatively narrow range of SSFR (<~1 dex) dominates the cosmic SFRD at 0.5<z<3.5. The SSFR of galaxies which 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.Comment: 16 pages, 13 figures, accepted for publication in Ap

The Number Density of Old Passively-Evolving Galaxies at z=1 in the Subaru/XMM-Newton Deep Survey Field

We obtained the number counts and the rest-frame B-band luminosity function of the color-selected old passively-evolving galaxies (OPEGs) at z=1 with very high statistical accuracy using a large and homogeneous sample of about 4000 such objects with z' <25 detected in the area of 1.03 deg^2 in the Subaru/XMM-Newton Deep Survey (SXDS) field. Our selection criteria are defined on the i'-z' and R-z' color-magnitude plane so that OPEGs at z=0.9-1.1 with formation redshift z_f=2-10 are properly sampled. The limiting magnitude corresponds to the luminosity of galaxies with M_*+3 at z=0. We made a pilot redshift observations for 99 OPEG candidates with 19 < z' < 22 and found that at least 78% (73/93) of the entire sample, or 95% (73/77) of these whose redshifts were obtained are indeed lie between z=0.87 and 1.12 and the most of their spectra show the continuum break and strong Ca H and K lines, indicating that these objects are indeed dominated by the old stellar populations. We then compare our results with the luminosity functions of the color- or the morphologically-selected early type galaxies at z=0 taking the evolutionary factor into account and found that the number density of old passive galaxies with sim M_* magnitude at z~1 averaged over the SXDS area is 40-60% of the equivalently red galaxies and 60-85% of the morphologically-selected E/S0 galaxies at z=0 depending on their luminosity evolution. It is revealed that more than half, but not all, of the present-day early-type galaxies had already been formed into quiescent passive galaxies at z=1.Comment: 28 pages, accepted for publication in Astrophysical Journal. The full version of the paper including Fig.3 and Fig.4 (large size) in full resolution is put at http://optik2.mtk.nao.ac.jp/~yamada/astronomy/sxdsred.htm

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, K=23.1 (5sigma, Vega magnitude) over 103 arcmin^2 (wide) and J=25.1, H=23.7, K=24.1 over 28 arcmin^2 (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 (10^9-10^10 Msun) 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 alpha ~- 1.3 at z~1 to alpha ~- 1.6 at z~3, and that the evolution of the number density of low-mass (10^9-10^10 Msun) galaxies is weaker than that of M* (~10^11 Msun) 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 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.Comment: 21 pages, 18 figures, accepted for publication in Ap

Evolution of the Fraction of Clumpy Galaxies at 0.2<z<1.0 in the COSMOS field

Using the Hubble Space Telescope/Advanced Camera for Surveys data in the COSMOS field, we systematically searched clumpy galaxies at 0.2<z<1.0 and investigated the fraction of clumpy galaxies and its evolution as a function of stellar mass, star formation rate (SFR), and specific SFR (SSFR). The fraction of clumpy galaxies in star-forming galaxies with Mstar > 10^9.5 Msun decreases with time from ~0.35 at 0.8<z<1.0 to ~0.05 at 0.2<z<0.4 irrespective of the stellar mass, although the fraction tends to be slightly lower for massive galaxies with Mstar > 10^10.5 Msun at each redshift. On the other hand, the fraction of clumpy galaxies increases with increasing both SFR and SSFR in all the redshift ranges we investigated. In particular, we found that the SSFR dependences of the fractions are similar among galaxies with different stellar masses, and the fraction at a given SSFR does not depend on the stellar mass in each redshift bin. The evolution of the fraction of clumpy galaxies from z~0.9 to z~0.3 seems to be explained by such SSFR dependence of the fraction and the evolution of SSFRs of star-forming galaxies. The fraction at a given SSFR also appears to decrease with time, but this can be due to the effect of the morphological K-correction. We suggest that these results are understood by the gravitational fragmentation model for the formation of giant clumps in disk galaxies, where the gas mass fraction is a crucial parameter.Comment: 14 Pages, 13 Figures, 1 Table, Accepted for publication in Ap