Cosmic star-formation activity at z = 2.2 probed by Hα emission-line galaxies

We present a pilot narrow-band survey of Hα emitters at z = 2.2 in the Great Observatories Origins Deep Survey North (GOODS-N) field with MOIRCS instrument on the Subaru Telescope. The survey reached a 3σ limiting magnitude of 23.6 (NB209), which corresponds to a 3σ limiting line flux of 2.5 × 10^-17 erg s^-1 cm^-2 over a 56arcmin² contiguous area (excluding a shallower area). From this survey, we have identified eleven Hα emitters and one AGN at z = 2.2 on the basis of narrow-band excesses and photometric redshifts. We obtained spectra for seven out of the new objects, including one AGN; also, an emission line above 3σ was detected from all of them. We estimated star-formation rates (SFR) and stellar masses (M_star) for individual galaxies. The average SFR and M_star are 27.8M_⊙ yr^-1 and 4.0 × 10¹⁰M_⊙, respectively. Their specific star-formation rates negatively correlate with their stellar masses. Fitting to a Schechter function yields the Hα luminosity function with logL = 42.82, logφ = -2.78, and α = -1.37. The average star-formation rate density in the survey volume is estimated to be 0.31M_⊙yr^-1 Mpc^-3 according to the Kennicutt relation between the Hα luminosity and the star-formation rate. We compared our Hα emitters at z = 2.2 in GOODS-N with narrow-band line emitters in other fields and clusters to see their time evolution and environmental dependence. We found that the star-formation activity is rapidly reduced from z = 2.5 to z = 0.8 in the cluster environment, while it only moderately changed in the field environment. This result suggests that the time scale of galaxy formation differs among different environments, and the star-forming activities in high density regions eventually overtake those in lower-density regions as a consequence of "galaxy-formation bias" at high redshifts.