Cooling cores were first discovered by Fabian and Nulsen (1977) and Cowie and Binney (1977), who argued that the high X-ray luminosity of bright X-ray cores of some clusters suggested that, in the absence of significant energy input, the gas cooling times were less than the ages of the clusters. The gas must then cool and flow inward toward the center of the cD galaxy. For detailed historical reviews of cooling in cluster cores see Fabian, Nulsen, and Canizares (1991) and Fabian (1994).

Cluster bservations implied that some cooling rates exceeded 100 solar masses per year. Although some star formation was detected in clusters with cooling cores and cool CO was detected, despite careful searches, the ultimate repository of the large amounts of cooling gas remained a mystery. Also, theoretical efforts to understand possible energy sources that could reheat the cooling gas were not successful.

With the launch of Chandra and XMM-Newton, observations showed that in fact the gas was cooling but appeared to stabilize at a temperature of about 1/3 of the cluster mean ( Peterson et al. 2003 ). Some source of energy was providing energy to the cooling gas. Over the subsequent years, studies with XMM-Newton and Chandra have shown that the energy source is almost certainly the supermassive black holes at the centers of bulge dominated (especially elliptical) galaxies (e.g., Churazov et al. 2002 and papers listed below).

At the same time that it was realized that AGN were providing energy to cooling cores, models of galaxy evolution tied to large numerical simulations (e.g., Croton et al. 2004; see also see also talk by Croton) showed that the same kind of AGN feedback could solve several puzzles of galaxy evolution: the exponential cut-off at the bright end of the galaxy luminosity function, that the most massive galaxies tend to be bulge-dominated, and that these same galaxies tend to be ``red and dead'' i.e., composed of older stellar populations compared to disk galaxies. Thus, AGN outbursts, best studied through X-ray observations of cooling cores, are central to our understanding galaxy evolution.

M87 is the second brightest extragalactic X-ray source and lies at the core of the nearby Virgo cluster. It is a classic cooling core system (e.g., Fabian 2004 ). With its well-studied supermassive black hole ( Macchetto et al. 1997) and jet ( Sparks et al. 1996 , Perlman et al. 2001 , Marshall al. 2002 ), its detailed radio observations showing multiple episodes of activity ( Owen, Eilek, & Kassim 2000), M87 is the best galaxy for studying the interaction between AGN outbursts and their effects on the surrounding gaseous atmospheres.

M87 was the first, and remains the only, galaxy where the X-ray observations have sufficient precision to accurately measure the properties of AGN-driven shocks. Chandra observations of M87 ( Forman et al. 2005 and Forman et al. 2007 ) show:

• a classical shock. The gas density and temperature jumps independently yield the same Mach number, M =1.2
• the AGN outbursts are "slow" and last 1-3 million years. A simple model applied to M87 shows the shock properties can be explained by a 1-3 million year long outburst of total energy 4-6x10⁵⁷ ergs that began 12 million years ago. Hence, the absence of large amounts of hot, shocked plasma in nearly all hot atmospheres in galaxies, groups, and clusters is consistent with the idea that most outbursts do not drive very strong shocks.
• at the present time, 50% of the outburst energy lies in the central cocoon of relativisitic plasma (seen in the radio), 25% of the energy resides in gas that was mildly heated by the passage of the shock, and 25% of the outburst energy lies in kinetic energy of the shock.
• the AGN energy output is dominated by mechanical (not radiative) power; the energy required to drive the observed shock is 4-6x10⁵⁷ ergs with comparable outbursts roughly every 10 million years. The time averaged AGN output of a few 10⁴³ ergs/sec closely matches the energy radiated by the gas in the cooling core.

Additional References for Detailed Reading on Cooling Cores and AGN Outbursts
UNDER CONSTRUCTION