ABELL-2163 - TEMPERATURE, MASS, AND HYDROSTATIC EQUILIBRIUM

Using ASCA data, we have measured the electron temperature in Abell 21 63 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(-1.2)(+1.9) keV, 11.5(-2.9)(+2.7) keV, and 3.8(-0.9)(+1.11) keV in the 0-3a(X) (0-3'.5), 3-6a(X) and 6-13a(X ) spherical shells, respectively. [Errors are 90% throughout the paper unless otherwise stated, and h = H-0(100 km s(-1) Mpc(-1))(-1).] Form ally applying the hydrostatic equilibrium and spherical symmetry assum ptions and using these data together with the Ginga spectral and the R OSA T imaging data, we were able to severely limit the possible bindin g mass distribution of the generic form rho = rho(0)(1 + r(2)/a(b)(2)) -n/2. All the allowed binding mass profiles are steeper than the gas d ensity profiles, and mass profiles with the same slope as gas are excl uded at a greater than 99% confidence. The total mass inside 0.5 h(-1) Mpc is 4.3 +/- 0.5 x 10(14) h(-1) M., of which 0.074h(-3/2) is gas, w hile inside 1.5h(-1) Mpc, the mass is 1.07 +/- 0.13 x 10(15) h(-1) M.. The strongest constraint on the mass profile is the observed quick dr op of the temperature at large radii, which can be reconciled only mar ginally 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 r ecent merger, the timescale for reaching electron-ion temperature equa lity via collisions is comparable to the merger timescale, so the meas ured electron temperature may give an underestimate of the gas pressur e there. Otherwise, if our low value is indeed representative of the g as temperature in the outer shell, the cluster atmosphere should be co nvectionally unstable, and gas turbulence should exist. Bulk motions o f the gas are also expected during the merger. Their existence would i ncrease the total gas pressure above that indicated by the observed te mperature. Thus, failure of the model in which dark matter and gas hav e the same distribution at the radii of interest, which is favored by hydrodynamic simulations, may be due to the neglect of these phenomena , which leads to an underestimate of the total density and an overesti mate of the baryonic fraction at large radii. The mass estimate at the smaller radius, where there is no evidence of departing from equilibr ium, is likely to be correct. Our measured electron temperatures, comb ined with the previously reported Sunyaev-Zeldovich decrement toward t his cluster and the ROSAT gas density profile, under the assumption of spherical symmetry, are consistent with a Hubble constant between 42 and 110 km s(-1) Mpc(-1) (68% interval), where the uncertainty is domi nated by that of the available SZ measurement.