Hα emitting galaxies and the star formation rate density at z ≃ 0.24

Abstract

We have carried out a survey searching for Hα emitting galaxies at z ≃ 0.24 a narrow band filter tuned with the redshifted line. The total sky area covered was 0.19 square within the redshift range 0.228 to 0.255 in a set of four fields in the ELAIS-N1 zone. This corresponds to a volume of 9.8 × 103 Mpc3 and a look-back time of 3.6 Gyr when H0 = 50 km s-1 Mpc-1 and q0 = 0.5 are assumed. A total of 52 objects selected as candidates for a broad band limiting magnitude of I ∼ 22.9, plus 16 detected only in the narrow band image for a narrow band limiting magnitude for object detection of 21.0. The threshold detection corresponds to about 20 Å equivalent width with an uncertainty of ∼±10 Å. Point-like objects (15) were excluded from our analysis using CLASS_STAR parameter from SExtractor. The contamination from other emission lines such as [O II]λ3727, Hβ and [O III]λλ4959, 5007 at redshifts, 1.2, 0.66 and 0.61 respectively is estimated, and found to be negligible at the flux limits of our sample. We find an extinction-corrected Hα luminosity density of (5.4 ± 1.1) × 1039 erg s-1 Mpc-3. This uncertainty takes into account the photometric and Poissonian errors only. Assuming a constant relation between the Hα luminosity and star formation rate, the SFR density in the covered volume is (0.043 ± 0.009) M⊙ yr-1 Mpc-3. This translates to (0.037 ± 0.009) M⊙ yr-1 Mpc-3 when the total density is corrected for the AGN contribution as estimated in the local Universe. This value is a factor ∼4 higher than the local SFR density. This results needs to be confirmed by future spectroscopic follow-up observations.