The loss of the intra-cluster medium in globular clusters

Stars in globular clusters (GCs) lose a non negligible amount of mass during their post-main sequence evolution. This material is then expected to build up a substantial intra-cluster medium (ICM) within the GC. However, the observed gas content in GCs is a couple of orders of magnitude below these expectations. Here we follow the evolution of this stellar wind material through hydrodynamical simulations to attempt to reconcile theoretical predictions with observations. We test different mechanisms proposed in the literature to clear out the gas such as ram-pressure stripping by the motion of the GC in the Galactic halo medium and ionisation by UV sources. We use the code ramses to run 3D hydrodynamical simulations to study for the first time the ICM evolution within discretised multi-mass GC models including stellar winds and full radiative transfer. We find that the inclusion of both ram-pressure and ionisation is mandatory to explain why only a very low amount of ionised gas is observed in the core of GCs. The same mechanisms operating in ancient GCs that clear the gas could also be efficient at younger ages, meaning that young GCs would not be able to retain gas and form multiple generations of stars as assumed in many models to explain multiple populations. However, this rapid clearing of gas is consistent with observations of young massive clusters.

UV-Bright Stellar Populations and Their Evolutionary Implications in the Collapsed-Core Cluster M15

We performed deep photometry of the central region of Galactic globular cluster M15 from archival Hubble Space Telescope data taken on the High Resolution Channel and Solar Blind Channel of the Advanced Camera for Surveys. Our data set consists of images in far-UV (FUV$_{140}$; F140LP), near-UV (NUV$_{220}$; F220W), and blue (B$_{435}$; F435W) filters. The addition of an optical filter complements previous UV work on M15 by providing an additional constraint on the UV-bright stellar populations. Using color-magnitude diagrams (CMDs) we identified several populations that arise from non-canonical evolution including candidate blue stragglers, extreme horizontal branch stars, blue hook stars (BHks), cataclysmic variables (CVs), and helium-core white dwarfs (He WDs). Due to preliminary identification of several He WD and BHk candidates, we add M15 as a cluster containing a He WD sequence and suggest it be included among clusters with a BHk population. We also investigated a subset of CV candidates that appear in the gap between the main sequence (MS) and WDs in FUV$_{140}-$NUV$_{220}$ but lie securely on the MS in NUV$_{220}-$B$_{435}$. These stars may represent a magnetic CV or detached WD-MS binary population. Additionally, we analyze our candidate He WDs using model cooling sequences to estimate their masses and ages and investigate the plausibility of thin vs. thick hydrogen envelopes. Finally, we identify a class of UV-bright stars that lie between the horizontal branch and WD cooling sequences, a location not usually populated on cluster CMDs. We conclude these stars may be young, low-mass He WDs.

A Spitzer Space Telescope Atlas of omega Centauri: The Stellar Population, Mass Loss, and the Intracluster Medium

We present a Spitzer Space Telescope imaging survey of the most massive Galactic globular cluster, omega Centauri, and investigate stellar mass loss at low metallicity and the intracluster medium (ICM). The survey covers approximately 3.2x the cluster half-mass radius at 3.6, 4.5, 5.8, 8, and 24 microns, resulting in a catalog of over 40,000 point-sources in the cluster. Approximately 140 cluster members ranging 1.5 dex in metallicity show a red excess at 24 microns, indicative of circumstellar dust. If all of the dusty sources are experiencing mass loss, the cumulative rate of loss is estimated at 2.9 - 4.2 x 10^(-7) solar masses per year, 63% -- 66% of which is supplied by three asymptotic giant branch stars at the tip of the Red Giant Branch (RGB). There is little evidence for strong mass loss lower on the RGB. If this material had remained in the cluster center, its dust component (> 1 x 10^(-4) solar masses) would be detectable in our 24 and 70 micron images. While no dust cloud located at the center of omega Cen is apparent, we do see four regions of very faint, diffuse emission beyond two half-mass radii at 24 microns. It is unclear whether these dust clouds are foreground emission or are associated with omega Cen. In the latter case, these clouds may be the ICM in the process of escaping from the cluster.

The Intra-Cluster Medium

The Intra-Cluster Medium (ICM) is a rarefied, hot, highly ionized, metal rich, weakly magnetized plasma. In these proceeding, after having reviewed some basic ICM properties, I discuss recent results obtained with the BeppoSAX, XMM-Newton and Chandra satellites. These results are summarized in the following five points. 1) Currently available hard X-ray data does not allow us to constrain B fields in radio halos, the advent of hard X-ray telescopes in a few years may change the situation substantially. 2) There is mounting evidence that temperature profiles of clusters at large radii decline; however investigation of the outermost regions will have to await a new generation of yet unplanned but technologically feasible experiments. 3) The ICM is polluted with metals, the enrichment has probably occurred early on in the clusters life. The abundance excess observed at the center of CC clusters is due to the giant elliptical always found in these systems. 4) Chandra and XMM-Newton observations of relaxed clusters have falsified the previously accepted cooling flow model, heating mechanisms that may offset the cooling are actively being sought. 5) The superb angular resolution of Chandra is allowing us to trace a previously unknown phenomenon intimately related to the formation of galaxy clusters and of their cores.

Metal Enrichment Processes in the Intra-Cluster Medium

We present numerical simulations of galaxy clusters which include interaction processes between the galaxies and the intra-cluster gas. The considered interaction processes are galactic winds and ram-pressure stripping, which both transfer metal-enriched interstellar medium into the intra-cluster gas and hence increase its metallicity. We investigate the efficiency and time evolution of the interaction processes by simulated metallicity maps, which are directly comparable to those obtained from X-ray observations. We find that ram-pressure stripping is more efficient than quiet (i.e. non-starburst driven) galactic winds in the redshift interval between 1 and 0. The expelled metals are not mixed immediately with the intra-cluster gas, but inhomogeneities are visible in the metallicity maps. Even stripes of higher metallicity that a single galaxy has left behind can be seen. The spatial distribution of the metals transported by ram-pressure stripping and by galactic winds are very different for massive clusters: the former process yields a centrally concentrated metal distribution while the latter results in an extended metal distribution.