Origin of Galaxies and Large Scale Structures
Riccardo Valdarnini
SISSA/ISAS, International School for Advanced Studies
Via Beirut 2, 34014 Trieste, Italy
My research topics in cosmology
are the study of the formation and evolution of large scale structure (LSS)
in the universe. Over the past decades
extended redshift surveys have put in evidence that the galaxy distribution
is not random but on large scales ( > 10 Mpc) is arranged in a web of sheetlike
structures , with voids between them and filaments at the intersections.
The size of these structures can be as large as 50100 Mpc.
According
to the gravitational instability paradigm the observed structures in the
universe have been formed through the growth in time of an initial density
perturbation field (x),
subject to gravitational instability. At very early epoch the field is
assumed to be a random Gaussian process. The time evolution of the density
field can be considered first linear ( <<1
) and then nonlinear ( >>1)
at late epochs, according to the considered length scale. The presentday
patterns revealed in the galaxy distribution are then connected to the
initial power spectrum P(k)= <(k)
^{2}>of
the density perturbations. Studies of LSS are important since can be used
to constrain the possible forms of P(k). Theoretically the shape
of P(k) is determined according to the matter content of the
universe.
Thus
LSS can provide clues about the most important problem in modern cosmology
: the dark matter problem. There is a wide observational evidence that
most of the mass in the universe is in the form of a dark nonbaryonic
collisionless material ( > 90 %). Many cosmological models have been proposed
to solve the problem.The standard approach is to test the consistency of
the matter distribution, predicted according to the theoretical P(k),
with the observed galaxy distribution at the present epoch.This approach
is complicated first by nonlinearity which develops at late epochs and
second by the dissipative processes of the baryonic material.
The
study of LSS is then a wide field and my research interest cover different
areas.

In the linear phases a theoretical
model which includes a contribution of massive neutrinos
to the matter content of the universe and to P(k) has been proposed.

In the nonlinear phase threedimensional
patterns formed by the galaxy distribution have been subject to statistical
studies and measured using various methods. The LSS clustering distribution
has been investigated with statistical methods
using Npoint correlation functions, fractals and morphological descriptors
based on the void probability function.

Finally density perturbation evolution
into nonlinear phases can be followed by means of numerical simulations.
In the last decade Nbody techniques have become in cosmology a standard
tool of research. The matter distribution is sampled at early epochs with
a finite particle set and the evolution of the system is followed in time
solving the gravitational field at each timestep.These integrations are
commonly referred as computer simulations,since represent a computer model
for the evolution of the universe. Computer Nbody codes have been widely
used to simulate the collisionless clustering evolution of galaxies, cluster
of galaxies and LSS. With increasing available computational power computer
codes have incorporated the hydrodynamical treatment for the gaseous matter
component,with the inclusion also of radiative processes for ionized atoms.
Of particular interest is the formation and evolution of galaxy clusters.
These object are the largest virialized bound objects known in the universe,
with masses in excess of 10^{14} M.
Their evolution is a strong function of the underlying cosmological model,
so that present day cluster number count and Xray temperature function
can be used to constrain the range of the density parameter _{m}.
Their internal dynamical state is also a function of the background cosmology
and is connected to the amount of observed substructures.A research
project is currently in progress in which large cluster
catalogs are generated from hydrodynamical simulations using
a TREESPH code and are compared with current data.