Search

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 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 MU3500 < -19.31 at z = 0.901 - 0.920, which includes 195 [OII] 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 < Σ10th < 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 [OII] emitters decreases from ~60% at z ~ 1.2 to ~30% at z ~ 0.9 independent of galaxy environment. The decrease of the [OII] 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. [ABSTRACT FROM AUTHOR]

In order to understand environmental effects on star formation in high-redshift galaxies, we investigate the physical relationships between the star formation activity, stellar mass, and environment for z ≃1.2 galaxies in the 2 deg² COSMOS field. We estimate star formation using the [OII]λ3727 emission line and environment from the local galaxy density. Our analysis shows that for massive galaxies (M* ≳ 1010 M⊙), the fraction of [O II] emitters in high-density environments (Σ10th ≲ 3.9 Mpc-2) is 1.7 ± 0.4 times higher than in low-density environments (Σ10th ≾ 1.5 Mpc-2), while the [OII] emitter fraction does not depend on environment for low-mass M*≲1010 M⊙ galaxies. In order to understand what drives these trends, we investigate the role of companion galaxies in our sample. We find that the fraction of [OII] emitters in galaxies with companions is 2.4 ± 0.5 times as high as that in galaxies without companions at M* ≥ 1010 M⊙. In addition, massive galaxies are more likely to have companions in high-density environments. However, although the number of star-forming galaxies increases for massive galaxies with close companions and in dense environments, the average star formation rate of star-forming galaxies at a given mass is independent of environment and the presence/absence of a close companion. These results suggest that interactions and/or mergers in a high-density environment could induce star formation in massive galaxies at z ~ 1.2, increasing the fraction of star-forming galaxies with M* ≳ 1010 M⊙. [ABSTRACT FROM AUTHOR]