Cosmic Star Formation Activity at z=2.2 Probed by H-alpha Emission Line
  Galaxies
  We present a pilot narrow-band survey of H-alpha 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 3 sigma limiting
magnitude of 23.6 (NB209) which corresponds to a 3 sigma limiting line flux of
2.5 x 10^-17 erg s^-1 cm^-2 over a 56 arcmnin^2 contiguous area (excluding a
shallower area). From this survey, we have identified 11 H-alpha emitters and
one AGN at z=2.2 on the basis of narrow-band excesses and photometric
redshifts. We obtained spectra for seven new objects among them, including one
AGN, and an emission line above 3 sigma is detected from all of them. We have
estimated star formation rates (SFR) and stellar masses (M_star) for individual
galaxies. The average SFR and M_star is 27.8M_solar yr^-1 and 4.0 x
10^10M_solar, respectivly. Their specific star formation rates are inversely
correlated with their stellar masses. Fitting to a Schechter function yields
the H-alpha luminosity function with log L = 42.82, log phi = -2.78 and alpha =
-1.37. The average star formation rate density in the survey volume is
estimated to be 0.31M_solar yr^-1Mpc^-3 according to the Kennicutt relation
between H-alpha luminosity and star formation rate. We compare our H-alpha
emitters at z=2.2 in GOODS-N with narrow-band line emitters in other field and
clusters to see their time evolution and environmental dependence. We find that
the star formation activity is reduced rapidly from z=2.5 to z=0.8 in the
cluster environment, while it is only moderately changed in the field
environment. This result suggests that the timescale of galaxy formation is
different 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.
