The VIMOS-VLT Deep Survey: the evolution of type-1 AGN
Creators: name: Bongiorno, Angela <1980>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2007
Subjects: FIS/05 Astronomia e astrofisica

description: Quasars and AGN play an important role in many aspects of the modern cosmology.
Of particular interest is the issue of the interplay between AGN activity and formation
and evolution of galaxies and structures. Studies on nearby galaxies revealed that most
(and possibly all) galaxy nuclei contain a super-massive black hole (SMBH) and that
between a third and half of them are showing some evidence of activity (Kormendy and
Richstone, 1995). The discovery of a tight relation between black holes mass and velocity
dispersion of their host galaxy suggests that the evolution of the growth of SMBH and
their host galaxy are linked together. In this context, studying the evolution of AGN,
through the luminosity function (LF), is fundamental to constrain the theories of galaxy
and SMBH formation and evolution. Recently, many theories have been developed to
describe physical processes possibly responsible of a common formation scenario for
galaxies and their central black hole (Volonteri et al., 2003; Springel et al., 2005a; Vittorini
et al., 2005; Hopkins et al., 2006a) and an increasing number of observations in different
bands are focused on collecting larger and larger quasar samples. Many issues remain
however not yet fully understood.
In the context of the VVDS (VIMOS-VLT Deep Survey), we collected and studied
an unbiased sample of spectroscopically selected faint type-1 AGN with a unique and
straightforward selection function. Indeed, the VVDS is a large, purely magnitude
limited spectroscopic survey of faint objects, free of any morphological and/or color preselection.
We studied the statistical properties of this sample and its evolution up to
redshift z  4.
Because of the contamination of the AGN light by their host galaxies at the faint
magnitudes explored by our sample, we observed that a significant fraction of AGN in
our sample would be missed by the UV excess and morphological criteria usually adopted
for the pre-selection of optical QSO candidates. If not properly taken into account, this
failure in selecting particular sub-classes of AGN could, in principle, affect some of the
conclusions drawn from samples of AGN based on these selection criteria.
The absence of any pre-selection in the VVDS leads us to have a very complete sample of
AGN, including also objects with unusual colors and continuum shape. The VVDS AGN
sample shows in fact redder colors than those expected by comparing it, for example,
with the color track derived from the SDSS composite spectrum. In particular, the faintest
objects have on average redder colors than the brightest ones. This can be attributed to
both a large fraction of dust-reddened objects and a significant contamination from the
host galaxy. We have tested these possibilities by examining the global spectral energy
distribution of each object using, in addition to the U, B, V, R and I-band magnitudes,
also the UV-Galex and the IR-Spitzer bands, and fitting it with a combination of AGN
and galaxy emission, allowing also for the possibility of extinction of the AGN flux.
We found that for 44% of our objects the contamination from the host galaxy is not
negligible and this fraction decreases to 21% if we restrict the analysis to a bright subsample
(M1450