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

Abstract

We investigate the close environment of 203 Spitzer 24 μm -selected sources at 0.6 < z < 1.0 using zCOSMOS-bright redshifts and spectra of I < 22.5 AB mag galaxies, over 1.5 deg^2 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 large-scale 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.

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