The close environment of 24μm galaxies at 0.6 < z < 1.0 in the COSMOS field

Abstract

We investigate the close environment of 203 Spitzer 24μm-selected sources at 0.6 < z < 1.0 using zCOSMOSbright redshifts and spectra of I < 22.5 AB mag galaxies, over 1.5 deg2 of the COSMOS field. We quantify the degree of passivity of the luminous and ultra-luminous IR galaxy (LIRG and ULIRG, respectively) environments by analyzing the fraction of close neighbors with Dn (4000) > 1.4. We find that LIRGs at 0.6 < z < 0.8 live in more passive environments than those of other optical galaxies that have the same stellar mass distribution. Instead, ULIRGs inhabit more active regions (e.g., LIRGs and ULIRGs at 0.6 < z < 0.8 have, respectively, (42.0 ± 4.9)% and (24.5 ± 5.9)% of neighbors with Dn (4000) > 1.4 within 1 Mpc and ± 500 km s-1). The contrast between the activities of the close environments of LIRGs and ULIRGs appears especially enhanced in the COSMOS field density peak at z ∼ 0.67, because LIRGs on this peak have a larger fraction of passive neighbors, while ULIRGs have as active close environments as those outside the largescale structure. The differential environmental activity is related to the differences in the distributions of stellar mass ratios between LIRGs/ULIRGs and their close neighbors, as well as in the general local density fields. At 0.8 < z < 1.0, instead, we find no differences in the environment densities of ULIRGs and other similarly massive galaxies, in spite of the differential activities. We discuss a possible scenario to explain these findings.