Annu. Rev. Astron. Astrophys. 1994. 32: 277-318 Copyright © 1994 by Annual Reviews. All rights reserved

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 6 ₂^1/3 t₁₀^-1/3 T₈^-1/6 km s^-1, where = 100 ₂ M yr^-1, t₁₀ is the cooling time t_a at the edge of the flow in units of 10¹⁰ yr, and T = 10⁸ T₈ K.

The consistency between estimates of 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. Total mass deposition rate obtained from fits to X-ray spectra (spectral) compared to those from deprojections of the X-ray images (image), from White et al (1991).