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Register a new userGalaxy cluster outskirts: a universal entropy profile for relaxed clusters?
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We fit a functional form for a universal ICM entropy profile to the scaled entropy profiles of a catalogue of X-ray galaxy cluster outskirts results, which are all relaxed cool core clusters at redshift below 0.25. We also investigate the functional form suggested by Lapi et al. and Cavaliere et al. for the behaviour of the entropy profile in the outskirts and find it to fit the data well outside 0.3r200 . We highlight the discrepancy in the entropy profile behaviour in the outskirts between observations and the numerical simulations of Burns et al., and show that the entropy profile flattening due to gas clumping calculated by Nagai & Lau is insufficient to match observations, suggesting that gas clumping alone cannot be responsible for all of the entropy profile flattening in the cluster outskirts. The entropy profiles found with Suzaku are found to be consistent with ROSAT, XMM-Newton and Planck results.
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Until recently, only about 10% of the total intracluster gas volume had been studied with high accuracy, leaving a vast region essentially unexplored. This is now changing and a wide area of hot gas physics and chemistry awaits discovery in galaxy cluster outskirts. Also, robust large-scale total mass profiles and maps are within reach. First observational and theoretical results in this emerging field have been achieved in recent years with sometimes surprising findings. Here, we summarize and illustrate the relevant underlying physical and chemical processes and review the recent progress in X-ray, Sunyaev--Zeldovich, and weak gravitational lensing observations of cluster outskirts, including also brief discussions of technical challenges and possible future improvements.
We present atmospheric gas entropy profiles for 40 early type galaxies and 110 clusters spanning several decades of halo mass, atmospheric gas mass, radio jet power, and galaxy type. We show that within $sim 0.1R_{2500}$ the entropy profiles of low-mass systems, including ellipticals, brightest cluster galaxies, and spiral galaxies, scale approximately as $Kpropto R^{2/3}$. Beyond $sim 0.1R_{2500}$ entropy profiles are slightly shallower than the $K propto R^{1.1}$ profile expected from gravitational collapse alone, indicating that heating by AGN feedback extends well beyond the central galaxy. We show that the $Kpropto R^{2/3}$ entropy profile shape indicates that thermally unstable cooling is balanced by heating where the inner cooling and free-fall timescales approach a constant ratio. Hot atmospheres of elliptical galaxies have a higher rate of heating per gas particle compared to central cluster galaxies. This excess heating may explain why some central cluster galaxies are forming stars while most early-type galaxies have experienced no significant star formation for billions of years. We show that the entropy profiles of six lenticular and spiral galaxies follow the $R^{2/3}$ form. The continuity between central galaxies in clusters, giant ellipticals, and spirals suggests perhaps that processes heating the atmospheres of elliptical and brightest cluster galaxies are also active in spiral galaxies.
Knowledge of the structure of galaxy clusters is essential for an understanding of large scale structure in the universe, and may provide important clues to the nature of dark matter. Moreover, the shape of the dark matter distribution in the cluster core may offer insight into the structure formation process. Unfortunately, cluster cores also tend to be the site of complicated astrophysics. X-ray imaging spectroscopy of relaxed clusters, a standard technique for mapping their dark matter distributions, is often complicated by the presence of cool components in cluster cores, and the dark matter profile one derives for a cluster is sensitive to assumptions made about the distribution of this component. In addition, fluctuations in the temperature measurements resulting from normal statistical variance can produce results which are unphysical. We present here a procedure for extracting the dark matter profile of a spherically symmetric, relaxed galaxy cluster which deals with both of these complications. We apply this technique to a sample of galaxy clusters observed with the Chandra X-ray Observatory, and comment on the resulting mass profiles. For some of the clusters we compare their masses with those derived from weak and strong gravitational measurements.
One key ingredient in using galaxy clusters (GCs) as a precision cosmological probe in large X-ray surveys is to understand selection effects. The dependence of the X-ray emission on the square of the gas density leads to a predominant role of cool cores in the detection of GCs. The contribution of cool cores to the X-ray luminosity does not scale with GC mass and cosmology and therefore affects the use of X-ray GCs in producing cosmological constraints. One of the main science goals of the eROSITA mission is to constrain cosmology with a wide X-ray survey. We propose an eROSITA GC detection scheme that avoids the use of X-ray GC centers in detection. We calculate theoretical expectations and characterize the performance of this scheme by simulations. Performing realistic simulations of point sources (PSs) in survey mode we search for spatial scales where the extended signal is uncontaminated by the PS flux. We derive a combination of scales and thresholds, which result in a clean extended source catalog. We design the output of the GC detection which enables calibrating the core-excised luminosity using external mass measurements. We provide a way to incorporate the results of this calibration in the production of final core-excised luminosity. Similarly to other GC detection pipelines, we sample the flux - core radius detection space of our method and find many similarities with the pipeline used in the 400d survey. Both detection methods require large statistics on compact GCs, in order to reduce the contamination from PSs. The benefit of our pipeline consists in the sensitivity to the outer GC shapes, which are characterized by large core sizes with little GC to GC variation at a fixed total mass. GC detection through cluster outskirts improves the GC characterization using eROSITA survey data and is expected to yield well characterized GC catalogs having simple selection functions.
We present cosmological constraints from measurements of the gas mass fraction, $f_{gas}$, for massive, dynamically relaxed galaxy clusters. Our data set consists of Chandra observations of 40 such clusters, identified in a comprehensive search of the Chandra archive, as well as high-quality weak gravitational lensing data for a subset of these clusters. Incorporating a robust gravitational lensing calibration of the X-ray mass estimates, and restricting our measurements to the most self-similar and accurately measured regions of clusters, significantly reduces systematic uncertainties compared to previous work. Our data for the first time constrain the intrinsic scatter in $f_{gas}$, $(7.4pm2.3)$% in a spherical shell at radii 0.8-1.2 $r_{2500}$, consistent with the expected variation in gas depletion and non-thermal pressure for relaxed clusters. From the lowest-redshift data in our sample we obtain a constraint on a combination of the Hubble parameter and cosmic baryon fraction, $h^{3/2}Omega_b/Omega_m=0.089pm0.012$, that is insensitive to the nature of dark energy. Combined with standard priors on $h$ and $Omega_b h^2$, this provides a tight constraint on the cosmic matter density, $Omega_m=0.27pm0.04$, which is similarly insensitive to dark energy. Using the entire cluster sample, extending to $z>1$, we obtain consistent results for $Omega_m$ and interesting constraints on dark energy: $Omega_Lambda=0.65^{+0.17}_{-0.22}$ for non-flat $Lambda$CDM models, and $w=-0.98pm0.26$ for flat constant-$w$ models. Our results are both competitive and consistent with those from recent CMB, SNIa and BAO data. We present constraints on models of evolving dark energy from the combination of $f_{gas}$ data with these external data sets, and comment on the possibilities for improved $f_{gas}$ constraints using current and next-generation X-ray observatories and lensing data. (Abridged)
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