THE CLOSE ENVIRONMENT OF 24 μm GALAXIES AT 0.6 < 1.0 IN THE COSMOS FIELD

We investigate the close environment of 203 Spitzer 24 mu m-selected sources at 0.6 &lt; z &lt; 1.0 using zCOSMOS-bright redshifts and spectra of I &lt; 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 D-n (4000) &gt; 1.4. We find that LIRGs at 0.6 &lt; z &lt; 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 &lt; z &lt; 0.8 have, respectively, (42.0 +/- 4.9)% and (24.5 +/- 5.9)% of neighbors with D-n (4000) &gt; 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 similar to 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 &lt; z &lt; 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.