Environmental effects on star formation activity at z ∼ 0.9 in the cosmos field

We investigated the fraction of [O II] emitters in galaxies at z ∼ 0.9 as a function of the local galaxy density in the Hubble Space Telescope (HST) COSMOS 2 deg² field. [O II] emitters are selected by the narrowband 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 [O II] emitters, to directly compare the results with our previous study at z ∼ 1.2. We found that the fraction is almost constant at 0.3 Mpc^-2 < Σ_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 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 [O II] emitters decreases from ∼60% at z ∼ 1.2 to ∼30% at z ∼ 0.9 independent of galaxy environment. The decrease of the [O II] emitter fraction could be explained mainly by the rapid decrease of star formation activity in the universe from z ∼ 1.2 to z ∼ 0.9.