180 research outputs found
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
On the Discrepancy between Theoretical and X-Ray Concentration-Mass Relations for Galaxy Clusters
[Abridged] In the past 15 years, the concentration-mass relation has been
investigated diffusely in theoretical studies. On the other hand, only recently
has this relation been derived from X-ray observations. When that happened, the
results caused a certain level of concern: the X-ray normalizations and slopes
were found significantly dissimilar from those predicted by theory.
We analyzed 52 objects, simulated each time with different physical recipes
for the baryonic component, as well as 60 synthetic X-ray images, to determine
if these discrepancies are real or artificial. In particular, we investigate
how the simulated concentration-mass relation depends (1) on the radial range
used to derive the concentration, (2) on the presence of baryons in the
simulations, and on the prescription used to reproduce the gas. Finally, we
evaluate (3) how the results differ when adopting an X-ray approach for the
analysis and (4) how the selection functions based on X-ray luminosity can
impact the results. All effects studied go in the direction of alleviating the
discrepancy between observations and simulations, although with different
significance: while the fitting radial range and the baryonic component play
only a minor role, the X-ray approach and selection function have profound
repercussion on the resulting concentration-mass relation.Comment: 15 pages, 11 figures, 3 tables, ApJ in press. Significant extension
of the study of the selection-function influence and more attentive treatment
of errors (results unchanged
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
Improving fast generation of halo catalogs with higher-order Lagrangian perturbation theory
We present the latest version of Pinocchio, a code that generates catalogues
of DM haloes in an approximate but fast way with respect to an N-body
simulation. This code version extends the computation of particle and halo
displacements up to 3rd-order Lagrangian Perturbation Theory (LPT), in contrast
with previous versions that used Zeldovich approximation (ZA).
We run Pinocchio on the same initial configuration of a reference N-body
simulation, so that the comparison extends to the object-by-object level. We
consider haloes at redshifts 0 and 1, using different LPT orders either for
halo construction - where displacements are needed to decide particle accretion
onto a halo or halo merging - or to compute halo final positions.
We compare the clustering properties of Pinocchio haloes with those from the
simulation by computing the power spectrum and 2-point correlation function
(2PCF) in real and redshift space (monopole and quadrupole), the bispectrum and
the phase difference of halo distributions. We find that 2LPT and 3LPT give
noticeable improvement. 3LPT provides the best agreement with N-body when it is
used to displace haloes, while 2LPT gives better results for constructing
haloes. At the highest orders, linear bias is typically recovered at a few per
cent level.
In Fourier space and using 3LPT for halo displacements, the halo power
spectrum is recovered to within 10 per cent up to Mpc. The
results presented in this paper have interesting implications for the
generation of large ensemble of mock surveys aimed at accurately compute
covariance matrices for clustering statistics.Comment: 20 pages, 20 figures, submitted to 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
Cosmological simulations of black hole growth: AGN luminosities and downsizing
In this study, we present a detailed, statistical analysis of black hole
growth and the evolution of active galactic nuclei (AGN) using cosmological
hydrodynamic simulations run down to . The simulations self-consistently
follow radiative cooling, star formation, metal enrichment, black hole growth
and associated feedback processes from both supernovae typeII/Ia and AGN. We
consider two simulation runs, one with a large co-moving volume of $(500\
\mathrm{Mpc})^3(68\ \mathrm{Mpc})^3z=3.0z=3-4$. We also perform
a direct comparison with the observed soft and hard X-ray luminosity functions
of AGN, including an empirical correction for a torus-level obscuration, and
find a similarly good agreement. These results nicely demonstrate that the
observed "anti-hierarchical" trend in the AGN number density evolution (i.e.
the number densities of luminous AGN peak at higher redshifts than those of
faint AGN) is self-consistently predicted by our simulations. Implications of
this downsizing behaviour on active black holes, their masses and
Eddington-ratios are discussed. Overall, the downsizing behaviour in the AGN
number density as a function of redshift can be mainly attributed to the
evolution of the gas density in the resolved vicinity of a (massive) black
hole. (shortened)Comment: 24 pages, 15 figures, 1 table, accepted for publication in MNRAS, the
analysis is updated using a simulation run with a cosmological volume of
(500Mpc)^3 containing 2*1,564^3 particle
- …