Annu. Rev. Astron. Astrophys. 2003. 41: 191-239
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1. INTRODUCTION

Recent Chandra and XMM X-ray observations of the hot gas in elliptical galaxies and galaxy clusters have radically upset the traditional concept of cooling flows. On the other hand, these same observations may reveal new ways of understanding the origin and evolution of this gas. The most unexpected new discovery has been the absence in XMM spectra of emission from gas at intermediate or low temperatures, implying that the cooling gas is somehow hidden from view or that the cooling rate is much less than previously thought. Chandra X-ray images show that the hot gas deep inside E and cD galaxies near the centers of cooling flows is often highly disturbed. Evidently, massive black holes, thought to inhabit the cores of all stellar bulges, may become energy sources when stimulated by inflowing hot gas.

Can the central heating visible in Chandra images explain the absence of cooling in XMM spectra? This is one of many fundamental cooling flow questions that we review here. There is also a problem with terminology. Since the spectral evidence for cooling has weakened, the "cooling flow" concept may no longer be appropriate. Nevertheless, in keeping with accepted usage we occasionally use "cooling flow" when "galactic flow" might be preferred. In any case, the hot gas in and around elliptical galaxies is of particular astronomical interest because its origin and metal enrichment are closely entwined with the merger origin of central elliptical (E) galaxies in galaxy groups. A better understanding of the gas is likely to clarify baryonic structure formation in general. Fortunately, even with our currently incomplete understanding of its origin and evolution, the hot gas can contribute important information about the stellar kinematics and mass to light ratios in elliptical galaxies.

This review is necessarily limited to elliptical galaxies of moderate or high luminosity. X-ray emission from hot gas in low luminosity E galaxies is difficult to observe since it is masked by X-rays from stellar sources. We also emphasize whenever possible those X-ray ellipticals that are relatively undisturbed by recent mergers, stripping interactions with hot cluster gas or powerful radio sources. Although the X-ray properties of S0 galaxies can resemble those of E galaxies, we restrict our discussion to E galaxies owing to the uniformity of their optical structures and their less controversial origin.

The evolution and physical condition of the hot gas in rich clusters of galaxies and in individual E galaxies are similar in many ways. But there are some important distinctions. Because smaller structures are often older in our hierarchical universe, the hot gas in elliptical galaxies and their associated galaxy groups may on average be less disturbed by ongoing mergers than their younger, more massive cluster counterparts. X-ray bright E galaxies that are rather isolated and, one hopes, undisturbed may contain in their metallicity gradient unique information about star and galaxy formation in the distant past. This vital information is lost when such groups merge and mix to form rich clusters. Another key attribute of hot gas on galactic scales is the possible importance of stellar mass loss and continued Type Ia supernova activity in supplying and enriching the hot gas. Finally, the properties of the hot gas can be determined with more confidence on galactic scales where the gravitational potential is better known from the stellar light.

Notwithstanding their astrophysical importance, proximity and relative ease of interpretation, far less Chandra and XMM data are available for E galaxies and their groups than for galaxy clusters. Both telescopes were launched in 1999. As we write this review in the Fall of 2002 no XMM spectrum of a bright, relatively isolated, group-centered elliptical galaxy has appeared in the standard journals, but we expect this data to appear very soon.

Our review is not intended to be a comprehensive review of all the literature that has appeared since the excellent previous cooling flow reviews by Sarazin (1986), Fabbiano (1989), Sarazin (1990) and Fabian (1994). This is due in part to the rapid increase in the quality of the observations which tend to eclipse previous studies only a few years older. Our primary emphasis is to compare observations with theoretical expectations and to bring attention to the frequent dissonance between the two. This approach is complementary to the recent ARAA review of the X-ray properties of galaxy groups by Mulchaey (2000) who presents a more complete discussion of the observations.

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