SUZAKUOBSERVATIONS OF THE OUTSKIRTS OF A1835: DEVIATION FROM HYDROSTATIC EQUILIBRIUM

Abstract

We present results of four-pointing Suzaku X-ray observations (total ∼200 ks) of the intracluster medium (ICM) in the Abell 1835 galaxy cluster (kT ∼ 8 keV, z = 0.253) out to the virial radius (r vir ∼ 2.9 Mpc) and beyond. Faint X-ray emission from the ICM out to r vir is detected. The temperature gradually decreases with radius from ∼8 keV in the inner region to ∼2 keV at r vir . The entropy profile is shown to flatten beyond r 500 , in disagreement with the r 1.1 dependence predicted from the accretion shock heating model. The thermal pressure profile in the range 0.3r 500 < ∼ r < ∼ r vir agrees well with that obtained from the stacked Sunyaev-Zel'dovich effect observations with the Planck satellite. The hydrostatic mass profile in the cluster outskirts (r 500 < ∼ r < ∼ r vir ) falls well short of the weak lensing one derived from Subaru/Suprime-Cam observations, showing an unphysical decrease with radius. The gas mass fraction at r vir defined with the lensing total mass agrees with the cosmic baryon fraction from the WMAP 7-year data. All these results indicate, rather than the gas-clumping effect, that the bulk of the ICM in the cluster outskirts is far from hydrostatic equilibrium and infalling matter retained some of its kinetic energy. Finally, combining with our recent Suzaku and lensing analysis of Abell 1689, a cluster of similar mass, temperature, and redshift, we show that the cluster temperature distribution in the outskirts is significantly correlated with the galaxy density field in the surrounding large-scale environment at (1-2)r vir .

Suzaku X-Ray Observations of the Nearest Non-Cool Core Cluster, Antlia: Dynamically Young but With Remarkably Relaxed Outskirts

“…Mass determination using weak lensing has provided important insights to the origins of the gas fraction bias and entropy deficit in some massive clusters (Kawaharada et al 2010;Ichikawa et al 2013;Okabe et al 2014). For Hydra A, Abell 1689, Abell 1835, and Abell 478, where the first three are in the sample of Walker et al (2013), the HSE masses determined from X-ray are significantly smaller than the weak lensing masses near R 200 .…”

“…Okabe et al (2014) also argue that the entropy deficit near R 200 is primarily due to the steepening in temperature measurements, rather than a shallower density slope. Thus, the joint weak lensing and X-ray studies suggest that the breakdown of HSE is more important than clumping in cluster outskirts (Ichikawa et al 2013). Additional pressure support from turbulence or bulk motions (e.g., Lau et al 2009), higher ion temperature due to non-equipartition, and cosmic rays (e.g., Laganá et al 2010;Vazza et al 2012) may be responsible for the non-HSE effects.…”

“…breakdown of the hydrostatic equilibrium (HSE) approximation if only the thermal pressure is considered (e.g., Kawaharada et al 2010;Ichikawa et al 2013;Okabe et al 2014). More than a dozen clusters have been studied with spatially and spectrally resolved gas profiles out to ∼R 200 to understand these anomalous effects (see again the review by Reiprich et al 2013).…”

The XMM cluster outskirts project (X‐COP)

“…More recently, several works reported a deficit of entropy in massive clusters around the virial radius (see Reiprich et al 2013, for a review), which has been interpreted as a lack of thermalization of the ICM induced, e.g., by an incomplete virialization of the gas (e.g. Bonamente et al 2013;Ichikawa et al 2013;Kawaharada et al 2010), non-equilibration between electrons and ions (Hoshino et al 2010), non-equilibrium ionization (e.g. Fujita et al 2008), or weakening of the accretion shocks (Lapi et al 2010).…”

The Intragroup Versus the Intracluster Medium

“…If so, outer slope flattening will arise for r > r 500 ≈ R/2 (see Appendix A); there, a is prone to taper off or even bend down, as indeed it is observed in several instances (e.g., Kawaharada et al 2010;Bonamente et al 2013;Walker et al 2012;Ichikawa et al 2013). …”