  We present a near-infrared spectroscopic study of a stellar mass selected
sample of galaxies at z~1 utilising the LIRIS multi-object spectrograph on the
WHT. We detect continuum, and the H alpha line for our sample, which is one of
the better direct tracers of star formation in external galaxies. We
spectroscopically measure the H alpha emission from 41 massive (M_{*}>10^{10.5}
Msol) galaxies taken from the POWIR Survey with spectroscopic redshifts
0.4<z_{spec}<1.4. We correct our H alpha fluxes for dust extinction by using
multi-wavelength data, and investigate SFR trends with mass and colour. We find
a drop in the fraction of massive galaxies with M_{*}>10^{11} Msol which are
detected in H alpha emission at z<0.9. We furthermore find that the fraction of
galaxies with H alpha emission drops steadily and significantly with redder
(U-B) colours at z~1, and that the SSFR drops with increasing (U-B) colour for
galaxies at all masses. By investigating the SFR-mass relation we find that the
SFR is roughly constant with mass, in possible contrast to previous work, and
that the specific star formation rate (SSFR) is lower in the most massive
galaxies. The scatter in the SFR vs. mass relationship is very small for those
systems with ongoing star formation which suggests that star formation in the
most massive galaxies at z~1 shuts off rather abruptly over <1 Gyr, without an
obvious gradual decline. We furthermore investigate the SFR as a function of
(U-B) colour divided into different mass bins, revealing a tracer of the epoch
of transition from star forming to passive, as a form of star formation
"downsizing". This suggests that the shut off of star formation occurs before
the change in a galaxy's colour. We find that galaxy stellar mass is the
primary driving mechanisms behind the star formation history for these galaxies
and discuss several possible mechanisms for regulating this process.
