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Date and Time of the Query: 2019-06-24 T18:38:12 PDT
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For refcode 1988A&A...194...79S:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1988A&A...194...79S Extensive gaseous haloes surrounding giant elliptical galaxies: evidence from depolarization in radio galaxies R.G. Strom, and W.J. Jagers Netherlands Foundation for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands Sterrewacht, Postbus 9513, 2300 RA Leiden, The Netherlands Received July 6, accepted October 8, 1987 Summary. Radio polarization measurements have been used to investigate large scale gaseous components associated with some thirteen double radio sources. At 49 cm a significant proportion of the bridge emission, roughly centered on the parent galaxy, is invariably found to be unpolarized. We present evidence that this lack of polarization at long wavelengths is the result of differential Faraday rotation in a large scale halo associated with the central (usually elliptical) galaxy. The haloes, which extend beyond 100 kpc, appear to be the outer envelopes of hot gas such as that observed in the form of extended X-ray emission associated with a number of nearby early-type galaxies. The depolarization observed in the two largest sources indicates decreasing gas density with increasing radius, with a typical value of 3 10^-5^ cm^-3^ at 100 kpc. Combining radio and X-ray density estimates, we find that the behavior is consistent with gas in hydrostatic equilibrium trapped in the gravitational potential well of a central galaxy. The data imply total masses for these galaxies of several times 10^12^ M_sun_, while the mass detected in the haloes is but a fraction of this. Only for radii exceeding 1 Mpc would the halo mass approach that of a giant elliptical galaxy. There is no evidence for substantial amounts of additional ("dark") matter mixed in with the gas detected by depolarization. The surface densities implied (4 10^19^ cm^-2^ at a radius of 200 kpc for z~0.1) are not unlike those found in the Ly{alpha} forest of absorption features in distant quasars, although other facts suggest that the two phenomena may not be related. Key words: coronae of elliptical galaxies - polarization - Faraday effect
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