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 <-22.15).
Our estimated integral surface density at IAB < 24.0 is 500 AGN per square degree,
which represents the highest surface density of a spectroscopically confirmed sample of
optically selected AGN.
We derived the luminosity function in B-band for 1.0 < z < 3.6 using the 1/Vmax
estimator. Our data, more than one magnitude fainter than previous optical surveys, allow
us to constrain the faint part of the luminosity function up to high redshift. A comparison
of our data with the 2dF sample at low redshift (1 < z < 2.1) shows that the VDDS
data can not be well fitted with the pure luminosity evolution (PLE) models derived by
previous optically selected samples. Qualitatively, this appears to be due to the fact that
our data suggest the presence of an excess of faint objects at low redshift (1.0 < z < 1.5)
with respect to these models.
By combining our faint VVDS sample with the large sample of bright AGN extracted
from the SDSS DR3 (Richards et al., 2006b) and testing a number of different
evolutionary models, we find that the model which better represents the combined
luminosity functions, over a wide range of redshift and luminosity, is a luminosity
dependent density evolution (LDDE) model, similar to those derived from the major Xsurveys.
Such a parameterization allows the redshift of the AGN density peak to change
as a function of luminosity, thus fitting the excess of faint AGN that we find at 1.0 < z <
1.5.
On the basis of this model we find, for the first time from the analysis of optically
selected samples, that the peak of the AGN space density shifts significantly towards
lower redshift going to lower luminosity objects. The position of this peak moves from
z 2.0 for MB <-26.0 to z 0.65 for -22< MB <-20.
This result, already found in a number of X-ray selected samples of AGN, is consistent
with a scenario of “AGN cosmic downsizing”, in which the density of more luminous
AGN, possibly associated to more massive black holes, peaks earlier in the history of
the Universe (i.e. at higher redshift), than that of low luminosity ones, which reaches its
maximum later (i.e. at lower redshift).
This behavior has since long been claimed to be present in elliptical galaxies and it is not
easy to reproduce it in the hierarchical cosmogonic scenario, where more massive Dark
Matter Halos (DMH) form on average later by merging of less massive halos.
| Identifer | oai:union.ndltd.org:unibo.it/oai:amsdottorato.cib.unibo.it:345 |
| Date | 11 April 2007 |
| Creators | Bongiorno, Angela <1980> |
| Contributors | Marano, Bruno |
| Publisher | Alma Mater Studiorum - Università di Bologna |
| Source Sets | Università di Bologna |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, PeerReviewed |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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