187 research outputs found
An analytical model for the accretion of dark matter subhalos
An analytical model is developed for the mass function of cold dark matter
subhalos at the time of accretion and for the distribution of their accretion
times. Our model is based on the model of Zhao et al. (2009) for the median
assembly histories of dark matter halos, combined with a simple log-normal
distribution to describe the scatter in the main-branch mass at a given time
for halos of the same final mass. Our model is simple, and can be used to
predict the un-evolved subhalo mass function, the mass function of subhalos
accreted at a given time, the accretion-time distribution of subhalos of a
given initial mass, and the frequency of major mergers as a function of time.
We test our model using high-resolution cosmological -body simulations, and
find that our model predictions match the simulation results remarkably well.
Finally, we discuss the implications of our model for the evolution of subhalos
in their hosts and for the construction of a self-consistent model to link
galaxies and dark matter halos at different cosmic times.Comment: 14 pages, 10 figures (caption for figure 10 fixed). Accepted for
publication in Ap
The size evolution of galaxy discs formed within Lambda Cold Dark Matter haloes
By means of galaxy evolutionary models, we explore the direct consequences of
the LCDM cosmogony on the size evolution of galactic discs, avoiding
intentionally the introduction of intermediate (uncertain) astrophysical
processes. Based on the shape of the rotation curves and guided by a simplicity
criterion, we adopt an average galaxy mass baryon fraction of 0.03. In order to
study general behaviors, only models with the average initial conditions are
analyzed. The stellar and B-band effective radii, R* and RB, of individual
galaxies grow significantly with time (inside-out disc formation) with laws
that are weakly dependent on mass, M*,or luminosity, LB. However, the change of
R* with z at fixed M* is slow; for z<2.5, R*(M*=const) ~ (1+z)^-0.4. On the
other hand, the change of RB with z at a fixed LB is strong and resembles the
RB decreasing law of the individual models; roughly RB(LB=const) ~ (1+z)^-0.85
for z0.75. We find also that at z=0, R* ~ M*^0.38
and RB ~ LB^0.40, remaining the slopes of these relations almost the same up to
z ~ 3. Our predictions are in reasonable agreement with observational
inferences on the typical radius change with z of late-type galaxies more
luminous (massive) than high values imposed by the selection effects. The
models seem also to be consistent, within the large scatter, with the RB and LB
values obtained from non complete samples of sub-L* late-type galaxies with
available rest-frame photometric information at different z's. The properties
and evolution of the LCDM haloes seem to be the main drivers of galaxy disc
size evolution. Nevertheless, the models reveal a potential difficulty in
explaining the observed steepening of the RB-LB relation with respect to the
R*-M* one, an effect related to the well established color-magnitude relation.Comment: 7 pages, 4 figures. MNRAS in press. A few typos corrected in the
text, references corrected and updated, 1 more adde
On the Angular Correlation Function of SZ Clusters : Extracting cosmological information from a 2D catalog
We discuss the angular correlation function of Sunyaev-Zel'dovich
(SZ)-detected galaxy clusters as a cosmological probe. As a projection of the
real-space cluster correlation function, the angular function samples the
underlying SZ catalog redshift distribution. It offers a way to study cosmology
and cluster evolution directly with the two-dimensional catalog, even before
extensive follow-up observations, thereby facilitating the immediate scientific
return from SZ surveys. As a simple illustration of the information content of
the angular function, we examine its dependence on the parameter pair Om_m,
sigma_8 in flat cosmologies. We discuss sources of modeling uncertainty and
consider application to the future Planck SZ catalog, showing how these two
parameters and the normalization of the SZ flux-mass relation can be
simultaneously found when the local X-ray cluster abundance constraint is
included.Comment: 11 pages, 5 figures. A&A, 410, 767; corrected typo, published versio
Impact of Supernova feedback on the Tully-Fisher relation
Recent observational results found a bend in the Tully-Fisher Relation in
such a way that low mass systems lay below the linear relation described by
more massive galaxies. We intend to investigate the origin of the observed
features in the stellar and baryonic Tully-Fisher relations and analyse the
role played by galactic outflows on their determination. Cosmological
hydrodynamical simulations which include Supernova feedback were performed in
order to follow the dynamical evolution of galaxies. We found that Supernova
feedback is a fundamental process in order to reproduce the observed trends in
the stellar Tully-Fisher relation. Simulated slow rotating systems tend to have
lower stellar masses than those predicted by the linear fit to the massive end
of the relation, consistently with observations. This feature is not present if
Supernova feedback is turned off. In the case of the baryonic Tully-Fisher
relation, we also detect a weaker tendency for smaller systems to lie below the
linear relation described by larger ones. This behaviour arises as a result of
the more efficient action of Supernovae in the regulation of the star formation
process and in the triggering of powerful galactic outflows in shallower
potential wells which may heat up and/or expel part of the gas reservoir.Comment: 10 pages, 9 figures, accepted for publication in A&
A Bayesian approach to the semi-analytic model of galaxy formation: methodology
We believe that a wide range of physical processes conspire to shape the
observed galaxy population but we remain unsure of their detailed interactions.
The semi-analytic model (SAM) of galaxy formation uses multi-dimensional
parameterisations of the physical processes of galaxy formation and provides a
tool to constrain these underlying physical interactions. Because of the high
dimensionality, the parametric problem of galaxy formation may be profitably
tackled with a Bayesian-inference based approach, which allows one to constrain
theory with data in a statistically rigorous way. In this paper we develop a
SAM in the framework of Bayesian inference. We show that, with a parallel
implementation of an advanced Markov-Chain Monte-Carlo algorithm, it is now
possible to rigorously sample the posterior distribution of the
high-dimensional parameter space of typical SAMs. As an example, we
characterise galaxy formation in the current CDM cosmology using the
stellar mass function of galaxies as an observational constraint. We find that
the posterior probability distribution is both topologically complex and
degenerate in some important model parameters, suggesting that thorough
explorations of the parameter space are needed to understand the models. We
also demonstrate that because of the model degeneracy, adopting a narrow prior
strongly restricts the model. Therefore, the inferences based on SAMs are
conditional to the model adopted. Using synthetic data to mimic systematic
errors in the stellar mass function, we demonstrate that an accurate
observational error model is essential to meaningful inference.Comment: revised version to match published article published in MNRA
Fingerprints of the hierarchical building up of the structure on the gas kinematics of galaxies
Recent observational and theoretical works have suggested that the
Tully-Fisher Relation might be generalised to include dispersion-dominated
systems by combining the rotation and dispersion velocity in the definition of
the kinematical indicator. Mergers and interactions have been pointed out as
responsible of driving turbulent and disordered gas kinematics, which could
generate Tully-Fisher Relation outliers. We intend to investigate the gas
kinematics of galaxies by using a simulated sample which includes both, gas
disc-dominated and spheroid-dominated systems. Cosmological hydrodynamical
simulations which include a multiphase model and physically-motivated Supernova
feedback were performed in order to follow the evolution of galaxies as they
are assembled. Both the baryonic and stellar Tully-Fisher relations for gas
disc-dominated systems are tight while, as more dispersion-dominated systems
are included, the scatter increases. We found a clear correlation between
and morphology, with dispersion-dominated systems
exhibiting the larger values (). Mergers and interactions can affect the
rotation curves directly or indirectly inducing a scatter in the Tully-Fisher
Relation larger than the simulated evolution since . Kinematical
indicators which combine rotation velocity and dispersion velocity can reduce
the scatter in the baryonic and the stellar mass-velocity relations. Our
findings also show that the lowest scatter in both relations is obtained if the
velocity indicators are measured at the maximum of the rotation curve.
Moreover, the rotation velocity estimated at the maximum of the gas rotation
curve is found to be the best proxy for the potential well regardless of
morphology.Comment: 16 pages, 10 figures, accepted for publication in A&
Cluster abundances and S-Z power spectra: effects of non-Gaussianity and early dark energy
In the standard Lambda CDM cosmological model with a Gaussian primordial
density fluctuation field, the relatively low value of the mass variance
parameter (sigma_8=0.74{+0.05}{-0.06}, obtained from the WMAP 3-year data)
results in a reduced likelihood that the measured level of CMB anisotropy on
the scales of clusters is due to the Sunyaev-Zeldovich (S-Z) effect. To assess
the feasibility of producing higher levels of S-Z power, we explore two
alternative models which predict higher cluster abundance. In the first model
the primordial density field has a chi^2_1 distribution, whereas in the second
an early dark energy component gives rise to the desired higher cluster
abundance. We carry out the necessary detailed calculations of the levels of
S-Z power spectra, cluster number counts, and angular 2-point correlation
function of clusters, and compare (in a self-consistent way) their predicted
redshift distributions. Our results provide a sufficient basis upon which the
viability of the three models may be tested by future high quality
measurements.Comment: 12 pages, 5 figures, accepted for publication in MNRA
The Impact of Cooling and Feedback on Disc Galaxies
We present detailed, analytical models for the formation of disc galaxies to
investigate the impact that cooling and feedback have on their structural
properties. In particular, we investigate which observables extracted directly
from the models are best suited as virial mass estimators, and to what extent
they allow the recovery of the model input parameters regarding the feedback
and cooling efficiencies. Contrary to naive expectations, the luminosities and
circular velocities of disc galaxies are extremely poor indicators of total
virial mass. Instead, we show that the product of disc scale length and
rotation velocity squared yields a much more robust estimate. We show that
feedback can cause a narrow correlation between galaxy mass fraction and halo
spin parameter, similar to that found recently by van den Bosch, Burkert and
Swaters from an analysis of dwarf galaxy rotation curves. Finally we
investigate the impact that cooling and feedback have on the colors,
metallicities, star formation histories and Tully-Fisher relation of disc
galaxies.Comment: 20 pages, 12 figures. To be published in MNRA
Galaxy Formation in Preheated Intergalactic Media
We outline a scenario of galaxy formation in which the gas in galaxy-forming
regions was preheated to high entropy by vigorous energy feedback associated
with the formation of stars in old ellipticals and bulges and with AGN
activity. Such preheating likely occurred at redshifts z ~ 2-3, and can produce
the entropy excess observed today in low-mass clusters of galaxies without
destroying the bulk of the Lyman alpha forest. Subsequent galaxy formation is
affected by the preheating, because the gas no longer follows the dark matter
on galaxy scales. The hot gas around galaxy haloes has very shallow profiles
and emits only weakly in the X-ray. Cooling in a preheated halo is not
inside-out, because the cooling efficiency does not change significantly with
radius. Only part of the gas in a protogalaxy region can cool and be accreted
into the final galaxy halo. The accreted gas is likely in diffuse clouds and so
does not lose angular momentum to the dark matter. Cluster ellipticals are
produced by mergers of stellar systems formed prior to the preheating, while
large galaxy disks form in low-density environments where gas accretion can
continue to the present time.Comment: 11 pages, 7 figures, MNRAS submitte
The Atomic to Molecular Transition and its Relation to the Scaling Properties of Galaxy Disks in the Local Universe
We extend existing semi-analytic models of galaxy formation to track atomic
and molecular gas in disk galaxies. Simple recipes for processes such as
cooling, star formation, supernova feedback, and chemical enrichment of the
stars and gas are grafted on to dark matter halo merger trees derived from the
Millennium Simulation. Each galactic disk is represented by a series of
concentric rings. We assume that surface density profile of infalling gas in a
dark matter halo is exponential, with scale radius r_d that is proportional to
the virial radius of the halo times its spin parameter . As the dark
matter haloes grow through mergers and accretion, disk galaxies assemble from
the inside out. We include two simple prescriptions for molecular gas formation
processes in our models: one is based on the analytic calculations by Krumholz,
McKee & Tumlinson (2008), and the other is a prescription where the H_2
fraction is determined by the kinematic pressure of the ISM. Motivated by the
observational results of Leroy et al. (2008), we adopt a star formation law in
which in the regime where the molecular gas
dominates the total gas surface density, and where atomic hydrogen dominates. We then fit these models to
the radial surface density profiles of stars, HI and H_2 drawn from recent high
resolution surveys of stars and gas in nearby galaxies. We explore how the
ratios of atomic gas, molecular gas and stellar mass vary as a function of
global galaxy scale parameters, including stellar mass, stellar surface
density, and gas surface density. We elucidate how the trends can be understood
in terms of three variables that determine the partition of baryons in disks:
the mass of the dark matter halo, the spin parameter of the halo, and the
amount of gas recently accreted from the external environment.Comment: Made some minor changes according to the reviewer's suggestion.
Accepted by MNRA
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