122 research outputs found
An improved SPH scheme for cosmological simulations
We present an implementation of smoothed particle hydrodynamics (SPH) with
improved accuracy for simulations of galaxies and the large-scale structure. In
particular, we combine, implement, modify and test a vast majority of SPH
improvement techniques in the latest instalment of the GADGET code. We use the
Wendland kernel functions, a particle wake-up time-step limiting mechanism and
a time-dependent scheme for artificial viscosity, which includes a high-order
gradient computation and shear flow limiter. Additionally, we include a novel
prescription for time-dependent artificial conduction, which corrects for
gravitationally induced pressure gradients and largely improves the SPH
performance in capturing the development of gas-dynamical instabilities. We
extensively test our new implementation in a wide range of hydrodynamical
standard tests including weak and strong shocks as well as shear flows,
turbulent spectra, gas mixing, hydrostatic equilibria and self-gravitating gas
clouds. We jointly employ all modifications; however, when necessary we study
the performance of individual code modules. We approximate hydrodynamical
states more accurately and with significantly less noise than standard SPH.
Furthermore, the new implementation promotes the mixing of entropy between
different fluid phases, also within cosmological simulations. Finally, we study
the performance of the hydrodynamical solver in the context of radiative galaxy
formation and non-radiative galaxy cluster formation. We find galactic disks to
be colder, thinner and more extended and our results on galaxy clusters show
entropy cores instead of steadily declining entropy profiles. In summary, we
demonstrate that our improved SPH implementation overcomes most of the
undesirable limitations of standard SPH, thus becoming the core of an efficient
code for large cosmological simulations.Comment: 21 figures, 2 tables, accepted to MNRA
Cool Core Clusters from Cosmological Simulations
We present results obtained from a set of cosmological hydrodynamic
simulations of galaxy clusters, aimed at comparing predictions with
observational data on the diversity between cool-core (CC) and non-cool-core
(NCC) clusters. Our simulations include the effects of stellar and AGN feedback
and are based on an improved version of the smoothed particle hydrodynamics
code GADGET-3, which ameliorates gas mixing and better captures gas-dynamical
instabilities by including a suitable artificial thermal diffusion. In this
Letter, we focus our analysis on the entropy profiles, the primary diagnostic
we used to classify the degree of cool-coreness of clusters, and on the iron
profiles. In keeping with observations, our simulated clusters display a
variety of behaviors in entropy profiles: they range from steadily decreasing
profiles at small radii, characteristic of cool-core systems, to nearly flat
core isentropic profiles, characteristic of non-cool-core systems. Using
observational criteria to distinguish between the two classes of objects, we
find that they occur in similar proportions in both simulations and in
observations. Furthermore, we also find that simulated cool-core clusters have
profiles of iron abundance that are steeper than those of NCC clusters, which
is also in agreement with observational results. We show that the capability of
our simulations to generate a realistic cool-core structure in the cluster
population is due to AGN feedback and artificial thermal diffusion: their
combined action allows us to naturally distribute the energy extracted from
super-massive black holes and to compensate for the radiative losses of
low-entropy gas with short cooling time residing in the cluster core.Comment: 6 pages, 4 figures, accepted in ApJL, v2 contains some modifications
on the text (results unchanged
Properties of the diffuse X-ray background in a high-resolution hydrodynamical simulation
We study the properties of the diffuse X-ray background by using the results
of a cosmological hydrodynamical simulation of the concordance LambdaCDM model.
The simulation follows gravitational and gas dynamics and includes a treatment
of physical processes like radiative cooling, star formation and supernova
feedback. From the simulation outputs, we produce a set of two-dimensional maps
of the intergalactic medium X-ray emission integrated over redshift. We find
that the signal in the soft (0.5-2 keV) band is lognormally distributed with a
mean intensity of about 4 10^-12 erg s^-1 cm^-2 deg^-2; approximately 40 per
cent of the emission originates from warm-hot gas (defined as baryons with
10^5<T<10^7 K), and 90 per cent comes from structures at z<0.9. Since the
spectrum is soft, being mostly provided by the intergalactic medium at low
temperature, the total mean intensity in the hard (2-10 keV) X-ray band is
smaller by a factor of about 4. In order to constrain the physical processes
included in our simulation, we compare our results with the observed upper
limit (1.2 +/- 0.3) 10^-12 erg s^-1 cm^-2 deg^-2 of the soft X-ray emission due
to diffuse gas. To this purpose, we remove the contributions of observable
extended objects (groups and clusters of galaxies) from the simulated maps by
adopting different detectability criteria which are calibrated on the
properties of systems at intermediate redshifts observed by Chandra. We show
that the simulated diffuse soft X-ray emission is consistent with the present
observed upper limit. However, if future measurements will decrease the level
of the unresolved X-ray background by a factor of two, a more efficient
feedback mechanism should be required to suppress the soft emission of the gas
residing within filaments and group-size haloes.Comment: 10 pages, 9 figures. Accepted for pubblication in MNRAS. Added 2 new
sections and other minor changes due to referee repor
Cosmological hydrodynamical simulations of galaxy clusters: X-ray scaling relations and their evolution
We analyse cosmological hydrodynamical simulations of galaxy clusters to
study the X-ray scaling relations between total masses and observable
quantities such as X-ray luminosity, gas mass, X-ray temperature, and .
Three sets of simulations are performed with an improved version of the
smoothed particle hydrodynamics GADGET-3 code. These consider the following:
non-radiative gas, star formation and stellar feedback, and the addition of
feedback by active galactic nuclei (AGN). We select clusters with , mimicking the typical selection of
Sunyaev-Zeldovich samples. This permits to have a mass range large enough to
enable robust fitting of the relations even at . The results of the
analysis show a general agreement with observations. The values of the slope of
the mass-gas mass and mass-temperature relations at are 10 per cent lower
with respect to due to the applied mass selection, in the former case,
and to the effect of early merger in the latter. We investigate the impact of
the slope variation on the study of the evolution of the normalization. We
conclude that cosmological studies through scaling relations should be limited
to the redshift range , where we find that the slope, the scatter, and
the covariance matrix of the relations are stable. The scaling between mass and
is confirmed to be the most robust relation, being almost independent of
the gas physics. At higher redshifts, the scaling relations are sensitive to
the inclusion of AGNs which influences low-mass systems. The detailed study of
these objects will be crucial to evaluate the AGN effect on the ICM.Comment: 24 pages, 11 figures, 5 tables, replaced to match accepted versio
Diffuse light and galaxy interactions in the core of nearby clusters
The kinematics of the diffuse light in the densest regions of the nearby
clusters can be unmasked using the planetary nebulae (PNs) as probes of the
stellar motions. The position-velocity diagrams around the brightest cluster
galaxies (BCGs) identify the relative contributions from the outer halos and
the intracluster light (ICL), defined as the light radiated by the stars
floating in the cluster potential. The kinematics of the ICL can then be used
to asses the dynamical status of the nearby cluster cores and to infer their
formation histories. The cores of the Virgo and Coma are observed to be far
from equilibrium, with mergers currently on-going, while the ICL properties in
the Fornax and Hydra clusters show the presence of sub-components being
accreted in their cores, but superposed to an otherwise relaxed population of
stars. Finally the comparison of the observed ICL properties with those
predicted from Lambda-CDM simulations indicates a qualitative agreement and
provides insights on the ICL formation. Both observations and simulations
indicate that BCG halos and ICL are physically distinct components, with the
``hotter" ICL dominating at large radial distances from the BCGs halos as the
latter become progressively fainter.Comment: 14 pages, 5 figures. Invited review to appear in the proceedings of
"Galaxies and their masks" eds. Block, D.L., Freeman, K.C. and Puerari, I.,
2010, Springer (New York
Cluster Correlation in Mixed Models
We evaluate the dependence of the cluster correlation length r_c on the mean
intercluster separation D_c, for three models with critical matter density,
vanishing vacuum energy (Lambda = 0) and COBE normalized: a tilted CDM (tCDM)
model (n=0.8) and two blue mixed models with two light massive neutrinos
yielding Omega_h = 0.26 and 0.14 (MDM1 and MDM2, respectively). All models
approach the observational value of sigma_8 (and, henceforth, the observed
cluster abundance) and are consistent with the observed abundance of Damped
Lyman_alpha systems. Mixed models have a motivation in recent results of
neutrino physics; they also agree with the observed value of the ratio
sigma_8/sigma_25, yielding the spectral slope parameter Gamma, and nicely fit
LCRS reconstructed spectra. We use parallel AP3M simulations, performed in a
wide box (side 360/h Mpc) and with high mass and distance resolution, enabling
us to build artificial samples of clusters, whose total number and mass range
allow to cover the same D_c interval inspected through APM and Abell cluster
clustering data. We find that the tCDM model performs substantially better than
n=1 critical density CDM models. Our main finding, however, is that mixed
models provide a surprisingly good fit of cluster clustering data.Comment: 22 pages + 10 Postscript figures. Accepted for publication in Ap
Mismatch between X-ray and emission-weighted temperatures in galaxy clusters: cosmological implications
The thermal properties of hydrodynamical simulations of galaxy clusters are
usually compared to observations by relying on the emission-weighted
temperature T_ew, instead of on the spectroscopic X-ray temperature T_spec,
which is obtained by actual observational data. In a recent paper Mazzotta et
al. show that, if the cluster is thermally complex, T_ew fails at reproducing
T_spec, and propose a new formula, the spectroscopic-like temperature, T_sl,
which approximates T_spec better than a few per cent. By analyzing a set of
hydrodynamical simulations of galaxy clusters, we find that T_sl is lower than
T_ew by 20-30 per cent. As a consequence, the normalization of the M-T_sl
relation from the simulations is larger than the observed one by about 50 per
cent. If masses in simulated clusters are estimated by following the same
assumptions of hydrostatic equilibrium and \beta--model gas density profile, as
often done for observed clusters, then the M-T relation decreases by about 40
per cent, and significantly reduces its scatter. Based on this result, we
conclude that using the observed M-T relation to infer the amplitude of the
power spectrum from the X-ray temperature function could bias low \sigma_8 by
10-20 per cent. This may alleviate the tension between the value of \sigma_8
inferred from the cluster number density and those from cosmic microwave
background and large scale structure.Comment: Submitted for publication in APJL; 4 pages, 3 color figure
The effect of dwarf galaxies disruption in semi-analytic models
We present results for a galaxy formation model that includes a simple
treatment for the disruption of dwarf galaxies by gravitational forces and
galaxy encounters within galaxy clusters. This is implemented a posteriori in a
semi-analytic model by considering the stability of cluster dark matter
sub-haloes at z=0. We assume that a galaxy whose dark matter substructure has
been disrupted will itself disperse, while its stars become part of the
population of intracluster stars responsible for the observed intracluster
light. Despite the simplicity of this assumption, our results show a
substantial improvement over previous models and indicate that the inclusion of
galaxy disruption is indeed a necessary ingredient of galaxy formation models.
We find that galaxy disruption suppresses the number density of dwarf galaxies
by about a factor of two. This makes the slope of the faint end of the galaxy
luminosity function shallower, in agreement with observations. In particular,
the abundance of faint, red galaxies is strongly suppressed. As a result, the
luminosity function of red galaxies and the distinction between the red and the
blue galaxy populations in colour-magnitude relationships are correctly
predicted. Finally, we estimate a fraction of intracluster light comparable to
that found in clusters of galaxies.Comment: 7 pages, 6 figures, accepted for publication in MNRAS, 2 figures
changed and references adde
The Diffuse Light in Simulations of Galaxy Clusters
We study the properties of the diffuse light in galaxy clusters forming in a
large hydrodynamical cosmological simulation of the Lambda-CDM cosmology. The
simulation includes a model for radiative cooling, star formation in dense cold
gas, and feedback by SN-II explosions. We select clusters having mass M>10^(14)
h^(-1) Msun and study the spatial distribution of their star particles. While
most stellar light is concentrated in gravitationally bound galaxies orbiting
in the cluster potential, we find evidence for a substantial diffuse component,
which may account for the extended halos of light observed around central cD
galaxies. We find that more massive simulated clusters have a larger fraction
of stars in the diffuse light than the less massive ones. The intracluster
light is more centrally concentrated than the galaxy light, and the stars in
the diffuse component are on average older than the stars in cluster galaxies,
supporting the view that the diffuse light is not a random sampling of the
stellar population in the cluster galaxies. We thus expect that at least ~10%
of the stars in a cluster may be distributed as intracluster light, largely
hidden thus far due to its very low surface brightness.Comment: 4 pages, 3 figure
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