72 research outputs found
How does our choice of observable influence our estimation of the centre of a galaxy cluster? Insights from cosmological simulations
Galaxy clusters are an established and powerful test-bed for theories of both
galaxy evolution and cosmology. Accurate interpretation of cluster observations
often requires robust identification of the location of the centre. Using a
statistical sample of clusters drawn from a suite of cosmological simulations
in which we have explored a range of galaxy formation models, we investigate
how the location of this centre is affected by the choice of observable -
stars, hot gas, or the full mass distribution as can be probed by the
gravitational potential. We explore several measures of cluster centre: the
minimum of the gravitational potential, which would expect to define the centre
if the cluster is in dynamical equilibrium; the peak of the density; the centre
of BCG; and the peak and centroid of X-ray luminosity. We find that the centre
of BCG correlates more strongly with the minimum of the gravitational potential
than the X-ray defined centres, while AGN feedback acts to significantly
enhance the offset between the peak X-ray luminosity and minimum gravitational
potential. These results highlight the importance of centre identification when
interpreting clusters observations, in particular when comparing theoretical
predictions and observational data.Comment: 11 pages, 6 figures, MNRAS accepte
On the dynamical state of galaxy clusters: insights from cosmological simulations - II.
Using a suite of cosmology simulations of a sample of >120 galaxy clusters with log (MDM, vir) 64 14.5. We compare clusters that form in purely dark matter (DM) run and their counterparts in hydro-runs and investigate four independent parameters that are normally used to classify dynamical state. We find that the virial ratio \u3b7 in hydro-dynamical runs is \u2dc10 per cent lower than in the DM run, and there is no clear separation between the relaxed and unrelaxed clusters for any parameter. Further, using the velocity dispersion deviation parameter \u3b6, which is defined as the ratio between cluster velocity dispersion \u3c3 and the theoretical prediction \u3c3 _t = 1a{G M_{total}/R}, we find that there is a linear correlation between the virial ratio \u3b7 and this \u3b6 parameter. We propose to use this \u3b6 parameter, which can be easily derived from observed galaxy clusters, as a substitute of the \u3b7 parameter to quantify the cluster dynamical state
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
Intragroup diffuse light in compact groups of galaxies II. HCG 15, 35 and 51
This continuing study of intragroup light in compact groups of galaxies aims
to establish new constraints to models of formation and evolution of galaxy
groups, specially of compact groups, which are a key part in the evolution of
larger structures, such as clusters. In this paper we present three additional
groups (HCG 15, 35 and 51) using deep wide field and band images
observed with the LAICA camera at the 3.5m telescope at the Calar Alto
observatory (CAHA). This instrument provides us with very stable flatfielding,
a mandatory condition for reliably measuring intragroup diffuse light. The
images were analyzed with the OV\_WAV package, a wavelet technique that allows
us to uncover the intragroup component in an unprecedented way. We have
detected that 19, 15 and 26% of the total light of HCG 15, 35 and 51,
respectively, is in the diffuse component, with colours that are compatible
with old stellar populations and with mean surface brightness that can be as
low as . Dynamical masses, crossing times and
mass to light ratios were recalculated using the new group parameters. Also
tidal features were analyzed using the wavelet technique.Comment: 12 pages, 5 figures. Accepted for publication in MNRAS. See
http://www.eso.org/~cdarocha/publications/DaRochaetal2008_IGL_HCG.pdf for
full resolution version. Complementary reference adde
Diffuse light and building history of the galaxy cluster Abell 2667
We have searched for diffuse intracluster light in the galaxy cluster Abell
2667 (z=0.233) from HST images in three filters. We have applied to these
images an iterative multi-scale wavelet analysis and reconstruction technique,
which allows to subtract stars and galaxies from the original images. We detect
a zone of diffuse emission south west of the cluster center (DS1), and a second
faint object (ComDif), within DS1. Another diffuse source (DS2) may be
detected, at lower confidence level, north east of the center. These sources of
diffuse light contribute to 10-15% of the total visible light in the cluster.
Whether they are independent entities or are part of the very elliptical
external envelope of the central galaxy remains unclear. VLT VIMOS integral
field spectroscopy reveals a faint continuum at the positions of DS1 and ComDif
but do not allow to compute a redshift. A hierarchical substructure detection
method reveals the presence of several galaxy pairs and groups defining a
similar direction as the one drawn by the DS1-central galaxy-DS2 axis. The
analysis of archive XMM-Newton and Chandra observations shows X-ray emission
elongated in the same direction. The X-ray temperature map shows the presence
of a cool core, a broad cool zone stretching from north to south and hotter
regions towards the north east, south west and north west. This possibly
suggests shock fronts along these directions produced by infalling material.
These various data are consistent with a picture in which diffuse sources are
concentrations of tidal debris and harassed matter expelled from infalling
galaxies by tidal stripping and undergoing an accretion process onto the
central cluster galaxy; as such, they are expected to be found along the main
infall directions.Comment: Accepted for publication in Astronomy and Astrophysic
The optical/X-ray connection: ICM iron content and galaxy optical luminosity in 20 galaxy clusters
X-ray observations of galaxy clusters have shown that the intra-cluster gas
has iron abundances of about one third of the solar value. These observations
also show that part (if not all) of the intra-cluster gas metals were produced
within the member galaxies. We present a systematic analysis of 20 galaxy
clusters to explore the connection between the iron mass and the total
luminosity of early-type and late-type galaxies, and of the brightest cluster
galaxies (BCGs). From our results, the intra-cluster medium (ICM) iron mass
seems to correlate better with the luminosity of the BCGs than with that of the
red and blue galaxy populations. As the BCGs cannot produce alone the observed
amount of iron, we suggest that ram-pressure plus tidal stripping act together
to enhance, at the same time, the BCG luminosities and the iron mass in the
ICM. Through the analysis of the iron yield, we have also estimated that SN Ia
are responsible for more than 50% of the total iron in the ICM. This result
corroborates the fact that ram-pressure contributes to the gas removal from
galaxies to the inta-cluster medium, being very efficient for clusters in the
temperature range 2 < kT (keV)< 10Comment: Accepted for publication in MNRAS (11 pg, 9 figures and 3 tables
Mass assembly of galaxies: Smooth accretion versus mergers
Galaxies accrete their mass by means of both smooth accretion from the cosmic
web, and the mergers of smaller entities. We wish to quantify the respective
role of these two modes of accretion, which could determine the morphological
types of galaxies observed today. Multi-zoom cosmological simulations are used
to estimate as a function of time the evolution of mass in bound systems, for
dark matter as well as baryons. The baryonic contents of dark matter haloes are
studied. Merger histories are followed as a function of external density, and
the different ways in which mass is assembled in galaxies and the stellar
component accumulated are quantified. We find that most galaxies assemble their
mass through smooth accretion, and only the most massive galaxies also grow
significantly through mergers. The mean fraction of mass assembled by accretion
is 77 %, and by mergers 23 %. We present typical accretion histories of
hundreds of galaxies: masses of the most massive galaxies increase
monotonically in time, mainly through accretion, many intermediate-mass objects
also experience mass-loss events such as tidal stripping and evaporation.
However, our simulations suffer from the overcooling of massive galaxies caused
by the neglect of active galaxy nuclei (AGN) feedback. The time by which half
of the galay mass has assembled, both in dark matter and baryons, is a
decreasing function of mass, which is compatible with the observations of a
so-called downsizing. At every epoch in the universe, there are low-mass
galaxies actively forming stars, while more massive galaxies form their stars
over a shorter period of time within half the age of the universe.Comment: A&A Accepted, 19 pages, 17 figure
Fractals vs. halos: Asymptotic scaling without fractal properties
Precise analyses of the statistical and scaling properties of galaxy
distribution are essential to elucidate the large-scale structure of the
universe. Given the ongoing debate on its statistical features, the development
of statistical tools permitting to discriminate accurately different spatial
patterns is highly desiderable. This is specially the case when non-fractal
distributions have power law two-point correlation functions, which are usually
signatures of fractal properties. Here we review some possible methods used in
the literature and introduce a new variable called "scaling gradient". This
tool and the conditional variance are shown to be effective in providing an
unambiguous way for such a distinction. Their application is expected to be of
outmost importance in the analysis of upcoming galaxy catalogues.Comment: 7 pages, 3 figure
Metal enrichment processes
There are many processes that can transport gas from the galaxies to their
environment and enrich the environment in this way with metals. These metal
enrichment processes have a large influence on the evolution of both the
galaxies and their environment. Various processes can contribute to the gas
transfer: ram-pressure stripping, galactic winds, AGN outflows, galaxy-galaxy
interactions and others. We review their observational evidence, corresponding
simulations, their efficiencies, and their time scales as far as they are known
to date. It seems that all processes can contribute to the enrichment. There is
not a single process that always dominates the enrichment, because the
efficiencies of the processes vary strongly with galaxy and environmental
properties.Comment: 18 pages, 8 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 17; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
The probability of galaxy-galaxy strong lensing events in hydrodynamical simulations of galaxy clusters
Meneghetti et al. (2020) recently reported an excess of galaxy-galaxy strong
lensing (GGSL) in galaxy clusters compared to expectations from the LCDM
cosmological model. Theoretical estimates of the GGSL probability are based on
the analysis of numerical hydrodynamical simulations in the LCDM cosmology. We
quantify the impact of the numerical resolution and AGN feedback scheme adopted
in cosmological simulations on the predicted GGSL probability and determine if
varying these simulation properties can alleviate the gap with observations. We
repeat the analysis of Meneghetti et al. (2020) on cluster-size halos simulated
with different mass and force resolutions and implementing several independent
AGN feedback schemes. We find that improving the mass resolution by a factor of
ten and twenty-five, while using the same galaxy formation model that includes
AGN feedback, does not affect the GGSL probability. We find similar results
regarding the choice of gravitational softening. On the contrary, adopting an
AGN feedback scheme that is less efficient at suppressing gas cooling and star
formation leads to an increase in the GGSL probability by a factor between
three and six. However, we notice that such simulations form overly massive
subhalos whose contribution to the lensing cross-section would be significant
while their Einstein radii are too large to be consistent with the
observations. The primary contributors to the observed GGSL cross-sections are
subhalos with smaller masses, that are compact enough to become critical for
lensing. The population with these required characteristics appears to be
absent in simulations.Comment: 13 pages, 11 figures. Submitted for publication on Astronomy and
Astrophysic
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