The Role of Galaxy Interaction in Environmental Dependence of the Star Formation Activity at z ≃ 1.2

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 deg^2 COSMOS field. We estimate star formation using the [O II]λ3727 emission line and environment from the local galaxy density. Our analysis shows that for massive galaxies (M_* ≳ 10^(10) M_☉), the fraction of [O II] emitters in high-density environments (Σ_(10th) ≳ 3.9 Mpc^(–2)) is 1.7 ± 0.4 times higher than in low-density environments (Σ_(10th)≾ 1.5 Mpc^(–2)), while the [O II] emitter fraction does not depend on environment for low-mass M_* ≾ 10^(10) 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 [O II] emitters in galaxies with companions is 2.4 ± 0.5 times as high as that in galaxies without companions at M_* ≳ 10^(10) 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_* ≳ 10^(10) M_☉.