ENVIRONMENTAL EFFECTS ON THE STAR FORMATION ACTIVITY IN GALAXIES AT z ≃ 1.2 IN THE COSMOS FIELD*

We investigate the relation between the star formation activity in galaxies and environment at z ≃ 1.2 in the Cosmic Evolution Survey field, using the fraction of [O II] emitters and the local galaxy density. The fraction of [O II] emitters appears to be almost constant over the surface density of galaxies between 0.2 and 10 Mpc^(–2). This trend is different from that seen in the local universe where the star formation activity is weaker in higher density regions. To understand this difference between z ~ 1 and z ~ 0, we study the fraction of non-isolated galaxies as a function of local galaxy density. We find that the fraction of non-isolated galaxies increases with increasing density. Our results suggest that the star formation in galaxies at z ~ 1 is triggered by galaxy interaction and/or mergers.