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

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 MOIRCS Deep Survey project and the published Chandra X-ray source catalog. 61 X-ray sources with the 2-10 keV luminosity L_X = 10^{42}-10^{44} erg/s are identified with the K-selected galaxies and we found that they are exclusively (90%) associated with the massive objects with stellar mass larger than 10^{10.5} Msun. Our results are consistent with the idea that the M_BH/M_str ratio of the galaxies at z=2-4 is similar to the present-day value. On the other hand, the AGN detection rate among the very massive galaxies with the stellar mass larger than 10^{11} Msun is high, 33% (26/78). They are active objects in the sense that the black-hole mass accretion rate is ~ 1-50% of the Eddington limit if they indeed have similar M_BH/M_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.Comment: 33 pages, 12 figures, accepted for publication in Ap

Vibration measurement in the KAGRA cryostat

The Japanese gravitational wave observatory KAGRA will be operated at cryogenic temperatures to reduce thermal noise. Four main mirrors and their suspension systems, called cryogenic payloads, will be cooled in the cryostat. Vibrations of the cryostat and the cryocooler can contaminate the output of the detector. One of the noise paths is the heat link made from the pure soft metal between the cryogenic payload and cryocoolers to cool the payload. In order to evaluate this noise amplitude, we measured the vibration of the radiation shield at cryogenic temperatures at the cryostat production site in Yokohama, Japan. For this measurement, we developed cryogenic accelerometers. Based on the result of this measurement, we calculated the noise in the KAGRA interferometer. Our results show that with the current design, the seismic noise goal formulated for KAGRA cannot be achieved. Finally, we present a possible design optimization that is meant to reach the nominal sensitivity of the detector

ATHENA X-IFU 300 K–50 mK cryochain test results

International audienc

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 μm-source catalog of the Multiband Imaging Photometer for Spitzer on board the Spitzer Space Telescope. Hα 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 Hα 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 Hα 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 Hα 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 sun show SFRs higher than ~100 M sun 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 Hα-emission-line galaxies is 0.089-0.136 M sun 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. This study is based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan