Annu. Rev. Astron. Astrophys. 2000. 38: 289-335 Copyright © 2000 by Annual Reviews. All rights reserved

5.7. Cooling Flows

Galaxy groups display many of the signatures of cooling flows that have previously been observed in rich clusters and elliptical galaxies (Fabian 1994). The surface brightness profiles of the X-ray emission are sharply peaked, indicating that the gas density is rising rapidly towards the center of the group. In addition, at least half of all groups with measured temperature profiles show direct evidence for cooler gas in the central regions (Ponman & Bertram 1993; David et al 1994; Trinchieri et al 1997; Mulchaey & Zabludoff 1998; Helsdon & Ponman 2000). In some cases, the central gas is cooler than the mean gas temperature by nearly 50%. Cooling flow models also appear to provide a better fit to the ASCA spectra of groups than an isothermal plasma model (Buote 2000a). While these observations are consistent with the cooling flow interpretation, there are other possibilities. For example, Mulchaey & Zabludoff (1998) noted that the above features could also be explained if there is a distinct X-ray component associated with the central elliptical galaxy.

Perhaps the strongest case for a cooling flow in a low-mass system is the NGC 5044 group. David et al (1994) obtained a very deep ROSAT PSPC observation of this system that allowed the construction of a detailed temperature profile. They found evidence for a cooling flow with an essentially constant mass accretion rate from approximately 20 h^-1₁₀₀ kpc out to the cooling radius (~ 50-75 h^-1₁₀₀ kpc). This suggests a nearly homogeneous cooling flow. In contrast, the cooling flows in rich clusters tend to be inhomogeneous; a significant amount of the gas cools out at large radii (cf Fabian 1994). David et al (1994) suggest that gravitational heating is more important in the NGC 5044 group than in clusters because in groups the temperature of the hot gas is comparable to the virial temperature of the central galaxy, whereas for rich clusters the gas temperature is significantly higher. Therefore, most of the observed X-ray emission in the cooling flow region can be provided by the gravitational energy in groups, whereas mass deposition dominates in rich clusters.