1,460 research outputs found

### The Mass Function of Cosmic Structures with Non-Spherical Collapse

Non-spherical dynamical approximations and models for the gravitational
collapse are used to extend the well-known Press \& Schechter (PS) approach, in
order to determine analytical expressions for the mass function of cosmic
structures. The problem is rigorously set up by considering the intrinsic
Lagrangian nature of the mass function. The Lagrangian equations of motion of a
cold and irrotational fluid in single-stream regime show that the shear, which
is non-locally determined by all the matter field, is the quantity which
characterizes non-spherical perturbations. The Zel'dovich approximation, being
a self-consistent first-order Lagrangian and local one, is used as a suitable
guide to develop realistic estimates of the collapse time of a mass clump,
starting from the local initial values of density and shear. Both
Zel'dovich-based \an\ and models and the homogeneous ellipsoidal model predict
that more large-mass objects are expected to form than the usual PS relation.
In particular, the homogeneous ellipsoid model is consistent at large masses
with a Press \& Schechter mass function with a lower value of the \dc\
parameter, in the range 1.4$\div$1.6. This gives a dynamical explanation of why
lower \dc\ values have been found to fit the results of several N-body
simulations. When more small-scale structure is present, highly non-linear
dynamical effects can effectively slow down the collapse rate of a
perturbation, increasing the effective value of \dc. This may have interesting
consequences on the abundance of large-mass high-redshift objects.Comment: 16 pages+5 figures, uuencoded postscript file, submitted to Ap

### Observational Support for the Gurzadyan-Kocharyan Relation in Clusters of Galaxies

We show that observational data for four Abell clusters of galaxies support
the Gur\-za\-dyan-Kocharyan relation between the Hausdorff dimension and the
dynamical properties of a galaxy system. The Hausdorff dimension is calculated
using the two-point correlation function, while the dynamical parameters are
estimated using available data and reasonable assumptions on the mass function
of galaxies. This result can have essential consequences in the understanding
of the dynamical mechanisms that determine the fractal distribution of
galaxies.Comment: 5 pages, uuencoded postscript file with figures, SISSA Preprint
72/94/A, A&A Letters in pres

### A Lagrangian Dynamical Theory for the Mass Function of Cosmic Structures: I Dynamics

A new theory for determining the mass function of cosmic structures is
presented. It relies on a realistic treatment of collapse dynamics.
Gravitational collapse is analyzed in the Lagrangian perturbative framework.
Lagrangian perturbations provide an approximation of truncated type, i.e.
small-scale structure is filtered out. The collapse time is suitably defined as
the instant at which orbit crossing takes place. The convergence of the
Lagrangian series in predicting the collapse time of a homogeneous ellipsoid is
demonstrated; it is also shown that third-order calculations are necessary in
predicting collapse. Then, the Lagrangian prediction, with a correction for
quasi-spherical perturbations, can be used to determine the collapse time of a
homogeneous ellipsoid in a fast and precise way. Furthermore, ellipsoidal
collapse can be considered as a particular truncation of the Lagrangian series.
Gaussian fields with scale-free power spectra are then considered. The
Lagrangian series for the collapse time is found to converge when the collapse
time is not large. In this case, ellipsoidal collapse gives a fast and accurate
approximation of the collapse time; spherical collapse is found to poorly
reproduce the collapse time, even in a statistical sense. Analytical fits of
the distribution functions of the inverse collapse times, as predicted by the
ellipsoid model and by third-order Lagrangian theory, are given. These will be
necessary for a determination of the mass function, which will be given in
paper II.Comment: 18 pages, Latex, uses mn.sty and psfig, 7 postscript figures (fig. 2
and 3 not complete). Revised version, stylistic changes. MNRAS, in pres

### The Cosmological Mass Function with 1D Gravity

The cosmological mass function problem is analyzed in full detail in the case
of 1D gravity, with analytical, semi-analytical and numerical techniques. The
extended Press & Schechter theory is improved by detailing the relation between
smoothing radius and mass of the objects. This is done by introducing in the
formalism the concept of a growth curve for the objects. The predictions of the
extended Press & Schechter theory are compared to large N-body simulations of
flat expanding 1D universes with scale-free power spectra of primordial
perturbations. The collapsed objects in the simulations are located with a
clump-finding algorithm designed to find regions that have undergone orbit
crossing or that are in the multi-stream regime (these are different as an
effect of the finite size of the multi-stream regions). It is found that the
semi-analytical mass function theory, which has no free parameters, is able to
recover the properties of collapsed objects both statistically and object by
object. In particular, the predictions of regions in orbit crossing are
optimized by the use of Gaussian filtering, while the use of sharp k-space
filtering apparently allows to reproduce the larger multi-stream regions. The
mass function theory does not reproduce well the clumps found with the standard
friends-of-friends algorithm; however, the performance of this algorithm has
not been thoroughly tested in the 1D cosmology. Our preliminary analyses of the
3D case confirms that the techniques developed in this paper are precious in
understanding the cosmological mass function problem in 3D.Comment: 25 pages, revtex, postscript figures included, in press on Physical
Review

### Interpreting the possible break in the Black Hole - Bulge mass relation

Recent inspections of local available data suggest that the almost linear
relation between the stellar mass of spheroids ($M_{\rm sph}$) and the mass of
the super massive Black Holes (BHs) residing at their centres, shows a break
below $M_{\rm sph} \sim 10^{10}\ {\rm M}_\odot$, with a steeper, about
quadratic relation at smaller masses. We investigate the physical mechanisms
responsible for the change in slope of this relation, by comparing data with
the results of the semi-analytic model of galaxy formation MORGANA, which
already predicted such a break in its original formulation. We find that the
change of slope is mostly induced by effective stellar feedback in star-forming
bulges. The shape of the relation is instead quite insensitive to other
physical mechanisms connected to BH accretion such as disc instabilities,
galaxy mergers, Active Galactic Nucleus (AGN) feedback, or even the exact
modelling of accretion onto the BH, direct or through a reservoir of low
angular momentum gas. Our results support a scenario where most stars form in
the disc component of galaxies and are carried to bulges through mergers and
disc instabilities, while accretion onto BHs is connected to star formation in
the spheroidal component. Therefore, a model of stellar feedback that produces
stronger outflows in star-forming bulges than in discs will naturally produce a
break in the scaling relation. Our results point to a form of co-evolution
especially at lower masses, below the putative break, mainly driven by stellar
feedback rather than AGN feedback.Comment: MNRAS accepted, 10 pages, 6 figures, 1 tabl

### LA FUNZIONE DI MASSA COSMOLOGICA

1995/1996VIII Ciclo1969Versione digitalizzata della tesi di dottorato cartacea

### The Formation of Supermassive Black Holes from Population III.1 Seeds. I. Cosmic Formation Histories and Clustering Properties

We calculate cosmic distributions in space and time of the formation sites of
the first, "Pop III.1" stars, exploring a model in which these are the
progenitors of all supermassive black holes (SMBHs), seen in the centers of
most large galaxies. Pop III.1 stars are defined to form from primordial
composition gas in dark matter minihalos with $\sim10^6\:M_\odot$ that are
isolated from neighboring astrophysical sources by a given isolation distance,
$d_{\rm{iso}}$. We assume Pop III.1 sources are seeds of SMBHs, based on
protostellar support by dark matter annihilation heating that allows them to
accrete a large fraction of their minihalo gas, i.e., $\sim10^5\:M_\odot$.
Exploring $d_{\rm{iso}}$ from $10 - 100\:\rm{kpc}$ (proper distances), we
predict the redshift evolution of Pop III.1 source and SMBH remnant number
densities. The local, $z=0$ density of SMBHs constrains $d_{\rm{iso}}\lesssim
100\:\rm{kpc}$ (i.e., $3\:\rm{Mpc}$ comoving distance at $z\simeq30$). In our
simulated ($\sim60\:\rm{Mpc}$)$^3$ comoving volume, Pop III.1 stars start
forming just after $z=40$. Their formation is largely complete by $z\simeq25$
to $20$ for $d_{\rm{iso}}=100$ to $50\:\rm{kpc}$. We follow source evolution to
$z=10$, by which point most SMBHs reside in halos with $\gtrsim10^8\:M_\odot$.
Over this period, there is relatively limited merging of SMBHs for these values
of $d_{\rm{iso}}$. We also predict SMBH clustering properties at $z=10$:
feedback suppression of neighboring sources leads to relatively flat angular
correlation functions.Comment: 18 pages, 10 figures, MNRAS accepte

### Environmental Effects on Local Active Galactic Nuclei

Using an extensive sample of nearby galaxies (the Nearby Galaxies Catalog, by
Tully), we investigate the environment of the galaxies hosting low-luminosity
AGNs (Seyferts and LINERs). We define the local galaxy density, adopting a new
correction for the incompleteness of the galaxy sample at large distances. We
consider both a complete sample of bright and nearby AGNs, identified from the
nuclear spectra obtained in available wide optical spectroscopic surveys, and a
complete sample of nearby Seyferts. Basically, we compare the local galaxy
density distributions of the AGNs with those of non-AGN samples, chosen in
order to match the magnitude and morphological type distributions of the AGN
samples. We find, only for the early-type spirals more luminous than $\sim
M^*$, that both LINERs and Seyferts tend to reside in denser environments on
all the scales tested, from tenths of Mpc to a few Mpc; moreover Seyferts show
an enhanced small-scale density segregation with respect to LINERs. This gives
support to the idea that AGNs can be stimulated by interactions. On larger
scales, tens of Mpc, we find that the AGNs hosted in luminous early-type
spirals show a tendency to stay near the center of the Local Supercluster.
Finally we discuss the interpretations of our findings and their consequences
for some possible scenarios of AGN formation and evolution and for the problem
of how AGNs trace the large-scale structures.Comment: 16 pages+3 figures, uuencoded postscript file, preprint SISSA 76/94/A
, ApJ November 20, 199

### Mass function of dormant black holes and the evolution of the Active Galactic Nuclei

We derive the mass function of the relic black holes and compared with that
of the Massive Dark Objects in galaxies. Under the assumption that accretion
onto massive BH's powers the Active Galactic Nuclei, the mass function of the
BH responsibile for the past activity of QSO/AGN is computed. Our results
support the scenario in which the QSO phase has exclusively occurred in every
proto-elliptical.Comment: 10 pages, 8 Figures. Version improved with referee comments. J.
Accepted on MNRA

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