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Register a new userThe Perfect Slope: A new robust low-scatter X-ray mass indicator for clusters of galaxies
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This presentation is a Moriond version of our recent paper (Kravtsov, Vikhlinin & Nagai astro-ph/0603205) where we discussed X-ray proxies for the total cluster mass, including the spectral temperature (Tx), gas mass measured within r500 (Mg), and the new proxy, Yx, which is a simple product of Tx and Mg. We use mock Chandra images constructed for a sample of clusters simulated with high resolution in the concordance LambdaCDM cosmology. The simulated clusters exhibit tight correlations between the considered observables and total mass. The normalizations of the M500-Tx, Mg-Tx, and M500-Yx relations agree to better than =~ 10-15% with the current observational measurements of these relations. Our results show that Yx is the best mass proxy with a remarkably low scatter of only =~ 5-7% in M500 for a fixed Yx, at both low and high redshifts and regardless of whether clusters are relaxed or not. In addition, we show that redshift evolution of the Yx-M500 relation is close to the self-similar prediction, which makes Yx a very attractive mass indicator for measurements of the cluster mass function from X-ray selected samples.
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We present comparison of X-ray proxies for the total cluster mass, including the spectral temperature (Tx), gas mass measured within r500 (Mg), and the new proxy, Yx, which is a simple product of Tx and Mg and is related to the total thermal energy of the ICM. We use mock Chandra images constructed for a sample of clusters simulated with the eulerian N-body+gasdynamics adaptive mesh refinement ART code in the concordance LCDM cosmology. The simulations achieve high spatial and mass resolution and include radiative cooling, star formation, and other processes accompanying galaxy formation. Our analysis shows that simulated clusters exhibit a high degree of regularity and tight correlations between the considered observables and total mass. The normalizations of the M-Tx, Mg-Tx, and M-Yx relations agree to better than 10-15% with the current observational measurements of these relations. Our results show that Yx is the best mass proxy with a remarkably low scatter of only ~5-7% in M500 for a fixed Yx, at both low and high redshifts and regardless of whether clusters are relaxed or not. In addition, we show that redshift evolution of the Yx-M500 relation is close to the self-similar prediction, which makes Yx a very attractive mass indicator for measurements of the cluster mass function from X-ray selected samples.
We use a sample of 115 galaxy clusters at 0.1<z<1.3 observed with Chandra ACIS-I to investigate the relation between luminosity and Yx (the product of gas mass and temperature). The scatter in the relation is dominated by cluster cores, and a tight LY relation (11% intrinsic scatter in Lx) is recovered if sufficiently large core regions (0.15R500) are excluded. The intrinsic scatter is well described by a lognormal distribution and the relations are consistent for relaxed and disturbed/merging clusters. We investigate the LY relation in low-quality data (e.g. for clusters detected in X-ray survey data) by estimating Lx from soft band count rates, and find that the scatter increases somewhat to 21%. We confirm the tight correlation between Yx and mass and the self-similar evolution of that scaling relation out to z=0.6 for a subset of clusters in our sample with mass estimates from the literature. This is used to estimate masses for the entire sample and hence measure the LM relation. We find that the scatter in the LM relation is much lower than previous estimates, due to the full removal of cluster cores and more robust mass estimates. For high-redshift clusters the scatter in the LM relation remains low if cluster cores are not excluded. These results suggest that cluster masses can be reliably estimated from simple luminosity measurements in low quality data where direct mass estimates, or measurements of Yx are not possible. This has important applications in the estimation of cosmological parameters from X-ray cluster surveys.
We investigate the relationship between soft xray luminosity and mass for low redshift clusters of galaxies by comparing observed number counts to expectations of $Lambda$CDM cosmologies. We use a three-parameter model for the conditional probability of luminosity given mass and epoch, described as a log-normal distribution of fixed width centered on a power-law scaling relation, $L spropto M^prhoc^s(z)$. We use an ensemble of simulated clusters to argue that the observed, intrinsic variance in the temperature--luminosity relation is directly indicative of mass--luminosity variance, and derive $sigm se 0.43 pm 0.06$ from HIFLUGCS data. Adding this to the likelihood analysis results in best-fit estimates $p se 1.59 pm 0.05$, $lnlf se 1.34 pm 0.09$, and $sigm se 0.37 pm 0.05$ for self-similar redshift evolution in a concordance ($Omega_m se 0.3$, $Omega_Lambda se 0.7$, $sigma_8 se0.9$) universe. We show that the present-epoch intercept is very sensitive to power spectrum normalization, $lnlf spropto sigate^{-4}$, and the slope is weakly sensitive to the matter density, $p spropto Omega_m^{1/2}$. The intercept derived here is dimmer by a factor 2, and slope slightly steeper, than the L-M relation published using hydrostatic mass estimates of the HIFLUGCS sample. We show that this discrepancy is largely due to Malmquist bias of the xray flux-limited sample. In light of new WMAP constraints, we discuss the interplay between parameters and sources of systematic error, and offer a compromise model with $Omega_m se 0.24$, $sigma_8 se 0.85$, and somewhat lower scatter $sigm se 0.25$, in which hydrostatic mass estimates remain accurate to $ssim 15%$. We stress the need for independent calibration of the L-M relation via weak gravitational lensing.
We present results of Hubble Space Telescope and Chandra X-ray Observatory observations of globular clusters (GCs) and low-mass X-ray binaries (LMXBs) in the central regions of Centaurus A. Out of 440 GC candidates we find that 41 host X-ray point sources that are most likely LMXBs. We fit King models to our GC candidates in order to measure their structural parameters. We find that GCs that host LMXBs are denser and more compact, and have higher encounter rates and concentrations than the GC population as a whole. We show that the higher concentrations and masses are a consequence of the dependence of LMXB incidence on central density and size plus the general trend for denser GCs to have higher masses and concentrations. We conclude that neither concentration nor mass are fundamental variables in determining the presence of LMXBs in GCs, and that the more fundamental parameters relate to central density and size.
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.
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