145 research outputs found
XMM-Newton/SDSS: star formation efficiency in galaxy clusters and constraints on the matter density parameter
It is believed that the global baryon content of clusters of galaxies is
representative of the matter distribution of the universe, and can, therefore,
be used to reliably determine the matter density parameter Omega_m. This
assumption is challenged by the growing evidence from optical and X-ray
observations that the total baryon mass fraction increases towards rich
clusters. In this context, we investigate the dependence of stellar, and total
baryon mass fractions as a function of mass. To do so, we used a subsample of
nineteen clusters extracted from the X-ray flux limited sample HIFLUGCS that
have available DR-7 Sloan Digital Sky Survey (SDSS) data. From the optical
analysis we derived the stellar masses. Using XMM-Newton we derived the gas
masses. Then, adopting a scaling relation we estimate the total masses. Adding
the gas and the stellar mass fractions we obtain the total baryonic content
that we find to increase with cluster mass, reaching 7-year Wilkinson Microwave
Anisotropy Probe (WMAP-7) prediction for clusters with M_500 = 1.6 x 10^{15}
M_sun. We observe a decrease of the stellar mass fraction (from 4.5% to ~1.0%)
with increasing total mass where our findings for the stellar mass fraction
agree with previous studies. This result suggests a difference in the number of
stars formed per unit of halo mass, though with a large scatter for low-mass
systems. That is, the efficiency of star formation varies on cluster scale that
lower mass systems are likely to have higher star formation efficiencies. It
follows immediately that the dependence of the stellar mass fraction on total
mass results in an increase of the mass-to-light ratio from lower to higher
mass systems. We also discuss the consequences of these results in the context
of determining the cosmic matter density parameter Omega_m.Comment: Accepted for publication in ApJ, 11 pages, 5 figures.
http://stacks.iop.org/0004-637X/743/1
Stellar population and the origin of intra-cluster stars around brightest cluster galaxies: the case of NGC 3311
Context. We investigate the stellar population and the origin of diffuse
light around brightest cluster galaxies.
Aims. We study the stellar population of the dynamically hot stellar halo of
NGC 3311, the brightest galaxy in the Hydra I cluster, and that of photometric
substructures in the diffuse light to constrain the origin of these components.
Methods. We analyze absorption lines in medium-resolution, long-slit spectra
in the wavelength range 4800-5800 angstrom obtained with FORS2 at the Very
Large Telescope. We measure the equivalent width of Lick indices out to 20 kpc
from the center of NGC 3311 and fit them with stellar population models that
account for the [alpha/Fe] overabundance.
Results. Stars in the dynamically hot halo of NGC 3311 are old (age > 13
Gyr), metal-poor ([Z/H] ~ -0.35), and alpha-enhanced ([alpha/Fe] ~ 0.48).
Together with the high velocity dispersion, these measurements indicate that
the stars in the halo were accreted from the outskirts of other early-type
galaxies, with a possible contribution from dwarf galaxies. We identify a
region in the halo of NGC 3311 associated with a photometric substructure where
the stellar population is even more metal-poor ([Z/H] ~ -0.73). In this region,
our measurements are consistent with a composite stellar population superposed
along the line of sight, consisting of stars from the dynamically hot halo of
NGC 3311 and stars stripped from dwarf galaxies. The latter component
contributes < 28% to the local surface brightness.
Conclusions. The build-up of diffuse light around NGC 3311 is on-going. Based
on the observed stellar population properties, the dominant part of these stars
may have come from the outskirts of bright early-type galaxies, while stars
from stripped dwarf galaxies are presently being added.Comment: 8 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysic
The effect of dwarf galaxies disruption in semi-analytic models
We present results for a galaxy formation model that includes a simple
treatment for the disruption of dwarf galaxies by gravitational forces and
galaxy encounters within galaxy clusters. This is implemented a posteriori in a
semi-analytic model by considering the stability of cluster dark matter
sub-haloes at z=0. We assume that a galaxy whose dark matter substructure has
been disrupted will itself disperse, while its stars become part of the
population of intracluster stars responsible for the observed intracluster
light. Despite the simplicity of this assumption, our results show a
substantial improvement over previous models and indicate that the inclusion of
galaxy disruption is indeed a necessary ingredient of galaxy formation models.
We find that galaxy disruption suppresses the number density of dwarf galaxies
by about a factor of two. This makes the slope of the faint end of the galaxy
luminosity function shallower, in agreement with observations. In particular,
the abundance of faint, red galaxies is strongly suppressed. As a result, the
luminosity function of red galaxies and the distinction between the red and the
blue galaxy populations in colour-magnitude relationships are correctly
predicted. Finally, we estimate a fraction of intracluster light comparable to
that found in clusters of galaxies.Comment: 7 pages, 6 figures, accepted for publication in MNRAS, 2 figures
changed and references adde
Baryon fractions in clusters of galaxies: evidence against a preheating model for entropy generation
The Millennium Gas project aims to undertake smoothed-particle hydrodynamic
resimulations of the Millennium Simulation, providing many hundred massive
galaxy clusters for comparison with X-ray surveys (170 clusters with kTsl > 3
keV). This paper looks at the hot gas and stellar fractions of clusters in
simulations with different physical heating mechanisms. These fail to reproduce
cool-core systems but are successful in matching the hot gas profiles of
non-cool-core clusters. Although there is immense scatter in the observational
data, the simulated clusters broadly match the integrated gas fractions within
r500 . In line with previous work, however, they fare much less well when
compared to the stellar fractions, having a dependence on cluster mass that is
much weaker than is observed. The evolution with redshift of the hot gas
fraction is much larger in the simulation with early preheating than in one
with continual feedback; observations favour the latter model. The strong
dependence of hot gas fraction on cluster physics limits its use as a probe of
cosmological parameters.Comment: 16 pages, 18 figures, 4 tables. Accepted for publication in MNRA
The Science Case for an Extended Spitzer Mission
Although the final observations of the Spitzer Warm Mission are currently
scheduled for March 2019, it can continue operations through the end of the
decade with no loss of photometric precision. As we will show, there is a
strong science case for extending the current Warm Mission to December 2020.
Spitzer has already made major impacts in the fields of exoplanets (including
microlensing events), characterizing near Earth objects, enhancing our
knowledge of nearby stars and brown dwarfs, understanding the properties and
structure of our Milky Way galaxy, and deep wide-field extragalactic surveys to
study galaxy birth and evolution. By extending Spitzer through 2020, it can
continue to make ground-breaking discoveries in those fields, and provide
crucial support to the NASA flagship missions JWST and WFIRST, as well as the
upcoming TESS mission, and it will complement ground-based observations by LSST
and the new large telescopes of the next decade. This scientific program
addresses NASA's Science Mission Directive's objectives in astrophysics, which
include discovering how the universe works, exploring how it began and evolved,
and searching for life on planets around other stars.Comment: 75 pages. See page 3 for Table of Contents and page 4 for Executive
Summar
The Las Campanas/AAT Rich Cluster Survey III: Spectroscopic Studies of X-ray Bright Galaxy Clusters at z~0.1
[abridged] We present the analysis of the spectroscopic and photometric
catalogues of 11 X-ray luminous clusters at z=0.07-0.16 from the Las Campanas /
Anglo-Australian Telescope Rich Cluster Survey. Our spectroscopic dataset
consists of over 1600 galaxy cluster members, of which two thirds are outside
r_200. We assign cluster membership using a detailed mass model and expand on
our previous work on the cluster colour-magnitude relation where membership was
inferred statistically. We confirm that the modal colours of galaxies on the
colour magnitude relation become progressively bluer with increasing radius and
decreasing local galaxy density. Interpreted as an age effect, we hypothesize
that these trends in galaxy colour should be reflected in mean Hdelta
equivalent width. We confirm that passive galaxies in the cluster increase in
Hdelta line strength as dHdelta / d r_p = 0.35 +/- 0.06. A variation of star
formation rate, as measured by [OII], with increasing local density of the
environment is discernible and is shown to be in broad agreement with previous
studies from 2dFGRS and SDSS. We find that clusters at z~0.1 are less active
than their higher redshift analogues. We also investigate unusual populations
of blue and very red nonstarforming galaxies and we suggest that the former are
likely to be the progenitors of galaxies which will lie on the colour-magnitude
relation, while the colours of the latter possibly reflect dust reddening. The
cluster galaxies at large radii consist of both backsplash ones and those that
are infalling to the cluster for the first time. We make a comparison to the
field population at z~0.1 and examine broad differences between the two
populations. Individually, the clusters show significant variation in their
galaxy populations which reflects their recent infall histories.Comment: 25 pages, 16 figures. Accepted for publication in MNRA
Transiting Exoplanet Studies and Community Targets for JWST's Early Release Science Program
The James Webb Space Telescope will revolutionize transiting exoplanet
atmospheric science due to its capability for continuous, long-duration
observations and its larger collecting area, spectral coverage, and spectral
resolution compared to existing space-based facilities. However, it is unclear
precisely how well JWST will perform and which of its myriad instruments and
observing modes will be best suited for transiting exoplanet studies. In this
article, we describe a prefatory JWST Early Release Science (ERS) program that
focuses on testing specific observing modes to quickly give the community the
data and experience it needs to plan more efficient and successful future
transiting exoplanet characterization programs. We propose a multi-pronged
approach wherein one aspect of the program focuses on observing transits of a
single target with all of the recommended observing modes to identify and
understand potential systematics, compare transmission spectra at overlapping
and neighboring wavelength regions, confirm throughputs, and determine overall
performances. In our search for transiting exoplanets that are well suited to
achieving these goals, we identify 12 objects (dubbed "community targets") that
meet our defined criteria. Currently, the most favorable target is WASP-62b
because of its large predicted signal size, relatively bright host star, and
location in JWST's continuous viewing zone. Since most of the community targets
do not have well-characterized atmospheres, we recommend initiating preparatory
observing programs to determine the presence of obscuring clouds/hazes within
their atmospheres. Measurable spectroscopic features are needed to establish
the optimal resolution and wavelength regions for exoplanet characterization.
Other initiatives from our proposed ERS program include testing the instrument
brightness limits and performing phase-curve observations.(Abridged)Comment: This is a white paper that originated from an open discussion at the
Enabling Transiting Exoplanet Science with JWST workshop held November 16 -
18, 2015 at STScI (http://www.stsci.edu/jwst/science/exoplanets). Accepted
for publication in PAS
The impact of dust on the scaling properties of galaxy clusters
We investigate the effect of dust on the scaling properties of galaxy
clusters based on hydrodynamic N-body simulations of structure formation. We
have simulated five dust models plus a radiative cooling and adiabatic models
using the same initial conditions for all runs. The numerical implementation of
dust was based on the analytical computations of Montier and Giard (2004). We
set up dust simulations to cover different combinations of dust parameters that
put in evidence the effects of size and abundance of dust grains. Comparing our
radiative plus dust cooling runs to a purely radiative cooling simulation we
find that dust has an impact on cluster scaling relations. It mainly affects
the normalisation of the scalings (and their evolution), whereas it introduces
no significant differences on their slopes. The strength of the effect depends
critically on the dust abundance and grain size parameters as well as on the
cluster scaling. Indeed, cooling due to dust is effective at the cluster regime
and has a stronger effect on the "baryon driven" statistical properties of
clusters such as , , scaling relations. Major
differences, relative to the radiative cooling model, are as high as 25% for
the normalisation, and about 10% for the and
normalisations at redshift zero. On the other hand, we find that dust has
almost no impact on the "dark matter driven" scaling relation.
The effects are found to be dependent in equal parts on both dust abundances
and grain sizes distributions for the scalings investigated in this paper.
Higher dust abundances and smaller grain sizes cause larger departures from the
radiative cooling (i.e. with no dust) model.Comment: 12 pages, 6 figures, submitted to MNRA
Transmission Spectroscopy for the Warm Sub-Neptune HD 3167c: Evidence for Molecular Absorption and a Possible High-metallicity Atmosphere
We present a transmission spectrum for the warm (500−600 K) sub-Neptune HD 3167c obtained using the Hubble Space Telescope Wide Field Camera 3 infrared spectrograph. We combine these data, which span the 1.125–1.643 μm wavelength range, with broadband transit measurements made using Kepler/K2 (0.6–0.9 μm) and Spitzer/IRAC (4–5 μm). We find evidence for absorption by at least one of H₂O, HCN, CO₂, and CH₄ (Bayes factor 7.4; 2.5σ significance), although the data precision does not allow us to unambiguously discriminate between these molecules. The transmission spectrum rules out cloud-free hydrogen-dominated atmospheres with metallicities ≤100× solar at >5.8σ confidence. In contrast, good agreement with the data is obtained for cloud-free models assuming metallicities >700× solar. However, for retrieval analyses that include the effect of clouds, a much broader range of metallicities (including subsolar) is consistent with the data, due to the degeneracy with cloud-top pressure. Self-consistent chemistry models that account for photochemistry and vertical mixing are presented for the atmosphere of HD 3167c. The predictions of these models are broadly consistent with our abundance constraints, although this is primarily due to the large uncertainties on the latter. Interior structure models suggest that the core mass fraction is >40%, independent of a rock or water core composition, and independent of atmospheric envelope metallicity up to 1000× solar. We also report abundance measurements for 15 elements in the host star, showing that it has a very nearly solar composition
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