Evidence of Environmental Quenching at Redshift z ≈ 2

We report evidence of environmental quenching among galaxies at redshifts of ≈2, namely the probability that a galaxy quenches its star formation activity is enhanced in the regions of space in proximity of other quenched, more massive galaxies. The effect is observed as strong clustering of quiescent galaxies around quiescent galaxies on angular scales of θ ≤ 20 arcsec, corresponding to a proper (comoving) scale of 168 (502) kpc at z = 2. The effect is observed only for quiescent galaxies around other quiescent galaxies; the probability to find star-forming galaxies around quiescent or around star-forming ones is consistent with the clustering strength of galaxies of the same mass and at the same redshift, as observed in dedicated studies of galaxy clustering. The effect is mass dependent in the sense that the quenching probability is stronger for galaxies of smaller masses (M*< 10^(10) M☉) than for more massive ones, i.e., it follows the opposite trend with mass relative to gravitational galaxy clustering. The spatial scale where the effect is observed suggests that these environments are massive halos, in which case the observed effect would likely be satellite quenching. The effect is also redshift dependent in that the clustering strength of quiescent galaxies around other quiescent galaxies at z = 1.6 is ≈1.7× larger than that of the galaxies with the same stellar mass at z = 2.6. This redshift dependence allows for a crude estimate of the timescale of environmental quenching of low-mass galaxies, which is in the range of 1.5 ~ 4 Gyr, in broad agreement with other estimates and with our ideas on satellite quenching.