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

TL;DR

This study examines the environments of 24 micron-selected galaxies at redshifts 0.6 to 1.0 in the COSMOS field, revealing that LIRGs and ULIRGs occupy different local environments with varying activity levels, influenced by large-scale structures.

Contribution

It provides new insights into the environmental differences of LIRGs and ULIRGs at intermediate redshifts using extensive spectroscopic data from COSMOS.

Findings

LIRGs at 0.6<z<0.8 are in more passive environments than similar-mass optical galaxies.

ULIRGs at 0.6<z<0.8 are found in more active regions with more star-forming neighbours.

Environmental activity differences are linked to stellar mass ratios and local density variations.

Abstract

We investigate the close environment of 203 Spitzer 24 micron-selected sources at 0.6<z<1.0 using zCOSMOS-bright redshifts and spectra of I<22.5 AB mag galaxies, over 1.5 sq. deg. of the COSMOS field. We quantify the degree of passivity of the LIRG and ULIRG environments by analysing the fraction of close neighbours 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 neighbours with Dn (4000)>1.4 within 1 Mpc and +/- 500 km/s). 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 neighbours, 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 neighbours, 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.