Cluster induced quenching of galaxies in the massive cluster XMMXCS J2215.9?1738 at z ～ 1.5 traced by enhanced metallicities inside half R ₂₀₀ ?

摘要

Aims . Cluster environments at z ?< ?0.5 were found to increase the gas metallicities of galaxies which enter inner regions of the clusters where the density of the intracluster medium is high enough to remove their hot halo gas by ram-pressure stripping effects and to stop the inflow of pristine gas. To extend these studies to z ?> ?1, the most massive clusters known at these redshifts are the sites where these environmental effects should be more pronounced and more easily observed with present day telescopes. Methods . We explore the massive cluster XMMXCS J2215.9?1738 at z ?～?1.5 with KMOS spectroscopy of H α and [N?II]? λ ?6584 covering a region that corresponds to about one virial radius. Using published spectroscopic redshifts of 108 galaxies in and around the cluster we computed the location of galaxies in the projected velocity-versus-position phase-space to separate our cluster sample into a virialized region of objects accreted longer ago (roughly inside half R _(200)) and a region of infalling galaxies. We measured oxygen abundances for ten cluster galaxies with detected [N?II]? λ ?6584 lines in the individual galaxy spectra and compared the mass–metallicity relation of the galaxies inside half R _(200)with the infalling galaxies and a field sample at similar redshifts. Results . We find that the oxygen abundances of individual z ?～?1.5 star-forming cluster galaxies inside half R _(200)are comparable, at the respective stellar mass, to the higher local SDSS metallicity values. We compare our measurements with a field galaxy sample from the KMOS3D survey at similar redshifts. We find that the [N?II]? λ ?6584/H α line ratios inside half R _(200)are higher by 0.2 dex and that the resultant metallicities of the galaxies in the inner part of the cluster are higher by about 0.1 dex, at a given mass, than the metallicities of infalling galaxies and of field galaxies at z ?～?1.5. The enhanced metallicities of cluster galaxies at z ?～?1.5 inside 0.5 R _(200)indicate that the density of the intracluster medium in this massive cluster becomes high enough toward the cluster center such that the ram pressure exceeds the restoring pressure of the hot gas reservoir of cluster galaxies. This can remove the gas reservoir and initiate quenching; although the galaxies continue to form stars, albeit at slightly lower rates, using the available cold gas in the disk which is not stripped.