Revisiting thermodynamics at the outskirts of the Perseus cluster with Suzaku: importance of modeling the Hot Galactic gas

TL;DR

This work reevaluates the thermodynamics of the Perseus cluster outskirts using 65 Suzaku pointings beyond $1\,\mathrm{Mpc}$, emphasizing the significant impact of a foreground hot Galactic (HG) gas component near $1$ keV on ICM temperature and density measurements. By modeling the soft X-ray background with Local Hot Bubble, Milky Way Halo, the HG, and the cosmic X-ray background, the authors derive robust radial profiles for $kT_{ICM}$ and EM$_{ICM}$, finding a temperature slope of about $-0.67$ and a density slope near $-2.21$, with the entropy following $K\propto r^{0.81}$ close to the theoretical $1.1$ expectation. The hydrostatic mass within $r_{500}$ is $M_{500}\approx6.4\times10^{14}M_\odot$ at $r_{500}\approx59′$ (1.3 Mpc), and the gas fraction is $f_{gas}(r_{500})\approx0.13$ rising to $f_{gas}(r_{200})\approx0.18$, consistent with the cosmic baryon fraction when stellar mass is included. Most of the outskirts appear regular and near hydrostatic equilibrium, though two filament-directed arms show elevated emission measures, suggesting mild accretion from large-scale structure. The study underscores the necessity of accurate background modeling, particularly the HG component, for reliable ICM inferences at cluster outskirts.

Abstract

The thermodynamic properties of the intracluster medium (ICM) at the outskirts of galaxy clusters provide valuable insights into the growth of the dark matter halo and the heating of the ICM. Considering the results of the soft X-ray background study of non-cluster Suzaku fields, we revisit 65 Suzaku pointing observations of the Perseus cluster in eight azimuthal directions beyond 1 Mpc (0.8 $r_{500}$). A possible foreground component, whose spectrum is modeled as a 1 keV collisional ionization equilibrium plasma, significantly affects the temperature and density measurements of the ICM in cluster outskirts. The emission measures in the six arms are similar, showing that the radial slopes of temperature and density follow $r^{-0.67\pm0.25}$ and $r^{-2.21\pm 0.06}$, respectively. The radial pressure profile is close to the average profile measured by the Planck satellite. The resulting entropy slope is $\propto r^{0.81\pm 0.25}$, consistent with the theoretical slope of 1.1. The integrated gas fraction, the ratio of the integrated gas mass to the hydrostatic mass, is estimated to be 0.13$\pm$0.01 and 0.18$\pm$0.02 at $r_{500}$ and $r_{200}$, respectively, consistent with the cosmic baryon fraction. These results suggest that the ICM at the cluster outskirts is quite regular and close to hydrostatic equilibrium. The remaining two arms show that the emission measure is higher by a factor of 1.5-2, possibly due to accretion from filaments from the large-scale structure. A sudden drop in the emission measure also occurs in a direction toward one of the filaments.

Figures (24)

• Figure 1: The ROSAT soft-X-ray all-sky survey map in the 0.44--1.01 keV energy band Snowden1997 around the Perseus and AWM7 clusters. The squares show the XIS FOVs analyzed in this study. The inner and outer circles correspond to $r_{500}$ and $r_{200}$, respectively. Alt text: Soft X-ray map in Galactic coordinates showing the Perseus and AWM7 clusters.
• Figure 2: The representative XIS1 spectra (upper panels) of the Perseus cluster at $\sim 125^\prime$ in the S (a), and W (b) arms and at $\sim 116^\prime$ for the NW (c) and SE (d) arms, fitted with the background model and the residuals (lower panels). The contributions from the MWH (dotted red), the LHB (dashed magenta), the HG (dot-dashed green), the CXB (thick solid blue), O .8 .8 I (solid orange), and O .8 .8 VII (solid orange) are also shown. The contributions of the NXB components are shown as the gray solid lines. Alt text: Four subfigures, each consisting of two panels: the upper panels show the representative XIS1 spectra of the Perseus cluster, and the lower panels display the residuals. Each subfigure corresponds to a different arm direction (S, W, NW, SE) at radii of approximately 116$^\prime$--125$^\prime$.
• Figure 3: The radial profiles of EM$_{\rm HG}$ beyond 90$^\prime$ for the E (the red filled circle), NE (the blue filled diamonds), NW (the orange-filled squares), W (the brown downward triangles), SW (the gray pluses), S (the black stars), SE (the magenta crosses) arms, and the NW background region of the AWM7 cluster (the red open pentagon). Each dashed line represents the average value over the radial range covered by the corresponding data points with the same color. Alt text: Plot showing the radial profiles of EM$_{\rm HG}$ beyond 90$^\prime$ for multiple arms of the Perseus cluster and the NW background region of AWM7. A distinct symbol and color represent each arm.
• Figure 4: The representative XIS1 spectra (upper panels) and residuals (bottom panels). The contributions from the ICM (thick solid red) and the other components (the meaning of colors and line types are the same as in figure \ref{['fig:bgd']}) are also shown. Alt text: Eight subfigures, each consisting of two panels: the upper panels show the representative XIS1 spectra of the Perseus cluster, and the lower panels display the residuals. Each subfigure corresponds to a different arm direction at radii of approximately 64$^\prime$--88$^\prime$.
• Figure 5: The radial profiles of $kT_{\rm ICM}$ (upper panel) and EM$_{\rm ICM}$ (bottom panel) of the Perseus cluster for the E (the red filled circle), NE (the blue filled diamonds), N (the filled upward triangles), NW (the orange-filled squares), W (the brown downward triangles), SW (the gray pluses), S (the black stars), SE (the magenta crosses) arms. The open squares show the data from the NW arm observed in 2014. The gray solid and brown dashed lines are the best-fit power-law relations for the relaxed arms (except for the NW and W arms) and the W arm, respectively, over $r=$49$^\prime$--101$^\prime$. The black solid line in the upper panel represents the three-dimensional temperature profile (see section 3.5) and the dashed gray line in the bottom panel shows the best-fit EM$_{\rm ICM}$ profile for $r=63^\prime$--$101^\prime$ radial range. The two vertical dotted lines correspond to $r_{500}$ and $r_{200}$. The horizontal dashed line represents a flux level equal to 10% of the CXB. Alt text: Two-panel plot showing radial profiles of ICM temperature (top) and emission measure (bottom) in the Perseus cluster. Colored symbols represent different arm directions.