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We investigated the fraction of [OII] emitters in galaxies at z~0.9 as afunction of the local galaxy density in the Hubble Space Telescope (HST) COSMOS2 square degree field. [OII] emitters are selected by the narrow-band excesstechnique with the NB711-band imaging data taken with Suprime-Cam on the Subarutelescope. 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 compareresults with our previous study at z~1.2. We found that the fraction is almostconstant at 0.3 Mpc^-2 < Sigma_10th < 10 Mpc^-2. We also checked the fractionof galaxies with blue rest-frame colors of NUV-R < 2 in our photo-z selectedsample, and found that the fraction of blue galaxies does not significantlydepend on the local density. On the other hand, the semi-analytic model ofgalaxy formation predicted that the fraction of star-forming galaxies at z~0.9decreases with increasing the projected galaxy density even if the effects ofthe 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~0.9independent of the galaxy environment. The decrease of the [OII] emitterfraction could be explained mainly by the rapid decrease of the star formationactivity in the universe from z~1.2 to z~0.9.

Environmental Effects on the Star Formation Activity at z~0.9 in the COSMOS Field