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Published March 1, 2012 | Published
Journal Article Open

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_☉.

Additional Information

© 2012 American Astronomical Society. Received 2010 December 28; accepted 2011 December 9; published 2012 February 13. We thank the anonymous referee for very useful comments and suggestions. We also thank all members of the COSMOS team. This work was financially supported in part by the Japan Society for the Promotion of Science (Nos. 17253001, 19340046, 23244031, and 23654068). Y.I. is financially supported by the Japan Society for the Promotion of science (JSPS) through JSPS Research Fellowship for Young Scientists.

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Published - Ideue2012p17555Astrophys_J.pdf


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