4,797 research outputs found
Assembly Bias and Splashback in Galaxy Clusters
We use publicly available data for the Millennium Simulation to explore the
implications of the recent detection of assembly bias and splashback signatures
in a large sample of galaxy clusters. These were identified in the SDSS/DR8
photometric data by the redMaPPer algorithm and split into high- and
low-concentration subsamples based on the projected positions of cluster
members. We use simplified versions of these procedures to build cluster
samples of similar size from the simulation data. These match the observed
samples quite well and show similar assembly bias and splashback signals.
Previous theoretical work has found the logarithmic slope of halo density
profiles to have a well-defined minimum whose depth decreases and whose radius
increases with halo concentration. Projected profiles for the observed and
simulated cluster samples show trends with concentration which are opposite to
these predictions. In addition, for high-concentration clusters the minimum
slope occurs at significantly smaller radius than predicted. We show that these
discrepancies all reflect confusion between splashback features and features
imposed on the profiles by the cluster identification and concentration
estimation procedures. The strong apparent assembly bias is not reflected in
the three-dimensional distribution of matter around clusters. Rather it is a
consequence of the preferential contamination of low-concentration clusters by
foreground or background groups.Comment: 17 pages, 16 figures, 3 tables, accepted versio
Diffuse light in z~0.25 galaxy clusters: constraining tidal damage and the faint end of the Luminosity Function
The starlight coming from the intergalactic space in galaxy clusters and
groups witnesses the violent tidal interactions that galaxies experience in
these dense environments. Such interactions may be (at least partly)
responsible for the transformation of normal star-forming galaxies into passive
dwarf ellipticals (dEs). In this contribution we present the first systematic
study of the IntraCluster Light (ICL) for a statistically representative sample
(Zibetti et al. 2005), which comprises 683 clusters selected between z=0.2 and
0.3 from ~1500 deg^2 in the SDSS. Their ICL is studied by stacking the images
in the g-, r-, and i-band after masking out all galaxies and polluting sources.
In this way a very uniform background illumination is obtained, that allows us
to measure surface brightnesses as faint as 31 mag/arcsec^2 and to trace the
ICL out to 700 kpc from the central galaxy. We find that the local fraction of
light contributed by intracluster stars rapidly decreases as a function of the
clustercentric distance, from ~40% at 100 kpc to ~5% at 500 kpc. By comparing
the distribution and colours of the ICL and of the clusters galaxies, we find
indication that the main source of ICL are the stars stripped from galaxies
that plunge deeply into the cluster potential well along radial orbits. Thus,
if dEs are the remnants of these stripped progenitors we should expect similar
radial orbital anisotropies and correlations between the dE luminosity function
and the amount of ICL in different clusters. The diffuse emission we measure is
contaminated by faint unresolved galaxies: this makes our flux estimate depend
to some extent on the assumed luminosity function, but, on the other hand,
allows us to constrain the number of faint galaxies. Our present results
disfavour steep (alpha<-1.35) faint-end powerlaw slopes.Comment: 4 pages, 2 figures, use iauc.cls. Oral presentation to appear in the
proceedings of "IAU Colloquium 198 - Near-Field Cosmology with Dwarf
Elliptical Galaxies", Les Diablerets 14-18 March 2005, B. Binggeli and H.
Jerjen ed
Galaxy growth in the concordance CDM cosmology
We use galaxy and dark halo data from the public database for the Millennium
Simulation to study the growth of galaxies in the De Lucia et al. (2006) model
for galaxy formation. Previous work has shown this model to reproduce many
aspects of the systematic properties and the clustering of real galaxies, both
in the nearby universe and at high redshift. It assumes the stellar masses of
galaxies to increase through three processes, major mergers, the accretion of
smaller satellite systems, and star formation. We show the relative importance
of these three modes to be a strong function of stellar mass and of redshift.
Galaxy growth through major mergers depends strongly on stellar mass, but only
weakly on redshift. Except for massive systems, minor mergers contribute more
to galaxy growth than major mergers at all redshifts and at all stellar masses.
For galaxies significantly less massive than the Milky Way, star formation
dominates the growth at all epochs. For galaxies significantly more massive
than the Milky Way, growth through mergers is the dominant process at all
epochs. At a stellar mass of , star formation dominates
at and mergers at later times. At every stellar mass, the growth rates
through star formation increase rapidly with increasing redshift. Specific star
formation rates are a decreasing function of stellar mass not only at but
also at all higher redshifts. For comparison, we carry out a similar analysis
of the growth of dark matter halos. In contrast to the galaxies, growth rates
depend strongly on redshift, but only weakly on mass. They agree qualitatively
with analytic predictions for halo growth.Comment: 11 pages, 6 figure
How Rare is the Bullet Cluster?
The galaxy cluster 1E 0657-56 has a bullet-like subcluster that is moving
away from the centre of the main cluster at high speed. Markevitch et al.
(2004) recently estimated a relative velocity of V_bullet = 4500 +1100/-800
km/s, based on observations of the bow shock in front of the subcluster. The
weak lensing analysis of Clowe et al. (2004) indicates that a substantial
secondary mass peak is associated with this subcluster. We estimate the
likelihood of such a configuration by examining the distribution of subhalo
velocities for clusters in the Millennium Run, a large LCDM cosmological
simulation. We find that the most massive subhalo has a velocity as high as
that of the bullet subcluster in only about 1 out of every 100 cluster-sized
halos. This estimate is strongly dependent on the precise velocity adopted for
the bullet. One of the ten most massive subhalos has such a high velocity about
40% of the time. We conclude that the velocity of the bullet subcluster is not
exceptionally high for a cluster substructure, and can be accommodated within
the currently favoured LCDM comogony.Comment: 5 pages, 3 figures, accepted for publication in MNRA
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