29 March 2001CfA Colloquium

Title: Chandra observations of galaxy clusters: when arcsecond resolution really counts

Speaker: Alexey Vikhlinin

Abstract: I will describe Chandra observations of several clusters of galaxies and show that Chandra's arcsecond angular resolution enables studies of the physical processes in the intracluster medium in an unprecedented detail.

In Abell 3667, we observe the brightness edge which turns out to be a boundary of a large cool gas cloud moving through the hot ambient gas. The cloud velocity is accurately measured from the ratio of the interior and exterior pressures. We find M=1+-0.2 or v=1400+-300 km/s and observe two additional features expected for a slightly supersonic velocity --- a bow shock and gas compression at the leading edge of the cloud.

The "cold front" is remarkably sharp. The upper limit on its width, 3.5" or 5 kpc, is several times smaller than the Coulomb mean free path. Thus, we directly observe a suppression of the transport processes in the intracluster medium, most likely by magnetic fields. We further use the apparent dynamical stability of the cold front to measure the magnetic field strength. The estimated magnetic pressure is 10-20% of the thermal gas pressure in the region where the magnetic field is expected to be amplified. Therefore, magnetic fields are unimportant for the dynamics of the intracluster medium.

In the Coma cluster, we observe that both its dominant galaxies parts of their X-ray gas coronae. The interstellar gas with a temperature of 1-2 keV is confined by the hot intergalactic medium of the Coma cluster into compact clouds (only 3 kpc in radius) containing 10**8 Msun of gas. The physical state of the gas in these clouds appears to be determined by a delicate balance between the radiative cooling and suppressed (by a factor of 30--100) heat conduction through the interface between these clouds and the hot cluster gas.

Reference for students:

"Chandra Observation of Abell 2142: Survival of Dense Subcluster Cores in a Merger" Markevitch et al., 2000, astro-ph/0001269

"Chandra estimate of the magnetic field strength near the cold front in A3667" Vikhlinin, A., Markevitch, M. & Murray, S. 2000, astro-ph/0008499

"Zooming in on the Coma Cluster with Chandra: Compressed Warm Gas in the Brightest Cluster Galaxies" Vikhlinin, A. Markevitch, M., forman, W. & Jones, C. 2001, astro-ph/0102483

Lunch with the students will be on Friday, March 30th at 12:00 in A-101.