Title: The Metallicity of the Intracluster Medium Over Cosmic Time: Further Evidence for Early Enrichment,
Abstract: We use Chandra X-ray data to measure the metallicity of the intracluster
medium (ICM) in 245 massive galaxy clusters selected from X-ray and
Sunyaev-Zel'dovich (SZ) effect surveys, spanning redshifts $0<z<1.2$.
Metallicities were measured in three different radial ranges, spanning cluster
cores through their outskirts. We explore trends in these measurements as a
function of cluster redshift, temperature, and surface brightness "peakiness"
(a proxy for gas cooling efficiency in cluster centers). The data at large
radii (0.5--1 $r_{500}$) are consistent with a constant metallicity, while at
intermediate radii (0.1-0.5 $r_{500}$) we see a late-time increase in
enrichment, consistent with the expected production and mixing of metals in
cluster cores. In cluster centers, there are strong trends of metallicity with
temperature and peakiness, reflecting enhanced metal production in the
lowest-entropy gas. Within the cool-core/sharply peaked cluster population,
there is a large intrinsic scatter in central metallicity and no overall
evolution, indicating significant astrophysical variations in the efficiency of
enrichment. The central metallicity in clusters with flat surface brightness
profiles is lower, with a smaller intrinsic scatter, but increases towards
lower redshifts. Our results are consistent with other recent measurements of
ICM metallicity as a function of redshift. They reinforce the picture implied
by observations of uniform metal distributions in the outskirts of nearby
clusters, in which most of the enrichment of the ICM takes place before cluster
formation, with significant later enrichment taking place only in cluster
centers, as the stellar populations of the central galaxies evolve.