MOIRCS Deep Survey. VI. Near-Infrared Spectroscopy of K-Selected
  Star-Forming Galaxies at z~2
  We present near-infrared (NIR) spectroscopy for 37 BzK-color-selected
star-forming galaxies with MOIRCS on Subaru. The sample is drawn from the
Ks-band selected catalog of the MOIRCS Deep Survey (MODS) in the GOODS-N
region. About half of our samples are selected from the MIPS 24um-source
catalog. Ha 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 Ha luminosities. The extinction correction is estimated
from the SED fitting of multi-band photometric data covering UV to NIR
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 Ha 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.
The comparison between the Ha SFR and stellar mass estimated from SED fitting
shows no correlation between them. Some galaxies with stellar mass smaller than
~10^10 Msun show SFRs higher than ~100 Msun/yr. The specific SFRs (SSFRs) of
these galaxies are remarkably high. The average stellar-population age of these
high-SSFR galaxies 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. The total SFR density estimated from the Ha emission line galaxies is
0.089-0.136 Msun/yr/Mpc^3, which is consistent with the total SFR densities in
the literature. The metallicity of the high-SSFR galaxies is larger than that
expected from the mass-metallicity relation of UV-selected galaxies at z~2 by
Erb et al. (2006a). [Abridged]
