The Hα luminosity function and star formation rate at z ≈ 0.24 in the COSMOS 2 square degree field

To derive a new Hα luminosity function and to understand the clustering properties of star-forming galaxies at z ≈ 0.24, we have made a narrowband imaging survey for Hα-emitting galaxies in the Hubble Space Telescope (HST) COSMOS 2 square degree field. We used the narrowband filter NB816 (λ_c = 8150 Å, Δλ = 120 Å) and sampled Hα emitters with EW_obs( Hα+ [N II]) > 12 Å in a redshift range between z = 0.233 and 0.251 corresponding to a depth of 70 Mpc. We obtained 980 Hα-emitting galaxies in a sky area of 5540 arcmin², corresponding to a survey volume of 3.1 × 10 ⁴ Mpc³. We derive a Hα luminosity function with a best-fit Schechter function parameter set of α = -1.35 _-0.13^+0.11, log φ* = -2.65_-0.38 ^+0.27 and log L*(ergs s^-1) = 41.94 _-0.23^+0.38. The Hα luminosity density is 2.7 _-0.6^+0.7 × 10³⁹ ergs s^-1 Mpc^-3. After subtracting the AGN contribution (15%) to the Hα luminosity density, the star formation rate density is evaluated as 1.8 _-0.4^+0.7 × 10^-2 M_⊙ yr^-1 Mpc^-3. The angular two-point correlation function of Hα-emitting galaxies of log L(Hα) > 39.8 is well fit by a power-law form of w(θ) = 0.013_-0.001 ^+0.002θ^-0.88±0.03, corresponding to the correlation function of ξ(r) = (r/1.9 Mpc)^-1.88. We also find that the Hα emitters with higher Hα luminosity are more strongly clustered than those with lower luminosity.