Environmental Effects on the Star Formation Activity at z~0.9 in the COSMOS Field

TL;DR

This study examines how environmental density influences star formation activity at z~0.9 in the COSMOS field, finding that the fraction of star-forming galaxies remains constant across densities, contrasting with model predictions.

Contribution

It provides observational evidence that the fraction of star-forming galaxies at z~0.9 is environment-independent, challenging existing semi-analytic galaxy formation models.

Findings

The fraction of [OII] emitters is about 30% at z~0.9, down from 60% at z~1.2.

The fraction of blue, star-forming galaxies does not significantly vary with local galaxy density.

Star formation activity declines rapidly from z~1.2 to z~0.9 regardless of environment.

Abstract

We investigated the fraction of [OII] emitters in galaxies at z~0.9 as a function of the local galaxy density in the Hubble Space Telescope (HST) COSMOS 2 square degree field. [OII] emitters are selected by the narrow-band excess technique with the NB711-band imaging data taken with Suprime-Cam on the Subaru telescope. We carefully selected 614 photo-z selected galaxies with M_U3500 < -19.31 at z=0.901-0.920, which includes 195 [OII] emitters, to directly compare results with our previous study at z~1.2. We found that the fraction is almost constant at 0.3 Mpc^-2 < Sigma_10th < 10 Mpc^-2. We also checked the fraction of galaxies with blue rest-frame colors of NUV-R < 2 in our photo-z selected sample, and found that the fraction of blue galaxies does not significantly depend on the local density. On the other hand, the semi-analytic model of galaxy formation predicted that the fraction of star-forming galaxies at z~0.9 decreases with increasing the projected galaxy density even if the effects of the projection and the photo-z error in our analysis were taken into account. The fraction of [OII] emitters decreases from ~60% at z~1.2 to ~30% at z~0.9 independent of the galaxy environment. The decrease of the [OII] emitter fraction could be explained mainly by the rapid decrease of the star formation activity in the universe from z~1.2 to z~0.9.