Suzaku observations of outskirts of nearby clusters and groups: I. electron density and gas fraction to the virial radius

TL;DR

This study analyzes Suzaku observations of 14 nearby clusters and groups to map the intracluster medium out to the virial radius $r_{200}$, deriving robust radial profiles of emission measure, electron density, and temperature across a wide halo mass range. By meticulously modeling all background components, including the Local Hot Bubble, Milky Way Halo, a super-virial HG component, the cosmic X-ray background, and the non-X-ray background, the authors obtain reliable outskirts measurements and quantify systematic effects. They find that relaxed clusters exhibit self-similar emission-measure and density profiles with $n_e\propto R^{-2}$ beyond $\sim0.4\,r_{500}$, while gas fractions increase with radius and mass, approaching the cosmic mean near $r_{200}$ for massive systems but remaining sub-cosmic for low-mass halos. The results support a picture in which non-gravitational feedback, especially AGN activity, regulates baryon retention and shapes the baryon content of halos from groups to clusters, with important implications for cosmology and structure formation. The work emphasizes the critical role of careful background modeling in X-ray analyses of cluster outskirts and provides empirical constraints on pressure profiles, hydrostatic masses, and baryon partitioning across a broad mass spectrum.

Abstract

We present an analysis of Suzaku observations of 14 nearby galaxy clusters and groups (z < 0.06), extending radial coverage out to the virial radius (approximately r200). The sample spans a wide mass range, from M500 about 2x10^13 to 7x10^14 solar masses, and includes well-studied systems such as Coma, Perseus, and Virgo. We carefully modeled all background components, including the soft X-ray foregrounds (the Local Hot Bubble, Milky Way Halo, and super-virial temperature components), the cosmic X-ray background, and the non-X-ray background, and assessed their effects on the derived properties of the intracluster medium (ICM). We constructed radial profiles of emission measure, electron density, and temperature. Temperatures decrease smoothly with radius, typically dropping to about one-third to half of their peak values near r200. For relaxed clusters, the emission measure profiles outside the core regions are well described by a beta model with beta around 0.6-0.7, while groups show slightly flatter slopes of beta around 0.4-0.65. Beyond r2500, electron density profiles follow a power-law decline with a slope close to 2. At r500 and r200, the electron density and the gas mass fraction show a tight correlation with the system mass, except for three clusters with bright subclusters. In massive clusters, the gas fraction increases with radius and approaches the cosmic baryon fraction near r200. In contrast, lower-mass systems exhibit gas fractions of around 0.1 at r200. The observed mass dependence of gas fractions suggests that feedback and related processes play an increasingly important role toward the group scale, shaping the connection between baryons and dark matter halos.

Figures (24)

• Figure 1: Suzaku XIS mosaic images in the 0.5--5.0 keV energy band. While the exposure time and instrumental background were corrected, the effect of the vignetting was not corrected. The images were smoothed by a Gaussian of $\sigma=$16 pixels $\approx 17\arcsec$. The numbers below the color bars have units of counts Ms$^{-1}$ pixel$^{-1}$. The green circles correspond the $r_{500}$ and $2 r_{500}$, respectively. The dashed circles indicate the point sources excluded from the analysis. The yellow boxes and annuli showed the background region where the background component parameters were estimated. Alt text: Mosaic X-ray images of the sample except for the Virgo cluster observed with Suzaku XIS in the 0.5--5.0 keV band. The images are displayed in color, with the scale bar representing counts per pixel per megasecond. The horizontal and vertical axes show right ascension and declination.
• Figure 2: The same figure as figure \ref{['fig:image']}, but (left) north arm, (middle) south arm, (right top) east arm, and (right bottom) west arm of the Virgo cluster. Alt text: The same as figure \ref{['fig:image']}, but showing the Virgo cluster.
• Figure 3: The representative XIS1 spectra of the background regions of the Perseus (110$'$--130$'$), A2199 (90$'$ offset pointings), Virgo (240$'$--300$'$ east), and Antlia (130$'$ offset). The contributions of the LXB (dotted lines), MWH (dashed lines), HG (dot-dashed lines), CXB (dot-dot-dashed lines), O I (light green solid lines), O VII (green solid lines), and NXB (gray solid lines) components are shown. The bottom panels show the residuals of the fit. Alt text: Four subfigures, each consisting of two panels: the upper panels show the representative XIS1 spectra of the background regions and contributions of the spectral components with different colors, and the lower panels display the residuals.
• Figure 4: The representative XIS1 spectra of the Perseus cluster at 80$'$--100$'$ (1.7--2.2 Mpc) and A2199 at 36$'$--48$'$ (1.3--1.8 Mpc) The contributions of the ICM, LXB, MWH, HG, CXB, O I, O VII, and NXB components are shown. The bottom panels show the residuals of the fit. Alt text: Four subfigures, each consisting of two panels: the upper panels show the representative XIS1 and XIS3 spectra of the Perseus cluster and A2199 and contributions of the spectral components with different colors, and the lower panels display the residuals.
• Figure 5: Radial profiles of the emission measure for the ICM, EM$_{\rm HG08}$ (filled symbols) and EM$_{\rm Z03}$ (open symbols), with representative $\beta$-model profiles. The dashed line in the top left panel shows the best-fit emission measure profile of the Perseus cluster excluding the W and NW arms from Matsushita2025. The vertical lines indicate $r_{500}$ of representative clusters. Alt text: Six-panel figures showing radial profiles of ICM emission measure. The vertical axis is the emission measure in units of cm$^{-6}$ pc, and the horizontal axis is the radius in units of kpc. Different systems are plotted with colored symbols, and the representative $\beta$-model curves are overlaid.