We have conducted an H \alpha survey of 38 0.77 \leq z \leq 1 galaxies over \sim 100 arcmin ^ { 2 } of the Hubble Deep Field North and Flanking Fields , to determine star formation rates ( SFRs ) , with the near-infrared multi-object spectrograph CIRPASS on the WHT .
This represents the first successful application of this technique to observing high redshift galaxies .
Stacking the spectra in the rest-frame to infer a total SFR for the field , we find a lower limit ( uncorrected for dust reddening ) on the star formation rate density at redshift z = 1 of 0.04M _ { \odot } yr ^ { -1 } Mpc ^ { -3 } .
This implies rapid evolution in the star formation rate density from z = 0 to z = 1 which is proportional to ( 1 + z ) ^ { 3.1 } .