We have carried out a survey searching for H \alpha emitting galaxies at z \simeq 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 \cdot 10 ^ { 3 } Mpc ^ { 3 } and a look-back time of 3.6 Gyr when H _ { \mathrm { 0 } } =50km s ^ { -1 } Mpc ^ { -1 } and q _ { \mathrm { 0 } } =0.5 are assumed .
A total of 52 objects are selected as candidates for a broad band limiting magnitude of I \sim 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 20Å equivalent width with an uncertainty of \sim \pm 10Å .
Point-like objects ( 15 ) were excluded from our analysis using CLASS_STAR parameter from SExtractor .
The contamination from other emission lines such as \mathrm { [ \ion { O } { ii } ] \lambda \mathrm { 3727 } } , \mathrm { H } \beta and \mathrm { [ \ion { O } { iii } ] \lambda \lambda \mathrm { 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 \pm 1.1 ) \cdot 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 \pm 0.009 ) M _ { \sun } yr ^ { -1 } Mpc ^ { -3 } .
This translates to ( 0.037 \pm 0.009 ) 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 \sim 4 higher than the local SFR density .
This result needs to be confirmed by future spectroscopic follow-up observations .