  We investigated the fraction of [OII] emitters in galaxies at z~0.9 as a
function of the local galaxy density in the Hubble Space Telescope (HST) COSMOS
2 square degree field. [OII] emitters are selected by the narrow-band excess
technique with the NB711-band imaging data taken with Suprime-Cam on the Subaru
telescope. We carefully selected 614 photo-z selected galaxies with M_U3500 <
-19.31 at z=0.901-0.920, which includes 195 [OII] emitters, to directly compare
results with our previous study at z~1.2. We found that the fraction is almost
constant at 0.3 Mpc^-2 < Sigma_10th < 10 Mpc^-2. We also checked the fraction
of galaxies with blue rest-frame colors of NUV-R < 2 in our photo-z selected
sample, and found that the fraction of blue galaxies does not significantly
depend on the local density. On the other hand, the semi-analytic model of
galaxy formation predicted that the fraction of star-forming galaxies at z~0.9
decreases with increasing the projected galaxy density even if the effects of
the projection and the photo-z error in our analysis were taken into account.
The fraction of [OII] emitters decreases from ~60% at z~1.2 to ~30% at z~0.9
independent of the galaxy environment. The decrease of the [OII] emitter
fraction could be explained mainly by the rapid decrease of the star formation
activity in the universe from z~1.2 to z~0.9.
