  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 [OII] emission
line and environment from the local galaxy density. Our analysis shows that for
massive galaxies M_*>10^10 M_sun, the fraction of [OII] emitters in
high-density environments is 1.7 times higher than in low-density environments,
while the [OII] emitter fraction does not depend on environment for low-mass
M_* < 10^10 M_sun 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 times as high as
that in galaxies without companions at M_* > 10^10 M_sun. 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 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_sun.
