We make a search for Halpha emitting galaxies at z=0.24 in the Subaru Deep Field (SDF) using the archival data set obtained with the Subaru Telescope. We carefully select Halpha emitters in the narrowband filter NB816, using B, V, Rc, i, and z broad-
band colors. We obtain a sample of 258 emitting galaxies with observed equivalent widths of (Halpha+[NII]6548,6584) greater than 12 angstrom. We also analyze a sample of Halpha emitters taken from the Sloan Digital Sky Survey (SDSS) to constrain the luminous end of Halpha luminosity function. Using the same selection criteria as for the SDF, and after excluding AGNs, we obtain 317 Halpha emitting star-forming galaxies. Combining these two samples of Halpha emitters found in both SDF and SDSS, we derive a Halpha luminosity function with best-fit Schechter function parameters of alpha = -1.31^+0.17_-0.17, log phi^* = -2.46^+0.34_-0.40 Mpc^-3, log L^* = 41.99^+0.08_-0.07 ergs s^-1. An extinction-corrected Halpha luminosity density is 4.45^+2.96_-1.75 x 10^39 ergs s^-1 Mpc^-3. Using the Kennicutt relation between the Halpha luminosity and star formation rate, the star formation rate density in the survey volume is estimated as 0.035^+0.024_-0.014 M_sun yr^-1 Mpc^-3. The angular two-point correlation function of Halpha emitters over 875 arcmin^2 at z = 0.24 is well fitted by a power-law form with w(theta) = 0.047^+0.017_-0.013 theta^-0.66 +- 0.08, corresponding to the correlation function of xi(r) = (r/2.6^+1.0_-0.8 Mpc)^(-1.66 +- 0.08). The small correlation length of Halpha emitters may imply the weak clustering of active star-forming galaxies.
To derive a new H$alpha$ luminosity function and to understand the clustering properties of star-forming galaxies at $z approx 0.24$, we have made a narrow-band imaging survey for H$alpha$ emitting galaxies in the HST COSMOS 2 square degree field. We
used the narrow-band filter NB816 ($lambda_c = 8150$ AA, $Delta lambda = 120$ AA) and sampled H$alpha$ emitters with $EW_{rm obs}(rm Halpha + [Ntextsc{ii}]) > 12$ AA in a redshift range between $z=0.233$ and $z=0.251$ corresponding to a depth of 70 Mpc. We obtained 980 H$alpha$ emitting galaxies in a sky area of 5540 arcmin$^2$, corresponding to a survey volume of $3.1 times 10^4 {rm Mpc^3}$. We derive a H$alpha$ luminosity function with a best-fit Schechter function parameter set of $alpha = -1.35^{+0.11}_{-0.13}$, $logphi_* = -2.65^{+0.27}_{-0.38}$, and $log L_* ({rm erg s^{-1}}) = 41.94^{+0.38}_{-0.23}$. The H$alpha$ luminosity density is $2.7^{+0.7}_{-0.6} times 10^{39}$ ergs s$^{-1}$ Mpc$^{-3}$. After subtracting the AGN contribution (15 %) to the H$alpha$ luminosity density, the star formation rate density is evaluated as $1.8^{+0.7}_{-0.4} times 10^{-2}$ $M_{sun}$ yr$^{-1}$ Mpc$^{-3}$. The angular two-point correlation function of H$alpha$ emitting galaxies of $log L({rm Halpha}) > 39.8$ is well fit by a power law form of $w(theta) = 0.013^{+0.002}_{-0.001} theta^{-0.88 pm 0.03}$, corresponding to the correlation function of $xi(r) = (r/1.9{rm Mpc})^{-1.88}$. We also find that the H$alpha$ emitters with higher H$alpha$ luminosity are more strongly clustered than those with lower luminosity.
