77 research outputs found
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
The effect of AGN feedback on the halo mass function
[Abridged.] We investigate baryon effects on the halo mass function (HMF),
with emphasis on the role played by AGN feedback. Halos are identified with
both Friends-of-Friends (FoF) and Spherical Overdensity (SO) algorithms. We
embed the standard SO algorithm into a memory-controlled frame program and
present the {\bf P}ython spher{\bf I}c{\bf A}l {\bf O}verdensity code ---
{\small PIAO}.
For both FoF and SO halos, the effect of AGN feedback is that of suppressing
the HMFs to a level even below that of Dark Matter simulations. The ratio
between the HMFs in the AGN and in the DM simulations is at
overdensity , a difference that increases at higher overdensity
, with no significant redshift and mass dependence. A decrease
of the halo masses ratio with respect to the DM case induces the decrease of
the HMF in the AGN simulation. The shallower inner density profiles of halos in
the AGN simulation witnesses that mass reduction is induced by the sudden
displacement of gas induced by thermal AGN feedback. We provide fitting
functions to describe halo mass variations at different overdensities, which
can recover the HMFs with a residual random scatter per cent for halo
masses larger than .Comment: 16 pages, 11 figures. Matches to MNRAS published version, typo
corrected in the fitting functio
The relation between velocity dispersion and mass in simulated clusters of galaxies: dependence on the tracer and the baryonic physics
[Abridged] We present an analysis of the relation between the masses of
cluster- and group-sized halos, extracted from CDM cosmological N-body
and hydrodynamic simulations, and their velocity dispersions, at different
redshifts from to . The main aim of this analysis is to understand
how the implementation of baryonic physics in simulations affects such
relation, i.e. to what extent the use of the velocity dispersion as a proxy for
cluster mass determination is hampered by the imperfect knowledge of the
baryonic physics. In our analysis we use several sets of simulations with
different physics implemented. Velocity dispersions are determined using three
different tracers, DM particles, subhalos, and galaxies.
We confirm that DM particles trace a relation that is fully consistent with
the theoretical expectations based on the virial theorem and with previous
results presented in the literature. On the other hand, subhalos and galaxies
trace steeper relations, and with larger values of the normalization. Such
relations imply that galaxies and subhalos have a per cent velocity
bias relative to the DM particles, which can be either positive or negative,
depending on halo mass, redshift and physics implemented in the simulation.
We explain these differences as due to dynamical processes, namely dynamical
friction and tidal disruption, acting on substructures and galaxies, but not on
DM particles. These processes appear to be more or less effective, depending on
the halo masses and the importance of baryon cooling, and may create a
non-trivial dependence of the velocity bias and the \soneD--\Mtwo relation
on the tracer, the halo mass and its redshift.
These results are relevant in view of the application of velocity dispersion
as a proxy for cluster masses in ongoing and future large redshift surveys.Comment: 13 pages, 16 figures. Minor modifications to match the version in
press on MNRA
Simulating realistic disk galaxies with a novel sub-resolution ISM model
We present results of cosmological simulations of disk galaxies carried out
with the GADGET-3 TreePM+SPH code, where star formation and stellar feedback
are described using our MUlti Phase Particle Integrator (MUPPI) model. This
description is based on simple multi-phase model of the interstellar medium at
unresolved scales, where mass and energy flows among the components are
explicitly followed by solving a system of ordinary differential equations.
Thermal energy from SNe is injected into the local hot phase, so as to avoid
that it is promptly radiated away. A kinetic feedback prescription generates
the massive outflows needed to avoid the over-production of stars. We use two
sets of zoomed-in initial conditions of isolated cosmological halos with masses
(2-3) * 10^{12} Msun, both available at several resolution levels. In all cases
we obtain spiral galaxies with small bulge-over-total stellar mass ratios (B/T
\approx 0.2), extended stellar and gas disks, flat rotation curves and
realistic values of stellar masses. Gas profiles are relatively flat, molecular
gas is found to dominate at the centre of galaxies, with star formation rates
following the observed Schmidt-Kennicutt relation. Stars kinematically
belonging to the bulge form early, while disk stars show a clear inside-out
formation pattern and mostly form after redshift z=2. However, the baryon
conversion efficiencies in our simulations differ from the relation given by
Moster et al. (2010) at a 3 sigma level, thus indicating that our stellar disks
are still too massive for the Dark Matter halo in which they reside. Results
are found to be remarkably stable against resolution. This further demonstrates
the feasibility of carrying out simulations producing a realistic population of
galaxies within representative cosmological volumes, at a relatively modest
resolution.Comment: 19 pages, 21 figures, MNRAS accepte
Brightest cluster galaxies in cosmological simulations: achievements and limitations of AGN feedback models
We analyze the basic properties of Brightest Cluster Galaxies (BCGs) produced
by state of the art cosmological zoom-in hydrodynamical simulations. These
simulations have been run with different sub-grid physics included. Here we
focus on the results obtained with and without the inclusion of the
prescriptions for supermassive black hole (SMBH) growth and of the ensuing
Active Galactic Nuclei (AGN) feedback. The latter process goes in the right
direction of decreasing significantly the overall formation of stars. However,
BCGs end up still containing too much stellar mass, a problem that increases
with halo mass, and having an unsatisfactory structure. This is in the sense
that their effective radii are too large, and that their density profiles
feature a flattening on scales much larger than observed. We also find that our
model of thermal AGN feedback has very little effect on the stellar velocity
dispersions, which turn out to be very large. Taken together, these problems,
which to some extent can be recognized also in other numerical studies
typically dealing with smaller halo masses, indicate that on one hand present
day sub-resolution models of AGN feedback are not effective enough in
diminishing the global formation of stars in the most massive galaxies, but on
the other hand they are relatively too effective in their centers. It is likely
that a form of feedback generating large scale gas outflows from BCGs
precursors, and a more widespread effect over the galaxy volume, can alleviate
these difficulties.Comment: 17 pages, 14 figures, accepted for publication on MNRAS, comments
welcom
A warm mode of gas accretion on forming galaxies
We present results from high--resolution cosmological hydrodynamical
simulations of a Milky--Way-sized halo, aimed at studying the effect of
feedback on the nature of gas accretion. Simulations include a model of
inter-stellar medium and star formation, in which SN explosions provide
effective thermal feedback. We distinguish between gas accretion onto the halo,
which occurs when gas particles cross the halo virial radius, and gas accretion
onto the central galaxy, which takes place when gas particles cross the inner
one-tenth of the virial radius. Gas particles can be accreted through three
different channels, depending on the maximum temperature value, ,
reached during the particles' past evolution: a cold channel for K, and a warm one for
intermediate values of . We find that the warm channel is at least
as important as the cold one for gas accretion onto the central galaxy. This
result is at variance with previous findings that the cold mode dominates gas
accretion at high redshift. We ascribe this difference to the different
supernova feedback scheme implemented in our simulations. While results
presented so far in the literature are based on uneffective SN thermal feedback
schemes and/or the presence of a kinetic feedback, our simulations include only
effective thermal feedback. We argue that observational detections of a warm
accretion mode in the high--redshift circum-galactic medium would provide
useful constraints on the nature of the feedback that regulates star formation
in galaxies.Comment: 6 pages, 3 figures, accepted for publication in ApJ
Diffuse stellar component in galaxy clusters and the evolution of the most massive galaxies at z<~1
The high end of the stellar mass function of galaxies is observed to have
little evolution since z~1. This represents a stringent constraint for
merger--based models, aimed at explaining the evolution of the most massive
galaxies in the concordance LambdaCDM cosmology. In this Letter we show that it
is possible to remove the tension between the above observations and model
predictions by allowing a fraction of stars to be scattered to the Diffuse
Stellar Component (DSC) of galaxy clusters at each galaxy merger, as recently
suggested by the analysis of N-body hydrodynamical simulations. To this
purpose, we use the MORGANA model of galaxy formation in a minimal version, in
which gas cooling and star formation are switched off after z=1. In this way,
any predicted evolution of the galaxy stellar mass function is purely driven by
mergers. We show that, even in this extreme case, the predicted degree of
evolution of the high end of the stellar mass function is larger than that
suggested by data. Assuming instead that a significant fraction, ~30 per cent,
of stars are scattered in the DSC at each merger event, leads to a significant
suppression of the predicted evolution, in better agreement with observational
constraints, while providing a total amount of DSC in clusters which is
consistent with recent observational determinations.Comment: 5 pages, figures included; ApJ Letters, in press. Revision: reference
adde
Kinetic AGN Feedback Effects on Cluster Cool Cores Simulated using SPH
We implement novel numerical models of AGN feedback in the SPH code GADGET-3,
where the energy from a supermassive black hole (BH) is coupled to the
surrounding gas in the kinetic form. Gas particles lying inside a bi-conical
volume around the BH are imparted a one-time velocity (10,000 km/s) increment.
We perform hydrodynamical simulations of isolated cluster (total mass 10^14 /h
M_sun), which is initially evolved to form a dense cool core, having central
T<10^6 K. A BH resides at the cluster center, and ejects energy. The
feedback-driven fast wind undergoes shock with the slower-moving gas, which
causes the imparted kinetic energy to be thermalized. Bipolar bubble-like
outflows form propagating radially outward to a distance of a few 100 kpc. The
radial profiles of median gas properties are influenced by BH feedback in the
inner regions (r<20-50 kpc). BH kinetic feedback, with a large value of the
feedback efficiency, depletes the inner cool gas and reduces the hot gas
content, such that the initial cool core of the cluster is heated up within a
time 1.9 Gyr, whereby the core median temperature rises to above 10^7 K, and
the central entropy flattens. Our implementation of BH thermal feedback (using
the same efficiency as kinetic), within the star-formation model, cannot do
this heating, where the cool core remains. The inclusion of cold gas accretion
in the simulations produces naturally a duty cycle of the AGN with a
periodicity of 100 Myr.Comment: 22 pages, 11 figures, version accepted for publication in MNRAS,
references and minor revisions adde
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