Annu. Rev. Astron. Astrophys. 2003. 41: 191-239 Copyright © 2003 by Annual Reviews. All rights reserved

9. ROTATION AND MAGNETIC FIELDS

If the classical cooling inflow hypothesis is correct, both rotation and magnetic fields should be amplified near the center of the flow. Mass and angular momentum conservation in the hot gas within slowly rotating elliptical galaxies ultimately results in extended, massive disks of rotationally supported cold gas in the equatorial plane. As the hot interstellar gas approaches the disks, its density and thermal X-ray emission increase, resulting in X-ray images that are considerably flattened toward the equatorial plane out to an effective radius or beyond (Kley & Mathews 1995; Brighenti & Mathews 1996; 1997b). Such X-ray disks have never been observed (e.g. Hanlan & Bregman 2000). This problem can be avoided if the gas is flowing outward, but we have seen that this requires that the heating be fine tuned. Alternatively, cooling inflows that rotate can have nearly circular X-ray images (1) if the mass of hot gas is depleted by spatially extended (multiphase) mass dropout or, in view of XMM observations to the contrary, (2) if angular momentum is transported outward by subsonic turbulent diffusion (Brighenti & Mathews 2000). If the random velocities of the diffuse H + N[II] lines, ~ 150 km s^-1, are also present in the hot gas, they would be sufficient for this diffusion. Type Ia supernova rates of interest can maintain the centrally observed iron abundance gradient even in the presence of interstellar turbulence sufficient to circularize the X-ray image (Brighenti & Mathews 2000, Brüggen 2002).

Similarly, seed magnetic fields contained in the ejected envelopes of evolving stars or in relic radio sources can be amplified by the same interstellar turbulence. Ancient fields established by turbulent dynamos during the merging epoch of the E galaxy group could still be present in the hot gas. Further amplification occurs as these fields are compressed by an inward flow of hot gas (Soker & Sarazin 1990; Moss & Shukurov 1996; Christodoulou & Sarazin 1996; Mathews & Brighenti 1997; Godon, Soker & White 1998). Field stresses of ~ 50 µG, as observed in M87 (e.g. Owen, Eilek & Keel 1990), formed in this way (without a central AGN) can compete with the thermal pressure in the hot gas and influence the observed gas density gradient. The generic incompatibility of magnetic fields and spherical gravitating atmospheres may give rise to some of the central X-ray surface brightness irregularities observed. Although potentially very important, studies of magnetic fields in cooling flows have been limited by the lack of observations in normal (weak radio) E galaxies and by the theoretical and numerical complexities of field reconnection.