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

We have carried out a survey searching for H-alpha emitting galaxies at z similar or equal to 0.24 using a narrow band filter tuned with the redshifted line. The total sky area covered was 0.19 square degrees 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 x 10(3) Mpc(3) and a look-back time of 3.6 Gyr when H-0 = 50 km s(-1) Mpc(-1) and q(0) = 0.5 are assumed. A total of 52 objects are selected as candidates for a broad band limiting magnitude of I similar to 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 Angstrom equivalent width with an uncertainty of similar to +/- 10 Angstrom. 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]lambda 3727, H beta and [O III]lambda lambda 4959, 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 H alpha luminosity density of (5.4 +/- 1.1) x 10(39) erg s(-1) Mpc(-3). This uncertainty takes into account the photometric and Poissonian errors only. Assuming a constant relation between the H alpha luminosity and star formation rate, the SFR density in the covered volume is (0.043 +/- 0.009) M-circle dot yr(-1) Mpc(-3). This translates to (0.037 +/- 0.009) M-circle dot 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 similar to4 higher than the local SFR density. This result needs to be confirmed by future spectroscopic follow-up observations.