388 research outputs found
Pointing to the minimum scatter: the generalized scaling relations for galaxy clusters
We introduce a generalized scaling law, M_tot = 10^K A^a B^b, to look for the
minimum scatter in reconstructing the total mass of hydrodynamically simulated
X-ray galaxy clusters, given gas mass M_gas, luminosity L and temperature T. We
find a locus in the plane of the logarithmic slopes and of the scaling
relations where the scatter in mass is minimized. This locus corresponds to b_M
= -3/2 a_M +3/2 and b_L = -2 a_L +3/2 for A=M_gas and L, respectively, and B=T.
Along these axes, all the known scaling relations can be identified (at
different levels of scatter), plus a new one defined as M_tot ~ (LT)^(1/2).
Simple formula to evaluate the expected evolution with redshift in the
self-similar scenario are provided. In this scenario, no evolution of the
scaling relations is predicted for the cases (b_M=0, a_M=1) and (b_L=7/2,
a_L=-1), respectively. Once the single quantities are normalized to the average
values of the sample under considerations, the normalizations K corresponding
to the region with minimum scatter are very close to zero. The combination of
these relations allows to reduce the number of free parameters of the fitting
function that relates X-ray observables to the total mass and includes the
self-similar redshift evolution.Comment: 6 pages, 3 figures. MNRAS in pres
Weighing simulated galaxy clusters using lensing and X-ray
We aim at investigating potential biases in lensing and X-ray methods to
measure the cluster mass profiles. We do so by performing realistic simulations
of lensing and X-ray observations that are subsequently analyzed using
observational techniques. The resulting mass estimates are compared among them
and with the input models. Three clusters obtained from state-of-the-art
hydrodynamical simulations, each of which has been projected along three
independent lines-of-sight, are used for this analysis. We find that strong
lensing models can be trusted over a limited region around the cluster core.
Extrapolating the strong lensing mass models to outside the Einstein ring can
lead to significant biases in the mass estimates, if the BCG is not modeled
properly for example. Weak lensing mass measurements can be largely affected by
substructures, depending on the method implemented to convert the shear into a
mass estimate. Using non-parametric methods which combine weak and strong
lensing data, the projected masses within R200 can be constrained with a
precision of ~10%. De-projection of lensing masses increases the scatter around
the true masses by more than a factor of two due to cluster triaxiality. X-ray
mass measurements have much smaller scatter (about a factor of two smaller than
the lensing masses) but they are generally biased low by 5-20%. This bias is
ascribable to bulk motions in the gas of our simulated clusters. Using the
lensing and the X-ray masses as proxies for the true and the hydrostatic
equilibrium masses of the simulated clusters and averaging over the cluster
sample we are able to measure the lack of hydrostatic equilibrium in the
systems we have investigated.Comment: 27 pages, 21 figures, accepted for publication on A&A. Version with
full resolution images can be found at
http://pico.bo.astro.it/~massimo/Public/Papers/massComp.pd
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
Measuring the dark matter velocity anisotropy in galaxy clusters
The Universe contains approximately 6 times more dark matter than normal
baryonic matter, and a directly observed fundamental difference between dark
matter and baryons would both be significant for our understanding of dark
matter structures and provide us with information about the basic
characteristics of the dark matter particle. We discuss one distinctive feature
of dark matter structures in equilibrium, namely the property that a local dark
matter temperature may depend on direction. This is in stark contrast to
baryonic gases. We used X-ray observations of two nearby, relaxed galaxy
clusters, under the assumptions of hydrostatic equilibrium and identical dark
matter and gas temperatures in the outer cluster region, to measure this dark
matter temperature anisotropy beta_dm, with non-parametric Monte Carlo methods.
We find that beta_dm is greater than the value predicted for baryonic gases,
beta_gas=0, at more than 3 sigma confidence. The observed value of the
temperature anisotropy is in fair agreement with the results of cosmological
N-body simulations and shows that the equilibration of the dark matter
particles is not governed by local point-like interactions in contrast to
baryonic gases.Comment: 5 pages, 3 figures, extended discussions, matches accepted versio
Thermal Conduction in Simulated Galaxy Clusters
We study the formation of clusters of galaxies using high-resolution
hydrodynamic cosmological simulations that include the effect of thermal
conduction with an effective isotropic conductivity of 1/3 the classical
Spitzer value. We find that, both for a hot ( keV) and
several cold ( keV) galaxy clusters, the baryonic fraction
converted into stars does not change significantly when thermal conduction is
included. However, the temperature profiles are modified, particularly in our
simulated hot system, where an extended isothermal core is readily formed. As a
consequence of heat flowing from the inner regions of the cluster both to its
outer parts and into its innermost resolved regions, the entropy profile is
altered as well. This effect is almost negligible for the cold cluster, as
expected based on the strong temperature dependence of the conductivity. Our
results demonstrate that while thermal conduction can have a significant
influence on the properties of the intra--cluster medium of rich galaxy
clusters, it appears unlikely to provide by itself a solution for the
overcooling problem in clusters, or to explain the current discrepancies
between the observed and simulated properties of the intra--cluster medium.Comment: 4 Pages, 3 Figures, Submitted to ApJ-Letter
Spectroscopic-Like Temperature of Clusters of Galaxies and 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. Here we show that, if the
intra-cluster medium is thermally complex, T_ew fails at reproducing T_spec. We
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 also find that T_sl is lower
than T_ew by 20-30 per cent. As a consequence, the normalization of the M-T
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: 6 pages, 3 figures, to appear in the proceedings of the Rencontres du
Vietnam "New Views on the Universe
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
On the Baryon Fractions in Clusters and Groups of Galaxies
We present the baryon fractions of 2MASS groups and clusters as a function of
cluster richness using total and gas masses measured from stacked ROSAT X-ray
data and stellar masses estimated from the infrared galaxy catalogs. We detect
X-ray emission even in the outskirts of clusters, beyond r_200 for richness
classes with X-ray temperatures above 1 keV. This enables us to more accurately
determine the total gas mass in these groups and clusters. We find that the
optically selected groups and clusters have flatter temperature profiles and
higher stellar-to-gas mass ratios than the individually studied, X-ray bright
clusters. We also find that the stellar mass in poor groups with temperatures
below 1 keV is comparable to the gas mass in these systems. Combining these
results with individual measurements for clusters, groups, and galaxies from
the literature, we find a break in the baryon fraction at ~1 keV. Above this
temperature, the baryon fraction scales with temperature as f_b \propto
T^0.20\pm0.03. We see significantly smaller baryon fractions below this
temperature, and the baryon fraction of poor groups joins smoothly onto that of
systems with still shallower potential wells such as normal and dwarf galaxies
where the baryon fraction scales with the inferred velocity dispersion as f_b
\propto \sigma^1.6. The small scatter in the baryon fraction at any given
potential well depth favors a universal baryon loss mechanism and a preheating
model for the baryon loss. The scatter is, however, larger for less massive
systems. Finally, we note that although the broken power-law relation can be
inferred from data points in the literature alone, the consistency between the
baryon fractions for poor groups and massive galaxies inspires us to fit the
two categories of objects (galaxies and clusters) with one relation.Comment: 21 pages, 5 figures, ApJ in pres
Substructures in hydrodynamical cluster simulations
The abundance and structure of dark matter subhalos has been analyzed
extensively in recent studies of dark matter-only simulations, but
comparatively little is known about the impact of baryonic physics on halo
substructures. We here extend the SUBFIND algorithm for substructure
identification such that it can be reliably applied to dissipative
hydrodynamical simulations that include star formation. This allows, in
particular, the identification of galaxies as substructures in simulations of
clusters of galaxies, and a determination of their content of gravitationally
bound stars, dark matter, and hot and cold gas. Using a large set of
cosmological cluster simulations, we present a detailed analysis of halo
substructures in hydrodynamical simulations of galaxy clusters, focusing in
particular on the influence both of radiative and non-radiative gas physics,
and of non-standard physics such as thermal conduction and feedback by galactic
outflows. We also examine the impact of numerical nuisance parameters such as
artificial viscosity parameterizations. We find that diffuse hot gas is
efficiently stripped from subhalos when they enter the highly pressurized
cluster atmosphere. This has the effect of decreasing the subhalo mass function
relative to a corresponding dark matter-only simulation. These effects are
mitigated in radiative runs, where baryons condense in the central subhalo
regions and form compact stellar cores. However, in all cases, only a very
small fraction, of the order of one percent, of subhalos within the cluster
virial radii preserve a gravitationally bound hot gaseous atmosphere.
(abridged)Comment: improved manuscript, to appear in MNRA
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