5,036 research outputs found
Understanding Galaxy Formation and Evolution
The old dream of integrating into one the study of micro and macrocosmos is
now a reality. Cosmology, astrophysics, and particle physics intersect in a
scenario (but still not a theory) of cosmic structure formation and evolution
called Lambda Cold Dark Matter (LCDM) model. This scenario emerged mainly to
explain the origin of galaxies. In these lecture notes, I first present a
review of the main galaxy properties, highlighting the questions that any
theory of galaxy formation should explain. Then, the cosmological framework and
the main aspects of primordial perturbation generation and evolution are
pedagogically detached. Next, I focus on the ``dark side'' of galaxy formation,
presenting a review on LCDM halo assembling and properties, and on the main
candidates for non-baryonic dark matter. It is shown how the nature of
elemental particles can influence on the features of galaxies and their
systems. Finally, the complex processes of baryon dissipation inside the
non-linearly evolving CDM halos, formation of disks and spheroids, and
transformation of gas into stars are briefly described, remarking on the
possibility of a few driving factors and parameters able to explain the main
body of galaxy properties. A summary and a discussion of some of the issues and
open problems of the LCDM paradigm are given in the final part of these notes.Comment: 50 pages, 10 low-resolution figures (for normal-resolution, DOWNLOAD
THE PAPER (PDF, 1.9 Mb) FROM http://www.astroscu.unam.mx/~avila/avila.pdf).
Lectures given at the IV Mexican School of Astrophysics, July 18-25, 2005
(submitted to the Editors on March 15, 2006
Dark matter concentrations in galactic nuclei according to polytropic models
We calculate the radial profiles of galaxies where the nuclear region is
self-gravitating, consisting of self-interacting dark matter (SIDM) with
degrees of freedom. For sufficiently high density this dark matter becomes
collisional, regardless of its behaviour on galaxy scales. Our calculations
show a spike in the central density profile, with properties determined by the
dark matter microphysics, and the densities can reach the `mean density' of a
black hole (from dividing the black-hole mass by the volume enclosed by the
Schwarzschild radius). For a galaxy halo of given compactness
(), certain values for the dark matter entropy yield a dense
central object lacking an event horizon. For some soft equations of state of
the SIDM (e.g. ), there are multiple horizonless solutions at given
compactness. Although light propagates around and through a sphere composed of
dark matter, it is gravitationally lensed and redshifted. While some
calculations give non-singular solutions, others yield solutions with a central
singularity. In all cases the density transitions smoothly from the central
body to the dark-matter envelope around it, and to the galaxy's dark matter
halo. We propose that pulsar timing observations will be able to distinguish
between systems with a centrally dense dark matter sphere (for different
equations of state) and conventional galactic nuclei that harbour a
supermassive black hole.Comment: MNRAS accepted, 24 pages, 12 figure
A new gravitational N-body simulation algorithm for investigation of cosmological chaotic advection
Recently alternative approaches in cosmology seeks to explain the nature of
dark matter as a direct result of the non-linear spacetime curvature due to
different types of deformation potentials. In this context, a key test for this
hypothesis is to examine the effects of deformation on the evolution of large
scales structures. An important requirement for the fine analysis of this pure
gravitational signature (without dark matter elements) is to characterize the
position of a galaxy during its trajectory to the gravitational collapse of
super clusters at low redshifts. In this context, each element in an
gravitational N-body simulation behaves as a tracer of collapse governed by the
process known as chaotic advection (or lagrangian turbulence). In order to
develop a detailed study of this new approach we develop the COsmic LAgrangian
TUrbulence Simulator (COLATUS) to perform gravitational N-body simulations
based on Compute Unified Device Architecture (CUDA) for graphics processing
units (GPUs). In this paper we report the first robust results obtained from
COLATUS.Comment: Proceedings of Sixth International School on Field Theory and
Gravitation-2012 - by American Institute of Physic
Do micro brown dwarf detections explain the galactic dark matter?
Context: The baryonic dark matter dominating the structures of galaxies is
widely considered as mysterious, but hints for it have been in fact detected in
several astronomical observations at optical, infrared, and radio wavelengths.
We call attention to the nature of galaxy merging, the observed rapid
microlensing of a quasar, the detection of "cometary knots" in planetary
nebulae, and the Lyman-alpha clouds as optical phenomena revealing the compact
objects. Radio observations of "extreme scattering events" and "parabolic arcs"
and microwave observations of "cold dust cirrus" clouds are observed at 15 - 20
K temperatures are till now not considered in a unifying picture. Aims: The
theory of gravitational hydrodynamics predicts galactic dark matter arises from
Jeans clusters that are made up of almost a trillion micro brown dwarfs (mBDs)
of earth weight. It is intended to explain the aforementioned anomalous
observations and to make predictions within this framework. Methods: We employ
analytical isothermal modeling to estimate various effects. Results: Estimates
of their total number show that they comprise enough mass to constitute the
missing baryonic matter. Mysterious radio events are explained by mBD pair
merging in the Galaxy. The "dust" temperature of cold galaxy halos arises from
a thermostat setting due to a slow release of latent heat at the 14 K gas to
solid transition at the mBD surface. The proportionality of the central black
hole mass of a galaxy and its number of globular clusters is explained. The
visibility of an early galaxy at redshift 8.6 is obvious with most hydrogen
locked up in mBDs. Conclusions: Numerical simulations of various steps would
further test the approach. It looks promising to redo MACHO searches against
the Magellanic clouds.Comment: 12 pages A&A tex, 3 pdf figure
Constrained simulations of the Local Group: on the radial distribution of substructures
We examine the properties of satellites found in high resolution simulations
of the local group. We use constrained simulations designed to reproduce the
main dynamical features that characterize the local neighborhood, i.e. within
tens of Mpc around the Local Group (LG). Specifically, a LG-like object is
found located within the 'correct' dynamical environment and consisting of
three main objects which are associated with the Milky Way, M31 and M33. By
running two simulations of this LG from identical initial conditions - one with
and one without baryons modeled hydrodynamically - we can quantify the effect
of gas physics on the population of subhaloes in an environment similar
to our own. We find that above a certain mass cut, subhaloes in hydrodynamic simulations are more
radially concentrated than those in simulations with out gas. This is caused by
the collapse of baryons into stars that typically sit in the central regions of
subhaloes, making them denser. The increased central density of such a subhalo,
results in less mass loss due to tidal stripping than the same subhalo
simulated with only dark matter. The increased mass in hydrodynamic subhaloes
with respect to dark matter ones, causes dynamical friction to be more
effective, dragging the subhalo towards the centre of the host. This results in
these subhaloes being effectively more radially concentrated then their dark
matter counterparts.Comment: 12 pages, 9 figure
Cold Dark Matter Substructure and Galactic Disks I: Morphological Signatures of Hierarchical Satellite Accretion
(Abridged) We conduct a series of high-resolution, dissipationless N-body
simulations to investigate the cumulative effect of substructure mergers onto
thin disk galaxies in the context of the LCDM paradigm of structure formation.
Our simulation campaign is based on a hybrid approach. Substructure properties
are culled directly from cosmological simulations of galaxy-sized cold dark
matter (CDM) halos. In contrast to what can be inferred from statistics of the
present-day substructure populations, accretions of massive subhalos onto the
central regions of host halos, where the galactic disk resides, since z~1
should be common occurrences. One host halo merger history is subsequently used
to seed controlled numerical experiments of repeated satellite impacts on an
initially-thin Milky Way-type disk galaxy. We show that these accretion events
produce several distinctive observational signatures in the stellar disk
including: a ring-like feature in the outskirts; a significant flare; a central
bar; and faint filamentary structures that (spuriously) resemble tidal streams.
The final distribution of disk stars exhibits a complex vertical structure that
is well-described by a standard ``thin-thick'' disk decomposition. We conclude
that satellite-disk encounters of the kind expected in LCDM models can induce
morphological features in galactic disks that are similar to those being
discovered in the Milky Way, M31, and in other disk galaxies. These results
highlight the significant role of CDM substructure in setting the structure of
disk galaxies and driving galaxy evolution. Upcoming galactic structure surveys
and astrometric satellites may be able to distinguish between competing
cosmological models by testing whether the detailed structure of galactic disks
is as excited as predicted by the CDM paradigm.Comment: Accepted version to appear in ApJ, 24 pages, 8 figures, LaTeX (uses
emulateapj.cls). Comparison between the simulated ring-like features and the
Monoceros ring stellar structure in the Milky Way performed; conclusions
unaltere
A Systematic Look at the Effects of Radiative Feedback on Disc Galaxy Formation
Galaxy formation models and simulations rely on various feedback mechanisms
to reproduce the observed baryonic scaling relations and galaxy morphologies.
Although dwarf galaxy and giant elliptical properties can be explained using
feedback from supernova and active galactic nuclei, Milky Way-sized galaxies
still represent a challenge to current theories of galaxy formation. In this
paper, we explore the possible role of feedback from stellar radiation in
regulating the main properties of disk galaxies such as our own Milky Way. We
have performed a suite of cosmological simulations of the same halo selected based on its rather typical mass accretion
history. We have implemented radiative feedback from young stars using a crude
model of radiative transfer for ultraviolet (UV) and infrared (IR) radiation.
However, the model is realistic enough such that the dust opacity plays a
direct role in regulating the efficiency of our feedback mechanism. We have
explored various models for the dust opacity, assuming different constant dust
temperatures, as well as a varying dust temperature model. We find that while
strong radiative feedback appears as a viable mechanism to regulate the stellar
mass fraction in massive galaxies, it also prevents the formation of discs with
reasonable morphologies. In models with strong stellar radiation feedback,
stellar discs are systematically too thick while the gas disc morphology is
completely destroyed due to the efficient mixing between the feedback-affected
gas and its surroundings. At the resolution of our simulation suite, we find it
impossible to preserve spiral disc morphology while at the same time expelling
enough baryons to satisfy the abundance matching constraints.Comment: accepted to MNRA
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