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Register a new userThe Enrichment of the Intracluster Medium
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To determine the relative contributions of galactic and intracluster stars to the enrichment of the intracluster medium (ICM), we present X-ray surface brightness, temperature, and Fe abundance profiles for a set of twelve galaxy clusters for which we have extensive optical photometry. Assuming a standard IMF and simple chemical evolution model scaled to match the present-day cluster early-type SN Ia rate, the stars in the brightest cluster galaxy (BCG) plus the intracluster stars (ICS) generate 31^{+11}_{-9}%, on average, of the observed ICM Fe within r_{500} (~ 0.6 times r_{200}, the virial radius). An alternate, two-component SN Ia model (including both prompt and delayed detonations) produces a similar BCG+ICS contribution of 22^{+9}_{-9}%. Because the ICS typically contribute 80% of the BCG+ICS Fe, we conclude that the ICS are significant, yet often neglected, contributors to the ICM Fe within r_{500}. However, the BCG+ICS fall short of producing all the Fe, so metal loss from stars in other cluster galaxies must also contribute. By combining the enrichment from intracluster and galactic stars, we can account for all the observed Fe. These models require a galactic metal loss fraction (0.84^{+0.11}_{-0.14}) that, while large, is consistent with the metal mass not retained by galactic stars. The SN Ia rates, especially as a function of galaxy environment and redshift, remain a significant source of uncertainty in further constraining the metal loss fraction. For example, increasing the SN Ia rate by a factor of 1.8 -- to just within the 2 sigma uncertainty for present-day cluster early-type galaxies -- allows the combined BCG + ICS + cluster galaxy model to generate all the ICM Fe with a much lower galactic metal loss fraction (~ 0.35).
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The distribution of metals in the intracluster medium (ICM) of galaxy clusters provides valuable information on their formation and evolution, on the connection with the cosmic star formation and on the effects of different gas processes. By analyzing a sample of simulated galaxy clusters, we study the chemical enrichment of the ICM, its evolution, and its relation with the physical processes included in the simulation and with the thermal properties of the core. These simulations, consisting of re-simulations of 29 Lagrangian regions performed with an upgraded version of the SPH GADGET-3 code, have been run including two different sets of baryonic physics: one accounts for radiative cooling, star formation, metal enrichment and supernova (SN) feedback, and the other one further includes the effects of feedback from active galactic nuclei (AGN). In agreement with observations, we find an anti-correlation between entropy and metallicity in cluster cores, and similar radial distributions of heavy-element abundances and abundance ratios out to large cluster-centric distances (~R180). In the outskirts, namely outside of ~0.2R180, we find a remarkably homogeneous metallicity distribution, with almost flat profiles of the elements produced by either SNIa or SNII. We investigated the origin of this phenomenon and discovered that it is due to the widespread displacement of metal-rich gas by early (z>2-3) AGN powerful bursts, acting on small high-redshift haloes. Our results also indicate that the intrinsic metallicity of the hot gas for this sample is on average consistent with no evolution between z=2 and z=0, across the entire radial range.
We investigate the metal enrichment of the intracluster medium (ICM) in the framework of hierarchical models of galaxy formation. We calculate the formation and evolution of galaxies and clusters using a semi-analytical model which includes the effects of flows of gas and metals both into and out of galaxies. For the first time in a semi-analytical model, we calculate the production of both alpha and iron-peak elements based on theoretical models for the lifetimes and ejecta of type Ia and type II supernovae (SNe Ia and SNe II). It is essential to include the long lifetimes of the SNIa progenitors in order to correctly model the evolution of the iron-peak elements. We find that if all stars form with an IMF similar to that found in the solar neighbourhood, then the metallicities of O, Mg, Si and Fe in the ICM are predicted to be 2-3 times lower than observed values. In contrast, a model (also favoured on other grounds) in which stars formed in bursts triggered by galaxy mergers have a top-heavy IMF reproduces the observed ICM abundances of O, Mg, Si and Fe. The same model predicts ratios of ICM mass to total stellar luminosity in clusters which agree well with observations. According to our model, the bulk of the metals in clusters are produced by L* and brighter galaxies. [abridged]
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-Zeldovich (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.
We have investigated the baryon-mass content in a subsample of 19 clusters of galaxies extracted from the X-ray flux-limited sample HIFLUGCS according to their positions in the sky. For these clusters, we measured total masses and characteristic radii on the basis of a rich optical spectroscopic data set, the physical properties of the intracluster medium (ICM) using XMM-Newton and ROSAT X-ray data, and total (galaxy) stellar masses utilizing the SDSS DR7 multi-band imaging. The observed (hot) gas-mass fractions are almost constant in this mass range. We confirm that the stellar mass fraction decreases as the total mass increases and shows (20+/-4)% scatter; in addition, we show that it decreases as the central entropy increases. The latter behavior supports a twofold interpretation where heating from merging quenches the star-formation activity of galaxies in massive systems, and feedback from supernovae and/or radio galaxies drives a significant amount of gas to the regions beyond r_{500} or, alternatively, a substantially large amount of intracluster light (ICL) is associated with galaxies in nonrelaxed systems. Furthermore, less massive clusters are confirmed to host less gas per unit total mass; however, they exhibit higher mass fractions in metals, so that their ICM is more metal-rich. This again supports the interpretation that in the potential wells of low-mass systems the star-formation efficiency of galaxies was high or, alternatively, some gas is missing from the hot phase of the ICM. The former hypothesis is preferred as the main driver of the mass-dependent metal enrichment since the total mass-to-optical luminosity ratio increases as the total mass increases.
The cluster of galaxies MS 1512.4+3647 (z=0.372) was observed with Suzaku for 270 ks. Besides the Fe abundance, the abundances of Mg, Si, S, and Ni are separately determined for the first time in a medium redshift cluster (z>0.3). The derived abundance pattern of MS 1512.4+3647 is consistent with those of nearby clusters, suggesting that the system has similar contributions from supernovae (SNe) Ia and SNe II to nearby clusters. The number ratio of SNe II to SNe Ia is $sim3$. The estimated total numbers of both SNe II and SNe Ia against gas mass indicate similar correlation with those for the nearby clusters. The abundance results of MS 1512.4+3647 is consistent with the standard scenario that the SN II rate history roughly follows the star-formation history which has a peak at 1<z<2 and then declines by about one order of magnitude toward $zsim0$. The similar number of SNe Ia to the nearby clusters suggests that the SN Ia rate declines steeply from z=0.37 to z=0 and/or SN Ia explosions occurred predominantly at larger redshifts.
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