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Consider an isothermal galaxy in which a significant fraction fc of cooled gas remains as cold dusty clouds instead of rapidly forming stars. At the centre a black hole grows by accretion from the surrounding cold (and hot) gas. Assume that the nucleus also blows a wind of velocity vw which has a power Lw = alpha LEdd. Eventually the wind becomes powerful enough to blow away the surrounding gas and so shut off the accretion and further growth to the black hole and spheroid. The Magorrian et al (1998) black-hole - spheroid mass relation can then be obtained (Silk & Ress 1998; Fabian 1999; Blandford 1999).

The kinetic power of a wind at which it ejects cold gas of column density NH from a spheroid is given by

Equation 7 (7)


Equation 8 (8)

where sigma is the velocity dispersion within the spheroid. (I have used a force argument here, see Fabian 1999; Silk & Rees 1998 use an energy argument to obtain a limit of sigma5 / G, which is a factor sigma / vw smaller than the above Lw.) Ejection occurs when

Equation 9 (9)

Using the Faber-Jackson relation for spheroids (Msph propto sigma4) then yields, if vw/c fc/ alpha ~ 1

Equation 10 (10)

close to the Magorrian et al (1998) relation.

At that point the column density in to the accretion radius NH ~ NT = sigmaT-1, so the growth is (just) Compton thick. The growth of massive black holes is radiatively efficient, highly obscured and gives rise to much of the XRB. It is also intimately linked with the growth of galaxy spheroids, the main evolution of which is terminated by a quasar wind. X-ray observations probe best the underlying obscured nucleus at (rest frame) ebergies of about 30 keV. Indeed X-rays are the best diagnostic of the black hole accretion history of the Universe.

The optically bright quasar phase (from an outside observer's point of view) follows over the next few million years as the accretion disc around the black hole empties. The early phase as the wind clears the gas away can be identified with BAL quasars. The central engine is only revived after the quasar phase if a merger or other event brings in sufficient low angular momentum gas to fuel it.

Figure 3

Figure 3. The observed 2-10 keV flux as a function of redshift from a source of intrinsic (unabsorbed) 2-10 keV luminosity of 1045 erg s-1 with a column density of 1024.5 cm-2. Scattering fractions (by thin ionized gas) of 5, 1, 0.1 and 0.01 per cent are included (top to bottom). Note that the negatice K-correction means that sources at z ~ 0.1, 0.8 and 7 can have the same observed 2-10 keV flux. From Wilman & Fabian (1999).

The prospects of testing the above scenario and absorption models of the XRB are close at hand, with Chandra and XMM. They should detect large numbers of faint, but powerful absorbed sources in the 3-10 keV band, due to the negative K correction involved (see Fig. 3) and identify them with luminous FIR/sub-mm-emitting young galaxy spheroids.


I am grateful to Kazushi Iwasawa, Paul Nulsen and Richard Wilman for continued collaboration and the organisers for an interesting conference. The Royal Society is thanked for support.

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