Title:

Do we understand the cosmological evolution of AGN?



Guenther Hasinger
MPE, Garching



Abstract:

The majority of the X-ray background (XRB) below 10 keV has been
resolved by ROSAT, Chandra and XMM-Newton deep surveys. However, a
significant uncertainty still exists about the normalization of the
XRB. I will show very recent results from the Swift BAT telescope,
which basically confirm the low HEAO-1 normalization of the XRB, which
in turn means that a higher fraction of the XRB is resolved. The
luminosity function of X-ray selected AGN clearly shows a
luminosity-dependent evolutiion, leading to an "anti-hierarchical" or
"downsizing" behaviour, where the space density of luminous QSOs peak
rather early (z>2) and that of lower luminosity Seyferts rather late
(z<1) in the cosmic history. Assuming appropriate bolometric
corrections, this evolution patterns holds for all wavelengths (hard
X-ray, soft X-ray, optical, MIR) which are sampled well enough. An
important ingredient into the bolometric correction is the fraction of
obscured AGN with luminosity and redshift, and in particular the
contribution of the elusive population of Compton-thick objects. I
will show new results on these two questions. The "anti-hierarchical"
evolution of AGN can only be understood in terms of two radically
different modes of feeding the AGN. I will show that the violent
merger conjecture probably only applies to the early luminous QSO
evolution, while the general lower luminosity AGN must be fed through
a slower, more gentle feeding mechanism. Recent results point to the
direction that this AGN feeding mode may be connected to the
"re-building" of disks around bulges containg pre-fabricated black
holes.  The bulk of modern AGN appear in the so called "green valley",
which I interprete as objects coming back down from the red sequence
through the slow accretion of fresh gas.