173,968 research outputs found
Quasars at z=6: the survival of the fittest
The Sloan Digital Sky survey detected luminous quasars at very high redshift,
z>6. Follow-up observations indicated that at least some of these quasars are
powered by supermassive black holes (SMBHs) with masses in excess of billion
solar masses. SMBHs, therefore, seem to have already existed when the Universe
was less than 1 Gyr old, and the bulk of galaxy formation still has to take
place. We investigate in this paper to which extent accretion and dynamical
processes influence the early growth of SMBHs. We assess the impact of (i)
black hole mergers, (ii) the influence of the merging efficiency and (iii) the
negative contribution due to dynamical effects which can kick black holes out
of their host halos (gravitational recoil). We find that if accretion is always
limited by the Eddington rate via a thin disc, the maximum radiative efficiency
allowed to reproduce the LF at z=6 is of order 12%, when the adverse effect of
the gravitational recoil is taken into consideration. Dynamical effects cannot
be neglected in studies of high-redshift SMBHs. If black holes can accrete at
super-critical rate during an early phase, reproducing the observed SMBH mass
values is not an issue, even in the case that the recoil velocity is in the
upper limits range, as the mass ratios of merging binaries are skewed towards
low values, where the gravitational recoil effect is very mild. We propose that
SMBH growth at early times is very selective, and efficient only for black
holes hosted in high density peak halos.Comment: Accepted for publication in the ApJ. 9 pages, 6 b/w figure
Iron abundance distribution in the hot gas of merging galaxy clusters
We present XMM-Newton/EPIC observations of six merging galaxy clusters and
study the distributions of their temperature, iron (Fe) abundance and
pseudo-entropy along the merging axis. For the first time, we focus
simultaneously, and in a comprehensive way, on the chemical and thermodynamic
properties of the freshly collided intracluster medium (ICM). The Fe
distribution of these clusters along the merging axis is found to be in good
agreement with the azimuthally-averaged Fe abundance profile in typical
non-cool-core clusters out to . In addition to showing a moderate
central abundance peak, though less pronounced than in relaxed systems, the Fe
abundance flattens at large radii towards 0.2-0.3 . Although
this shallow metal distribution is in line with the idea that disturbed,
non-cool-core clusters originate from the merging of relaxed, cool-core
clusters, we find that in some cases, remnants of metal-rich and low entropy
cool cores can persist after major mergers. While we obtain a mild
anti-correlation between the Fe abundance and the pseudo-entropy in the (lower
entropy, = 200-500 keV cm) inner regions, no clear correlation is found
at (higher entropy, = 500-2300 keV cm) outer radii. The apparent
spatial abundance uniformity that we find at large radii is difficult to
explain through an efficient mixing of freshly injected metals, particularly in
systems for which the time since the merger is short. Instead, our results
provide important additional evidence in favour of the early enrichment
scenario - in which the bulk of the metals are released outside galaxies at
> 2-3 - and extend it from cool-core and (moderate) non-cool-core clusters to a
few of the most disturbed merging clusters as well. These results constitute a
first step towards a deeper understanding of the chemical history of merging
clusters.Comment: Accepted for publication in A&A, 21 pages with 17 figures and 19
table
The impact of cluster mergers on arc statistics
We study the impact of merger events on the strong lensing properties of
galaxy clusters. Previous lensing simulations were not able to resolve
dynamical time scales of cluster lenses, which arise on time scales which are
of order a Gyr. In this case study, we first describe qualitatively with an
analytic model how some of the lensing properties of clusters are expected to
change during merging events. We then analyse a numerically simulated lens
model for the variation in its efficiency for producing both tangential and
radial arcs while a massive substructure falls onto the main cluster body. We
find that: (1) during the merger, the shape of the critical lines and caustics
changes substantially; (2) the lensing cross sections for long and thin arcs
can grow by one order of magnitude and reach their maxima when the extent of
the critical curves is largest; (3) the cross section for radial arcs also
grows, but the cluster can efficiently produce this kind of arcs only while the
merging substructure crosses the main cluster centre; (4) while the arc cross
sections pass through their maxima as the merger proceeds, the cluster's X-ray
emission increases by a factor of . Thus, we conclude that accounting
for these dynamical processes is very important for arc statistics studies. In
particular, they may provide a possible explanation for the arc statistics
problem.Comment: 16 pages, submitted to MNRAS, revised version after referee'
Comments. Gzipped file including full resolution images can be downloaded at
http://dipastro.pd.astro.it/~cosmo/massimo/high-res-images.tar.g
Efficient Constellation-Based Map-Merging for Semantic SLAM
Data association in SLAM is fundamentally challenging, and handling ambiguity
well is crucial to achieve robust operation in real-world environments. When
ambiguous measurements arise, conservatism often mandates that the measurement
is discarded or a new landmark is initialized rather than risking an incorrect
association. To address the inevitable `duplicate' landmarks that arise, we
present an efficient map-merging framework to detect duplicate constellations
of landmarks, providing a high-confidence loop-closure mechanism well-suited
for object-level SLAM. This approach uses an incrementally-computable
approximation of landmark uncertainty that only depends on local information in
the SLAM graph, avoiding expensive recovery of the full system covariance
matrix. This enables a search based on geometric consistency (GC) (rather than
full joint compatibility (JC)) that inexpensively reduces the search space to a
handful of `best' hypotheses. Furthermore, we reformulate the commonly-used
interpretation tree to allow for more efficient integration of clique-based
pairwise compatibility, accelerating the branch-and-bound max-cardinality
search. Our method is demonstrated to match the performance of full JC methods
at significantly-reduced computational cost, facilitating robust object-based
loop-closure over large SLAM problems.Comment: Accepted to IEEE International Conference on Robotics and Automation
(ICRA) 201
Metal enrichment of the intra-cluster medium over a Hubble time for merging and relaxed galaxy clusters
We investigate the efficiency of galactic mass loss, triggered by
ram-pressure stripping and galactic winds of cluster galaxies, on the chemical
enrichment of the intra-cluster medium (ICM). We combine N-body and
hydrodynamic simulations with a semi-numerical galaxy formation model. By
including simultaneously different enrichment processes, namely ram-pressure
stripping and galactic winds, in galaxy-cluster simulations, we are able to
reproduce the observed metal distribution in the ICM. We find that the mass
loss by galactic winds in the redshift regime z>2 is ~10% to 20% of the total
galactic wind mass loss, whereas the mass loss by ram-pressure stripping in the
same epoch is up to 5% of the total ram-pressure stripping mass loss over the
whole simulation time. In the cluster formation epochs z<2 ram-pressure
stripping becomes more dominant than galactic winds. We discuss the
non-correlation between the evolution of the mean metallicity of galaxy
clusters and the galactic mass losses. For comparison with observations we
present two dimensional maps of the ICM quantities and radial metallicity
profiles. The shape of the observed profiles is well reproduced by the
simulations in the case of merging systems. In the case of cool-core clusters
the slope of the observed profiles are reproduced by the simulation at radii
below ~300 kpc, whereas at larger radii the observed profiles are shallower. We
confirm the inhomogeneous metal distribution in the ICM found in observations.
To study the robustness of our results, we investigate two different
descriptions for the enrichment process interaction.Comment: 11 pages, 13 figures, accepted for publication in A&A, high
resolution version can be found at
<http://astro.uibk.ac.at/~wolfgang/kapferer.pdf
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