Annu. Rev. Astron. Astrophys. 1994. 32: 277-318
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2.3 Summaty of X-Ray Data

The overwhelming evidence of the images and spectra shows that cooling does occur at a steady rate over long times (at least several billion years). Since gas is then cooling out of the hot phase at rates of hundreds of solar masses per year, an inflow must occur. We do not expect direct evidence of any inward flow since the velocity is highly subsonic over most of its volume at v0 approx 6 Mdot21/3 t10-1/3 T8-1/6 km s-1, where Mdot = 100 Mdot2 Msun yr-1, t10 is the cooling time ta at the edge of the flow in units of 1010 yr, and T = 108 T8 K.

The consistency between estimates of Mdot derived from spectral measurements and those derived from analysis of the surface brightness is shown in Figure 6. Agreement between these two methods gives further support for the existence of cooling flows. The spectral estimate uses emission lines and blends as a measure of the rate at which matter is cooling through a given temperature, whereas the estimate from the image is based on the rate with which matter must be cooling given its apparent density and temperature profiles.

Figure 6

Figure 6. Total mass deposition rate obtained from fits to X-ray spectra (Mdotspectral) compared to those from deprojections of the X-ray images (Mdotimage), from White et al (1991).

The X-ray data also show that the cooling-flow emission is absorbed, providing evidence for widespread cooled gas. The inferred mass of the absorbing matter is comparable to that expected from a persistent cooling flow.

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