THE ROLE OF GALAXY INTERACTION IN ENVIRONMENTAL DEPENDENCE OF THE STAR FORMATION ACTIVITY AT z similar or equal to 1.2

International audience; 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 similar or equal to 1.2 galaxies in the 2 deg(2) COSMOS field. We estimate star formation using the [O II]lambda 3727 emission line and environment from the local galaxy density. Our analysis shows that for massive galaxies (M-* greater than or similar to 10(10) M-circle dot), the fraction of [O II] emitters in high-density environments (Sigma(10th) greater than or similar to 3.9 Mpc(-2)) is 1.7 +/- 0.4 times higher than in low-density environments (Sigma(10th) less than or similar to 1.5 Mpc(-2)), while the [O II] emitter fraction does not depend on environment for low-mass M-* less than or similar to 10(10) M-circle dot 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-* greater than or similar to 10(10) M-circle dot. 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 similar to 1.2, increasing the fraction of star-forming galaxies with M-* greater than or similar to 10(10) M-circle dot.