3,156 research outputs found
Structural Properties of Central Galaxies in Groups and Clusters
Using a representative sample of 911 central galaxies (CENs) from the SDSS
DR4 group catalogue, we study how the structure of the most massive members in
groups and clusters depend on (1) galaxy stellar mass (Mstar), (2) dark matter
halo mass of the host group (Mhalo), and (3) their halo-centric position. We
establish and thoroughly test a GALFIT-based pipeline to fit 2D Sersic models
to SDSS data. We find that the fitting results are most sensitive to the
background sky level determination and strongly recommend using the SDSS global
value. We find that uncertainties in the background translate into a strong
covariance between the total magnitude, half-light size (r50), and Sersic index
(n), especially for bright/massive galaxies. We find that n depends strongly on
Mstar for CENs, but only weakly or not at all on Mhalo. Less (more) massive
CENs tend to be disk (spheroid)-like over the full Mhalo range. Likewise, there
is a clear r50-Mstar relation for CENs, with separate slopes for disks and
spheroids. When comparing CENs with satellite galaxies (SATs), we find that low
mass (<10e10.75 Msun/h^2) SATs have larger median n than CENs of similar Mstar.
Low mass, late-type SATs have moderately smaller r50 than late-type CENs of the
same Mstar. However, we find no size differences between spheroid-like CENs and
SATs, and no structural differences between CENs and SATs matched in both mass
and colour. The similarity of massive SATs and CENs shows that this distinction
has no significant impact on the structure of spheroids. We conclude that Mstar
is the most fundamental property determining the basic structure of a galaxy.
The lack of a clear n-Mhalo relation rules out a distinct group mass for
producing spheroids, and the responsible morphological transformation processes
must occur at the centres of groups spanning a wide range of masses. (abridged)Comment: 22 pages, 14 figures, submitted to MNRA
Design and Test of a Forward Neutron Calorimeter for the ZEUS Experiment
A lead scintillator sandwich sampling calorimeter has been installed in the
HERA tunnel 105.6 m from the central ZEUS detector in the proton beam
direction. It is designed to measure the energy and scattering angle of
neutrons produced in charge exchange ep collisions. Before installation the
calorimeter was tested and calibrated in the H6 beam at CERN where 120 GeV
electrons, muons, pions and protons were made incident on the calorimeter. In
addition, the spectrum of fast neutrons from charge exchange proton-lucite
collisions was measured. The design and construction of the calorimeter is
described, and the results of the CERN test reported. Special attention is paid
to the measurement of shower position, shower width, and the separation of
electromagnetic showers from hadronic showers. The overall energy scale as
determined from the energy spectrum of charge exchange neutrons is compared to
that obtained from direct beam hadrons.Comment: 45 pages, 22 Encapsulated Postscript figures, submitted to Nuclear
Instruments and Method
Cell transformation assays for prediction of carcinogenic potential: State of the science and future research needs
Copyright @ 2011 The Authors. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0), which permits
unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.Cell transformation assays (CTAs) have long been proposed as in vitro methods for the identification of potential chemical carcinogens. Despite showing good correlation with rodent bioassay data, concerns over the subjective nature of using morphological criteria for identifying transformed cells and a lack of understanding of the mechanistic basis of the assays has limited their acceptance for regulatory purposes. However, recent drivers to find alternative carcinogenicity assessment methodologies, such as the Seventh Amendment to the EU Cosmetics Directive, have fuelled renewed interest in CTAs. Research is currently ongoing to improve the objectivity of the assays, reveal the underlying molecular changes leading to transformation and explore the use of novel cell types. The UK NC3Rs held an international workshop in November 2010 to review the current state of the art in this field and provide directions for future research. This paper outlines the key points highlighted at this meeting
Identification of tidal features in deep optical galaxy images with Convolutional Neural Networks
Interactions between galaxies leave distinguishable imprints in the form of
tidal features which hold important clues about their mass assembly.
Unfortunately, these structures are difficult to detect because they are low
surface brightness features so deep observations are needed. Upcoming surveys
promise several orders of magnitude increase in depth and sky coverage, for
which automated methods for tidal feature detection will become mandatory. We
test the ability of a convolutional neural network to reproduce human visual
classifications for tidal detections. We use as training 6000 simulated
images classified by professional astronomers. The mock Hyper Suprime Cam
Subaru (HSC) images include variations with redshift, projection angle and
surface brightness ( =26-35 mag arcsec). We obtain
satisfactory results with accuracy, precision and recall values of Acc=0.84,
P=0.72 and R=0.85, respectively, for the test sample. While the accuracy and
precision values are roughly constant for all surface brightness, the recall
(completeness) is significantly affected by image depth. The recovery rate
shows strong dependence on the type of tidal features: we recover all the
images showing shell features and 87% of the tidal streams; these fractions are
below 75% for mergers, tidal tails and bridges. When applied to real HSC
images, the performance of the model worsens significantly. We speculate that
this is due to the lack of realism of the simulations and take it as a warning
on applying deep learning models to different data domains without prior
testing on the actual data.Comment: 13 pages, 10 figures, accepted for publication in MNRA
Mid-Infrared Galaxy Luminosity Functions from the AGN and Galaxy Evolution Survey
We present galaxy luminosity functions at 3.6, 4.5, 5.8, and 8.0 micron
measured by combining photometry from the IRAC Shallow Survey with redshifts
from the AGN and Galaxy Evolution Survey of the NOAO Deep Wide-Field Survey
Bootes field. The well-defined IRAC samples contain 3800-5800 galaxies for the
3.6-8.0 micron bands with spectroscopic redshifts and z < 0.6. We obtained
relatively complete luminosity functions in the local redshift bin of z < 0.2
for all four IRAC channels that are well fit by Schechter functions. We found
significant evolution in the luminosity functions for all four IRAC channels
that can be fit as an evolution in M* with redshift, \Delta M* = Qz. While we
measured Q=1.2\pm0.4 and 1.1\pm0.4 in the 3.6 and 4.5 micron bands consistent
with the predictions from a passively evolving population, we obtained
Q=1.8\pm1.1 in the 8.0 micron band consistent with other evolving star
formation rate estimates. We compared our LFs with the predictions of
semi-analytical galaxy formation and found the best agreement at 3.6 and 4.5
micron, rough agreement at 8.0 micron, and a large mismatch at 5.8 micron.
These models also predicted a comparable Q value to our luminosity functions at
8.0 micron, but predicted smaller values at 3.6 and 4.5 micron. We also
measured the luminosity functions separately for early and late-type galaxies.
While the luminosity functions of late-type galaxies resemble those for the
total population, the luminosity functions of early-type galaxies in the 3.6
and 4.5 micron bands indicate deviations from the passive evolution model,
especially from the measured flat luminosity density evolution. Combining our
estimates with other measurements in the literature, we found (53\pm18)% of the
present stellar mass of early-type galaxies has been assembled at z=0.7.Comment: 39 pages, 15 figures, submitted to ApJ (revised following the referee
report
Feedback and Recycled Wind Accretion: Assembling the z=0 Galaxy Mass Function
We analyse cosmological hydrodynamic simulations that include
observationally-constrained prescriptions for galactic outflows. If these
simulated winds accurately represent winds in the real Universe, then material
previously ejected in winds provides the dominant source of gas infall for new
star formation at redshifts z<1. This recycled wind accretion, or wind mode,
provides a third physically distinct accretion channel in addition to the "hot"
and "cold" modes emphasised in recent theoretical studies. Because of the
interaction between outflows and gas in and around halos, the recycling
timescale of wind material (t_rec) is shorter in higher-mass systems, which
reside in denser gaseous environments. In these simulations, this differential
recycling plays a central role in shaping the present-day galaxy stellar mass
function (GSMF). If we remove all particles that were ever ejected in a wind,
then the predicted GSMFs are much steeper than observed; galaxy masses are
suppressed both by the direct removal of gas and by the hydrodynamic heating of
their surroundings, which reduces subsequent infall. With wind recycling
included, the simulation that incorporates our favoured momentum-driven wind
scalings reproduces the observed GSMF for stellar masses 10^9 < M < 5x10^10
Msolar. At higher masses, wind recycling leads to excessive galaxy masses and
excessive star formation rates relative to observations. In these massive
systems, some quenching mechanism must suppress the re-accretion of gas ejected
from star-forming galaxies. In short, as has long been anticipated, the form of
the GSMF is governed by outflows; the unexpected twist here for our simulated
winds is that it is not primarily the ejection of material but how the ejected
material is re-accreted that governs the GSMF.Comment: 16 pages, 7 figures, accepted by MNRA
The IFMIF-DONES Project: Design Status and Main Achievements Within the EUROfusion FP8 Work Programme
International Fusion Materials Irradiation Facility-DEMO-Oriented NEutron Source (IFMIF-DONES) is a high-intensity neutron irradiation facility for qualification of fusion reactor materials, which is being designed as part of the European roadmap to fusion-generated electricity. Its main purpose is to study the behavior of materials properties under irradiation in a neutron flux able to simulate the same effects in terms of relevant nuclear responses as those expected in the first wall of the DEMO reactor which is envisaged to follow ITER. It is thus a key facility to support the design, licensing and safe operation of DEMO as well as of the fusion power plants that will be developed afterwards. The start of its construction is foreseen in the next few years. In this contribution, an overview of the IFMIF-DONES neutron source is presented together with a snapshot of the current engineering design status and of the relevant key results achieved within the EUROfusion Work Package Early Neutron Source (WPENS) as part of the 2014–2020 EURATOM Research and Training Programme, complementary to the EU Horizon 2020 Framework Programme (FP8). Moreover, some information on the future developments of the project are given
Balanced Schnyder woods for planar triangulations: an experimental study with applications to graph drawing and graph separators
In this work we consider balanced Schnyder woods for planar graphs, which are
Schnyder woods where the number of incoming edges of each color at each vertex
is balanced as much as possible. We provide a simple linear-time heuristic
leading to obtain well balanced Schnyder woods in practice. As test
applications we consider two important algorithmic problems: the computation of
Schnyder drawings and of small cycle separators. While not being able to
provide theoretical guarantees, our experimental results (on a wide collection
of planar graphs) suggest that the use of balanced Schnyder woods leads to an
improvement of the quality of the layout of Schnyder drawings, and provides an
efficient tool for computing short and balanced cycle separators.Comment: Appears in the Proceedings of the 27th International Symposium on
Graph Drawing and Network Visualization (GD 2019
Clues from nearby galaxies to a better theory of cosmic evolution
The great advances in the network of cosmological tests show that the
relativistic Big Bang theory is a good description of our expanding universe.
But the properties of nearby galaxies that can be observed in greatest detail
suggest a still better theory would more rapidly gather matter into galaxies
and groups of galaxies. This happens in theoretical ideas now under discussion.Comment: published in Natur
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