Abell 2163: Temperature, Mass, and Hydrostatic Equilibrium

Abstract

Using ASCA data, we have measured the electron temperature in Abell 2163 out to 1.5 h −1 Mpc (3/4 of the virial radius, or 10a x , where a x is the X-ray core-radius) from the center, in three radial bins. The obtained temperatures are 12.2 +1.9 −1.2 keV, 11.5 +2.7 −2.9 keV and 3.8 +1.1 −0.9 keV in the 0-3 a x (0-3.5 ′ ), 3-6 a x and 6-13 a x spherical shells, respectively 5 . Formally applying the hydrostatic equilibrium and spherical symmetry assumptions and using these data together with the Ginga spectral and the Rosat imaging data, we were able to severely limit the possible binding mass distribution of the generic form ρ = ρ 0 (1 + r 2 /a 2 b ) −n/2 . All the allowed binding mass profiles are steeper than the gas density profiles and mass profiles with the same slope as gas are excluded at a greater than 99% confidence. The total mass inside 0.The strongest constraint on the mass profile is the observed quick drop of the temperature at large radii, which can only marginally be reconciled with the Rosat detection of gas at an even greater radius. We note that in the outer part of this cluster, which is likely to be a recent merger, the timescale for reaching electron-ion temperature equality via collisions is comparable to the merger timescale, so the measured electron temperature may give an underestimate of the gas pressure there. Otherwise, if our low value is indeed representative of the gas temperature in the outer shell, the cluster atmosphere should be convectionally unstable and gas turbulence should exist. Bulk motions of the gas are also expected during the merger. Their existense would increase the total gas pressure above that indicated by the observed temperature. Thus, failure of the model in which dark matter and gas have the same distribution at the radii of interest, which is favored by hydrodynamic simulations, may be due to the neglect of these phenomena, leading to an underestimate of the total density and overestimate of the baryonic fraction at large radii. The mass estimate at the smaller radius, where there is no evidence of departing from equilibrium, is likely to be correct.Our measured electron temperatures, combined with the previously reported Sunyaev-Zel'dovich decrement toward this cluster and the Rosat gas density profile, under the assumption of spherical symmetry, are consistent with a 5 Errors are 90% throughout the paper unless otherwise stated, and h ≡ H 0 /100 km s −1 Mpc −1 .-3 -Hubble constant between 42-110 km s −1 Mpc −1 (68% interval), where the uncertainty is dominated by that of the available SZ measurement.

Outskirts of Galaxy Clusters

“…However, in cluster outskirts, there are some indications that density profiles may flatten (Section 4.1.1), temperature profiles may steepen (Section 4.1.2), and entropy profiles may turn over (Section 4.1.4). For instance, convectional instability was considered early on for A2163 by Markevitch et al (1996). In the presence of magnetic fields, however, the situation may be more complex due to other possible instabilities, in particular the magnetothermal instability (MTI, Balbus 2000) and the heat-flux-driven buoyancy instability (HBI, Quataert 2008).…”

A2163: Merger events in the hottest Abell galaxy cluster

“…Elbaz et al (1995) and Markevitch et al (1996), performed assuming different temperature gradients in the cluster outskirts. It is also interesting to note the discrepancy between X-ray measurements of the gas temperature and the indirect weak lensing temperature estimate of Cypriano et al (2004), kT SIS = 6.63 +2.03 −1.76 keV, obtained assuming energy equipartition between galaxies and gas.…”

Suzakumeasurement of Abell 2204's intracluster gas temperature profile out to 1800 kpc

“…X-rays are also a unique tool to study the physics of the hot cluster gas; e.g., gas temperature profiles allow constraints on (the suppression of) heat conduction. Consequently, constraining cluster temperature profiles has been the subject of many (partially contradictory) works in the recent past (e.g., Markevitch et al 1996;Fukazawa 1997;Markevitch et al 1998;Irwin et al 1999;White 2000;Irwin & Bregman 2000;Allen et al 2001;De Grandi & Molendi 2002;Zhang et al 2004;Vikhlinin et al 2005;Arnaud et al 2005;Kotov & Vikhlinin 2005;Piffaretti et al 2005;Hudson & Reiprich 2007;Pratt et al 2007;Snowden et al 2008;Leccardi & Molendi 2008).…”