The X-ray Background with Chandra: 1Msec observation of the
Chandra Deep Field South
Paolo Tozzi, Osservatorio Astronomico di Trieste, Italy
We present the main results from our 940 ksec observation of the
Chandra Deep Field South (CDFS). We extend the measurement of source
number counts to 5.5e-17 erg/cm^2/s in the soft 0.5-2 keV band and
4.5e-16 erg/cm^2/s in the hard 2-10 keV band. The hard band LogN-LogS
shows a significant flattening (slope~=0.6) below ~1e-14 erg/cm^2/s,
leaving at most 10-15% of the X-ray background (XRB) to be
resolved. This indicates that there is still a non negligible
population of faint hard sources to be discovered at energies not well
probed by Chandra. We use optical redshifts and identifications,
obtained with the VLT, for one quarter of the sample to characterize
the combined optical and X-ray properties of the sample. Different
source types are well separated in a parameter space which includes
X-ray luminosity, hardness ratio and R-K color. Type II objects, while
redder on average than the field population, have colors which are
consistent with being hosted by a range of galaxy types. Type II AGN
are mostly found at z<~1, in contrast with predictions based on AGN
population synthesis models, thus suggesting a revision of their
evolutionary parameters.