Chandra sample of nearby relaxed galaxy clusters: mass, gas fraction, and mass-temperature relation

TL;DR

This study analyzes Chandra observations of 13 nearby relaxed galaxy clusters to reconstruct three-dimensional gas and total mass profiles under hydrostatic equilibrium. Flexible analytic models for the gas density and temperature are projected to the X-ray data, with uncertainties quantified via extensive Monte Carlo simulations. The results show total density profiles consistent with the NFW form predicted by ΛCDM, and gas density/temperature profiles that are nearly self-similar at large radii, though the inner regions exhibit mass-dependent scatter. The mass–temperature relation has slope $α\approx1.5-1.6$ and a normalization about $30\%$ higher than earlier X-ray studies, while gas fractions rise with radius and cluster mass and remain below the universal baryon fraction within $r_{2500}$ but approach it near $r_{500}$; these findings have implications for Ω_m and the role of non-gravitational processes in clusters.

Abstract

We present gas and total mass profiles for 13 low-redshift, relaxed clusters spanning a temperature range 0.7-9 keV, derived from all available Chandra data of sufficient quality. In all clusters, gas temperature profiles are measured to large radii (Vikhlinin et al.) so that direct hydrostatic mass estimates are possible to nearly r_500 or beyond. The gas density was accurately traced to larger radii; its profile is not described well by a beta-model, showing continuous steepening with radius. The derived rho_tot profiles and their scaling with mass generally follow the Navarro-Frenk-White model with concentration expected for dark matter halos in LambdaCDM cosmology. In the inner region (r<0.1r_500), the gas density and temperature profiles exhibit significant scatter and trends with mass, but they become nearly self-similar at larger radii. Correspondingly, we find that the slope of the mass-temperature relation for these relaxed clusters is in good agreement with the simple self-similar behavior, M_500 ~ T^alpha, where alpha=(1.5-1.6)+-0.1, if the gas temperatures are measured excluding the central cool cores. The normalization of this M-T relation is significantly, by =~ 30%, higher than most previous X-ray determinations. We derive accurate gas mass fraction profiles, which show increase both with radius and cluster mass. The enclosed f_gas profiles within r_2500 =~ 0.4 r_500 have not yet reached any asymptotic value and are still far (by a factor of 1.5-2) from the Universal baryon fraction according to the CMB observations. The f_gas trends become weaker and its values closer to Universal at larger radii, in particular, in spherical shells r_2500<r<r_500.

Figures (23)

• Figure 1: Detected sources in the Chandra ACIS-I field of A1413. Yellow circles mark point sources. The only detectable extended X-ray source (other than A1413) is marked by the red circle. ACIS-I field of view is $16'\times16'$.
• Figure 2: Observed projected emissivity profile for A133. Chandra and ROSAT PSPC data are shown in red and green, respectively. Solid line shows the best fit to the 3D gas density model (eq. [\ref{['eq:density:model']}]). The slope of the emissivity profile steepens by $1$ at radius $r_{\text{br}} = r_s (\varepsilon-1)^{-1/\gamma}$.
• Figure 3: Results for A133. (a) --- Temperature profile. Observed projected temperatures are shown by crosses. Solid red and blue lines show the best fit 3D model and the corresponding projected profile, respectively. Dotted lines indicate the 68% CL uncertainty interval obtained from Monte-Carlo simulations (see text). Models are shown in the radial range $r_{\text{min}}-r_{\text{det}}$ (see text). (b) --- Mass and density profiles. $M(r)$ increases with radius and $\rho(r)$ decreases. Red and blue lines show results for the total mass and gas mass, respectively. (c) --- Gas mass fraction as a function of radius. Lines show the enclosed $f_{\text{gas}}=M_{\text{gas}}(<r)/M_{\text{tot}}(<r)$. The local gas fraction ($\rho_{\text{gas}}/\rho_{\text{tot}}$) in the radial range directly covered by the Chandra temperature data is shown by crosses. The vertical line shows the radius $r_{500}$ derived from the best fit mass model.
• Figure 4: Results for A262. See caption to Fig. \ref{['fig:a133:res']}.
• Figure 5: Results for A383. See caption for Fig. \ref{['fig:a133:res']}.