H alpha 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~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 10^3 Mpc^3 and a look-back time of 3.6 Gyr when H0=50 km s^{-1} Mpc^{-1} and q=0.5 are assumed. A total of 52 objects are 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 A equivalent width with an uncertainty of ~ 10 AA. 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] 3727, H beta and [OIII] 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 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 M_sun yr^{-1} Mpc^{-3}. This translates to 0.037 M_sun 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 result needs to be confirmed by future spectroscopic follow-up observations.