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The VIMOS-VLT Deep Survey: the evolution of type-1 AGNBongiorno, Angela <1980> 11 April 2007 (has links)
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.
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Statistical properties of Radio Halos and the re-acceleration modelCassano, Rossella <1978> 11 April 2007 (has links)
Galaxy clusters occupy a special position in the cosmic hierarchy as they are the
largest bound structures in the Universe. There is now general agreement on a
hierarchical picture for the formation of cosmic structures, in which galaxy clusters
are supposed to form by accretion of matter and merging between smaller units.
During merger events, shocks are driven by the gravity of the dark matter in the
diffuse barionic component, which is heated up to the observed temperature.
Radio and hard-X ray observations have discovered non-thermal components
mixed with the thermal Intra Cluster Medium (ICM) and this is of great importance
as it calls for a “revision” of the physics of the ICM. The bulk of present information
comes from the radio observations which discovered an increasing number of Mpcsized
emissions from the ICM, Radio Halos (at the cluster center) and Radio Relics
(at the cluster periphery). These sources are due to synchrotron emission from
ultra relativistic electrons diffusing through µG turbulent magnetic fields. Radio
Halos are the most spectacular evidence of non-thermal components in the ICM
and understanding the origin and evolution of these sources represents one of the
most challenging goal of the theory of the ICM.
Cluster mergers are the most energetic events in the Universe and a fraction of
the energy dissipated during these mergers could be channelled into the amplification
of the magnetic fields and into the acceleration of high energy particles via
shocks and turbulence driven by these mergers. Present observations of Radio
Halos (and possibly of hard X-rays) can be best interpreted in terms of the reacceleration
scenario in which MHD turbulence injected during these cluster mergers
re-accelerates high energy particles in the ICM. The physics involved in this scenario
is very complex and model details are difficult to test, however this model clearly
predicts some simple properties of Radio Halos (and resulting IC emission in the hard
X-ray band) which are almost independent of the details of the adopted physics. In
particular in the re-acceleration scenario MHD turbulence is injected and dissipated
during cluster mergers and thus Radio Halos (and also the resulting hard X-ray
IC emission) should be transient phenomena (with a typical lifetime <» 1 Gyr)
associated with dynamically disturbed clusters. The physics of the re-acceleration
scenario should produce an unavoidable cut-off in the spectrum of the re-accelerated
electrons, which is due to the balance between turbulent acceleration and radiative
losses. The energy at which this cut-off occurs, and thus the maximum frequency
at which synchrotron radiation is produced, depends essentially on the efficiency of
the acceleration mechanism so that observations at high frequencies are expected
to catch only the most efficient phenomena while, in principle, low frequency radio
surveys may found these phenomena much common in the Universe.
These basic properties should leave an important imprint in the statistical
properties of Radio Halos (and of non-thermal phenomena in general) which,
however, have not been addressed yet by present modellings.
The main focus of this PhD thesis is to calculate, for the first time, the expected
statistics of Radio Halos in the context of the re-acceleration scenario. In particular,
we shall address the following main questions:
• Is it possible to model “self-consistently” the evolution of these sources together
with that of the parent clusters?
• How the occurrence of Radio Halos is expected to change with cluster mass
and to evolve with redshift? How the efficiency to catch Radio Halos in galaxy
clusters changes with the observing radio frequency?
• How many Radio Halos are expected to form in the Universe? At which redshift
is expected the bulk of these sources?
• Is it possible to reproduce in the re-acceleration scenario the observed
occurrence and number of Radio Halos in the Universe and the observed
correlations between thermal and non-thermal properties of galaxy clusters?
• Is it possible to constrain the magnetic field intensity and profile in galaxy
clusters and the energetic of turbulence in the ICM from the comparison
between model expectations and observations?
Several astrophysical ingredients are necessary to model the evolution and
statistical properties of Radio Halos in the context of re-acceleration model and
to address the points given above. For these reason we deserve some space in this
PhD thesis to review the important aspects of the physics of the ICM which are of
interest to catch our goals. In Chapt. 1 we discuss the physics of galaxy clusters,
and in particular, the clusters formation process; in Chapt. 2 we review the main
observational properties of non-thermal components in the ICM; and in Chapt. 3 we
focus on the physics of magnetic field and of particle acceleration in galaxy clusters.
As a relevant application, the theory of Alfv´enic particle acceleration is applied
in Chapt. 4 where we report the most important results from calculations we have
done in the framework of the re-acceleration scenario. In this Chapter we show that
a fraction of the energy of fluid turbulence driven in the ICM by the cluster mergers
can be channelled into the injection of Alfv´en waves at small scales and that these
waves can efficiently re-accelerate particles and trigger Radio Halos and hard X-ray
emission.
The main part of this PhD work, the calculation of the statistical properties
of Radio Halos and non-thermal phenomena as expected in the context of the
re-acceleration model and their comparison with observations, is presented in
Chapts.5, 6, 7 and 8.
In Chapt.5 we present a first approach to semi-analytical calculations of
statistical properties of giant Radio Halos. The main goal of this Chapter is to model
cluster formation, the injection of turbulence in the ICM and the resulting particle
acceleration process. We adopt the semi–analytic extended Press & Schechter (PS)
theory to follow the formation of a large synthetic population of galaxy clusters and
assume that during a merger a fraction of the PdV work done by the infalling
subclusters in passing through the most massive one is injected in the form of
magnetosonic waves. Then the processes of stochastic acceleration of the relativistic
electrons by these waves and the properties of the ensuing synchrotron (Radio Halos)
and inverse Compton (IC, hard X-ray) emission of merging clusters are computed
under the assumption of a constant rms average magnetic field strength in emitting
volume. The main finding of these calculations is that giant Radio Halos are
naturally expected only in the more massive clusters, and that the expected fraction
of clusters with Radio Halos is consistent with the observed one.
In Chapt. 6 we extend the previous calculations by including a scaling of the
magnetic field strength with cluster mass. The inclusion of this scaling allows us to
derive the expected correlations between the synchrotron radio power of Radio Halos
and the X-ray properties (T, LX) and mass of the hosting clusters. For the first
time, we show that these correlations, calculated in the context of the re-acceleration
model, are consistent with the observed ones for typical µG strengths of the average
B intensity in massive clusters. The calculations presented in this Chapter allow
us to derive the evolution of the probability to form Radio Halos as a function of
the cluster mass and redshift. The most relevant finding presented in this Chapter
is that the luminosity functions of giant Radio Halos at 1.4 GHz are expected to
peak around a radio power » 1024 W/Hz and to flatten (or cut-off) at lower radio
powers because of the decrease of the electron re-acceleration efficiency in smaller
galaxy clusters. In Chapt. 6 we also derive the expected number counts of Radio
Halos and compare them with available observations: we claim that » 100 Radio
Halos in the Universe can be observed at 1.4 GHz with deep surveys, while more
than 1000 Radio Halos are expected to be discovered in the next future by LOFAR
at 150 MHz. This is the first (and so far unique) model expectation for the number
counts of Radio Halos at lower frequency and allows to design future radio surveys.
Based on the results of Chapt. 6, in Chapt.7 we present a work in progress on
a “revision” of the occurrence of Radio Halos. We combine past results from the
NVSS radio survey (z » 0.05 − 0.2) with our ongoing GMRT Radio Halos Pointed
Observations of 50 X-ray luminous galaxy clusters (at z » 0.2−0.4) and discuss the
possibility to test our model expectations with the number counts of Radio Halos
at z » 0.05 − 0.4.
The most relevant limitation in the calculations presented in Chapt. 5 and 6 is
the assumption of an “averaged” size of Radio Halos independently of their radio
luminosity and of the mass of the parent clusters. This assumption cannot be
released in the context of the PS formalism used to describe the formation process
of clusters, while a more detailed analysis of the physics of cluster mergers and of
the injection process of turbulence in the ICM would require an approach based on
numerical (possible MHD) simulations of a very large volume of the Universe which
is however well beyond the aim of this PhD thesis.
On the other hand, in Chapt.8 we report our discovery of novel correlations between
the size (RH) of Radio Halos and their radio power and between RH and the cluster
mass within the Radio Halo region, MH. In particular this last “geometrical”
MH − RH correlation allows us to “observationally” overcome the limitation of
the “average” size of Radio Halos. Thus in this Chapter, by making use of this
“geometrical” correlation and of a simplified form of the re-acceleration model based
on the results of Chapt. 5 and 6 we are able to discuss expected correlations
between the synchrotron power and the thermal cluster quantities relative to the
radio emitting region. This is a new powerful tool of investigation and we show that
all the observed correlations (PR − RH, PR − MH, PR − T, PR − LX, . . . ) now
become well understood in the context of the re-acceleration model. In addition, we
find that observationally the size of Radio Halos scales non-linearly with the virial
radius of the parent cluster, and this immediately means that the fraction of the
cluster volume which is radio emitting increases with cluster mass and thus that the
non-thermal component in clusters is not self-similar.
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A high energy perspective of the co-evolution of black holes and their host galaxiesCivano, Francesca Maria <1979> 11 April 2007 (has links)
Thanks to the Chandra and XMM–Newton surveys, the hard X-ray sky is
now probed down to a flux limit where the bulk of the X-ray background is
almost completely resolved into discrete sources, at least in the 2–8 keV band.
Extensive programs of multiwavelength follow-up observations showed that the large
majority of hard X–ray selected sources are identified with Active Galactic Nuclei (AGN)
spanning a broad range of redshifts, luminosities and optical properties. A sizable fraction
of relatively luminous X-ray sources hosting an active, presumably obscured, nucleus
would not have been easily recognized as such on the basis of optical observations because
characterized by “peculiar” optical properties. In my PhD thesis, I will focus the attention
on the nature of two classes of hard X-ray selected “elusive” sources: those characterized
by high X-ray-to-optical flux ratios and red optical-to-near-infrared colors, a fraction of
which associated with Type 2 quasars, and the X-ray bright optically normal galaxies,
also known as XBONGs. In order to characterize the properties of these classes of elusive
AGN, the datasets of several deep and large-area surveys have been fully exploited.
The first class of “elusive” sources is characterized by X-ray-to-optical flux ratios (X/O)
significantly higher than what is generally observed from unobscured quasars and Seyfert
galaxies. The properties of well defined samples of high X/O sources detected at bright
X–ray fluxes suggest that X/O selection is highly efficient in sampling high–redshift
obscured quasars. At the limits of deep Chandra surveys (∼10−16 erg cm−2 s−1), high
X/O sources are generally characterized by extremely faint optical magnitudes, hence
their spectroscopic identification is hardly feasible even with the largest telescopes. In
this framework, a detailed investigation of their X-ray properties may provide useful
information on the nature of this important component of the X-ray source population.
The X-ray data of the deepest X-ray observations ever performed, the Chandra deep
fields, allows us to characterize the average X-ray properties of the high X/O population.
The results of spectral analysis clearly indicate that the high X/O sources represent the
most obscured component of the X–ray background. Their spectra are harder (G ∼ 1)
than any other class of sources in the deep fields and also of the XRB spectrum (G ≈ 1.4).
In order to better understand the AGN physics and evolution, a much better knowledge
of the redshift, luminosity and spectral energy distributions (SEDs) of elusive AGN
is of paramount importance. The recent COSMOS survey provides the necessary
multiwavelength database to characterize the SEDs of a statistically robust sample of
obscured sources. The combination of high X/O and red-colors offers a powerful
tool to select obscured luminous objects at high redshift. A large sample of X-ray
emitting extremely red objects (R−K >5) has been collected and their optical-infrared
properties have been studied. In particular, using an appropriate SED fitting procedure,
the nuclear and the host galaxy components have been deconvolved over a large range of
wavelengths and ptical nuclear extinctions, black hole masses and Eddington ratios have
been estimated. It is important to remark that the combination of hard X-ray selection and
extreme red colors is highly efficient in picking up highly obscured, luminous sources at
high redshift.
Although the XBONGs do not present a new source population, the interest on the nature
of these sources has gained a renewed attention after the discovery of several examples
from recent Chandra and XMM–Newton surveys. Even though several possibilities
were proposed in recent literature to explain why a relatively luminous (LX = 1042 −
1043erg s−1) hard X-ray source does not leave any significant signature of its presence
in terms of optical emission lines, the very nature of XBONGs is still subject of debate.
Good-quality photometric near-infrared data (ISAAC/VLT) of 4 low-redshift XBONGs
from the HELLAS2XMMsurvey have been used to search for the presence of the putative
nucleus, applying the surface-brightness decomposition technique. In two out of the
four sources, the presence of a nuclear weak component hosted by a bright galaxy has
been revealed. The results indicate that moderate amounts of gas and dust, covering a
large solid angle (possibly 4p) at the nuclear source, may explain the lack of optical
emission lines. A weak nucleus not able to produce suffcient UV photons may provide
an alternative or additional explanation. On the basis of an admittedly small sample, we
conclude that XBONGs constitute a mixed bag rather than a new source population. When
the presence of a nucleus is revealed, it turns out to be mildly absorbed and hosted by a
bright galaxy.
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Multiwavelenght study of cluster mergers and consequences for the radio emission properties of galaxy clustersGiacintucci, Simona <1975> 11 April 2007 (has links)
In the present thesis a thourough multiwavelength analysis of a number of galaxy
clusters known to be experiencing a merger event is presented.
The bulk of the thesis consists in the analysis of deep radio observations of
six merging clusters, which host extended radio emission on the cluster scale. A
composite optical and X–ray analysis is performed in order to obtain a detailed and
comprehensive picture of the cluster dynamics and possibly derive hints about the
properties of the ongoing merger, such as the involved mass ratio, geometry and time
scale. The combination of the high quality radio, optical and X–ray data allows us
to investigate the implications of the ongoing merger for the cluster radio properties,
focusing on the phenomenon of cluster scale diffuse radio sources, known as radio
halos and relics.
A total number of six merging clusters was selected for the present study:
A3562, A697, A209, A521, RXCJ 1314.4–2515 and RXCJ 2003.5–2323.
All of them were known, or suspected, to possess extended radio emission on the
cluster scale, in the form of a radio halo and/or a relic.
High sensitivity radio observations were carried out for all clusters using the
Giant Metrewave Radio Telescope (GMRT) at low frequency (i.e. ≤ 610 MHz),
in order to test the presence of a diffuse radio source and/or analyse in detail the
properties of the hosted extended radio emission. For three clusters, the GMRT
information was combined with higher frequency data from Very Large Array (VLA)
observations. A re–analysis of the optical and X–ray data available in the public
archives was carried out for all sources. Propriety deep XMM–Newton and Chandra
observations were used to investigate the merger dynamics in A3562.
Thanks to our multiwavelength analysis, we were able to confirm the existence
of a radio halo and/or a relic in all clusters, and to connect their properties and
origin to the reconstructed merging scenario for most of the investigated cases.
• The existence of a small size and low power radio halo in A3562 was successfully
explained in the theoretical framework of the particle re–acceleration model
for the origin of radio halos, which invokes the re–acceleration of pre–existing
relativistic electrons in the intracluster medium by merger–driven turbulence.
• A giant radio halo was found in the massive galaxy cluster A209, which has
likely undergone a past major merger and is currently experiencing a new
merging process in a direction roughly orthogonal to the old merger axis. A
giant radio halo was also detected in A697, whose optical and X–ray properties
may be suggestive of a strong merger event along the line of sight. Given the
cluster mass and the kind of merger, the existence of a giant radio halo in both
clusters is expected in the framework of the re–acceleration scenario.
• A radio relic was detected at the outskirts of A521, a highly dynamically
disturbed cluster which is accreting a number of small mass concentrations.
A possible explanation for its origin requires the presence of a merger–driven
shock front at the location of the source. The spectral properties of the relic
may support such interpretation and require a Mach number M <
∼ 3 for the
shock.
• The galaxy cluster RXCJ 1314.4–2515 is exceptional and unique in hosting two
peripheral relic sources, extending on the Mpc scale, and a central small size
radio halo. The existence of these sources requires the presence of an ongoing
energetic merger. Our combined optical and X–ray investigation suggests that
a strong merging process between two or more massive subclumps may be
ongoing in this cluster. Thanks to forthcoming optical and X–ray observations,
we will reconstruct in detail the merger dynamics and derive its energetics, to
be related to the energy necessary for the particle re–acceleration in this cluster.
• Finally, RXCJ 2003.5–2323 was found to possess a giant radio halo. This source
is among the largest, most powerful and most distant (z=0.317) halos imaged
so far. Unlike other radio halos, it shows a very peculiar morphology with
bright clumps and filaments of emission, whose origin might be related to the
relatively high redshift of the hosting cluster. Although very little optical and
X–ray information is available about the cluster dynamical stage, the results
of our optical analysis suggest the presence of two massive substructures which
may be interacting with the cluster. Forthcoming observations in the optical
and X–ray bands will allow us to confirm the expected high merging activity
in this cluster.
Throughout the present thesis a cosmology with H0 = 70 km s−1 Mpc−1,
m=0.3
and
=0.7 is assumed.
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Looking for the building blocks of the Galactic halo: variable stars in Fornax and bootes Dwarfs and in NGC 2419Greco, Claudia <1979> 12 April 2007 (has links)
No description available.
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On the nature of High Frequency Peaker radio sourcesOrienti, Monica <1979> 12 April 2007 (has links)
No description available.
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Probing the innermost regions around supermassive black holes through X–ray spectral variabilityPonti, Gabriele <1977> 12 April 2007 (has links)
No description available.
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Modeling the signal of the cosmic web with hydrodynamical simulationsRoncarelli, Mauro <1978> 12 April 2007 (has links)
No description available.
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High energy cosmologyAlberghi, Gian Luigi <1971> 28 March 2008 (has links)
No description available.
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A spectroscopic and photometric study of MSP companions in Galactic Globular ClustersCocozza, Gabriele <1974> 28 March 2008 (has links)
This Thesis is devoted to the study of the optical companions of Millisecond
Pulsars in Galactic Globular Clusters (GCs) as a part of a large project started at
the Department of Astronomy of the Bologna University, in collaboration with other
institutions (Astronomical Observatory of Cagliari and Bologna, University of Virginia),
specifically dedicated to the study of the environmental effects on passive stellar evolution
in galactic GCs.
Globular Clusters are very efficient “Kilns” for generating exotic object, such as
Millisecond Pulsars (MSP), low mass X-ray binaries(LMXB) or Blue Straggler Stars
(BSS). In particular MSPs are formed in binary systems containing a Neutron Star which
is spun up through mass accretion from the evolving companion (e.g. Bhattacharia & van
den Heuvel 1991). The final stage of this recycling process is either the core of a peeled
star (generally an Helium white dwarf) or a very light almos exhausted star, orbiting a very
fast rotating Neutron Star (a MSP). Despite the large difference in total mass between the
disk of the Galaxy and the Galactic GC system (up a factor 103), the percentage of fast
rotating pulsar in binary systems found in the latter is very higher. MSPs in GCs show spin
periods in the range 1.3 ÷ 30ms, slowdown rates ˙P 1019 s/s and a lower magnetic field,
respect to ”normal” radio pulsars, B 108 gauss . The high probability of disruption of a
binary systems after a supernova explosion, explain why we expect only a low percentage
of recycled millisecond pulsars respect to the whole pulsar population. In fact only the
10% of the known 1800 radio pulsars are radio MSPs.
Is not surprising, that MSP are overabundant in GCs respect to Galactic field, since
in the Galactic Disk, MSPs can only form through the evolution of primordial binaries,
and only if the binary survives to the supernova explosion which lead to the neutron star
formation. On the other hand, the extremely high stellar density in the core of GCs,
relative to most of the rest of the Galaxy, favors the formation of several different binary
systems, suitable for the recycling of NSs (Davies at al. 1998).
In this thesis we will present the properties two millisecond pulsars companions
discovered in two globular clusters, the Helium white dwarf orbiting the MSP PSR 1911-5958A in NGC 6752 and the second case of a tidally deformed star orbiting an eclipsing
millisecond pulsar, PSR J1701-3006B in NGC6266
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