Secondary evolution of galaxies investigated by N-body simulations
Creators: name: Coppola, Giuseppina
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2008

description: In the ΛCDM cosmology, merging is one of the most important physical
processes that drives the formation and evolution of galaxies. In the present
work, we use N-body techniques to investigate some of currently open issues
related to the formation and evolution of galaxies along the Hubble sequence
(see Chapter 1). In particular, we address (i) the role of dissipation-less
merging on the scaling relations and internal color gradients of early-type
galaxies, by modeling these systems with two-component Sérsic models; (ii)
the formation and survival of cold disks in the merging of late-type gas-rich
systems, and (iii) the different merging history of galaxy types through cosmological
simulations.
We present new spherical, isotropic, non rotating, two-component (dark +
stellar matter) models of early-type galaxies (see Chapter 2). In order to
realistically describe the observed light profile of early-type systems and the
shape of the mass profile of galaxy dark matter haloes predicted by recent numerical
simulation results, both components of these models are described by
a deprojected Sérsic law. We perform a detailed analysis of structural properties
and distribution function of these models, proving that they represent
physically admissible and stable systems. The free parameters of the models
are derived from observational properties of early-type galaxies. We perform
discrete realizations of the two-component Sérsic models, analyzing in detail
how to derive an optimal softening length for the gravitational potential of
these discrete systems. The models are then used to simulate dry mergers of
early-type galaxy systems, by means of the N-body simulation code Gadget-2
(Springel 2005, see Chapter 3). The mergers are performed with progenitors
spanning a wide range of galaxy luminosities and with a variety of initial
orbital parameters. We find that dissipation-less merging preserves the Fundamental
Plane relation of early-type galaxies, in agreement with previous
works. However, in contrast to previous findings, we find that dissipationless
merging also moves galaxies along other observed correlations, such as
the Kormendy, the Faber-Jackson, and the luminosity–size relations. Hence,
we conclude that all the above correlations are preserved after dissipationless encounters of early-type galaxies. For the first time, we are also able
to perform a detailed analysis of how dissipationless merging affects internal
stellar population gradients of ETGs. We find that the metallicity profiles,
initially assigned to the merging progenitors, can be significantly flattened
after the encounters. The amount of flattening is larger for low mass-ratio
mergers (down to a minor-merger ratio of 1:4), and also becomes larger as
the mass of the progenitors decreases. Remarkably, this allows the existence
of shallow stellar population gradients in ETGs to be explained as a result
of galaxy-galaxy merging.
The second issue we have addressed is the possibility of rebuilding late-type
systems starting from merging of disk galaxies. Recent pioneering works
have shown that, in merger simulations with a significant stellar feedback,
even a major merger can produce a disk-dominated remnant. These works
show that a combination of strong stellar feedback (in very peculiar conditions)
and a large gas content are essential ingredients to the survival of disks
after a merging process. However, merger remnants result to have a large
bulge component, which can likely describe only early spiral galaxy types. In
contrast, our simulations show that disk formation through merging of gas
rich systems might be an important ingredient of galaxy formation theories
in more general conditions. Using realistic galaxy models (M33-like) whose
main novelty is that of having a hot gaseous halo component (in addition
to cold gas in the disk), we performed a set of hydrodynamical merger simulations.
We show that mergers between these progenitors, whose baryonic
component mainly consists of gas, produce late-type galaxies rather than
elliptical/S0 systems. We interpret this result by the fact that gas cooling
from the halo has a crucial role in producing the disk-dominated remnants.
In fact, we find that gas particles in the halo have a temperature very close
to the peak of the gas cooling function. Hence, the hot gas cools very rapidly
after merging, acquiring angular momentum from the orbit, and settles on
the final disk.
Finally, we have studied the formation and evolution of different galaxy types,
with the main focus of studying the properties of S0 galaxies, in N-body cosmological
simulations. From a large cosmological simulation, we have selected
a cluster of galaxies with size and velocity dispersion similar to the Virgo cluster,
developing an original scheme to identify elliptical, S0, and spiral galaxy
candidates. With this scheme, we have derived the morphology-radius relation
and the velocity distribution of galaxies in the simulated cluster at
redshift z = 0, and compared them to observational results. The first results
presented in this work show a relatively good match of the simulated and
observed morphology-radius relation, but unfortunately, we also find that
our simulations suffers of the low resolution problem. The number of substructures that we included in the analysis is lower than that expected from
the total abundance of Virgo cluster members. As a future work, we plan
to re-do the analysis on different simulated clusters with improved mass and
spatial resolution.
Conclusions for each part of the present work are reported at the end of each
chapter of the thesis.