  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.
