70 research outputs found
Dark matter distribution in galaxy groups from combined strong lensing and dynamics analysis
Using a combined analysis of strong lensing and galaxy dynamics, we
characterize the mass distributions and M/L ratios of galaxy groups, which form
an important transition regime in Lambda-CDM cosmology. By mapping the
underlying mass distribution, we test whether groups are dark matter dominated
as hypothesized by the standard cosmogony, or isothermal as observed in baryon
rich field galaxies. We present our lensing + galaxy dynamics formalism built
around the dark matter dominant NFW and Hernquist distributions, compared
against the Isothermal Sphere observed in galaxy scale objects. We show that
mass measurement in the core of the group (r ~ 0.2 r_{vir}), determined jointly
from a lens model and from differential velocity dispersion estimates, may
effectively distinguish between these density distributions. We apply our
method to MOS observations of two groups, SL2SJ1430+5546 and SL2SJ1431+5533,
drawn from our CFHTLS lens catalog. With the measured lensing and dynamical
masses, combined with a maximum likelihood estimator built around our model, we
estimate the concentration index characterizing each density distribution and
the corresponding virial mass of each group. Our results indicate that both
groups are dark matter dominant, and reject the Isothermal distribution at >>3
sigma level. For both groups, the estimated i-band M/L ratios of ~260
Msun/Lsun, are similar to other published values for groups. The Gaussian
distributions of the velocities of their member galaxies support a high degree
of virialization. The differences in their virial masses, 2.8 and 1.6 x 10^14
Msun, and velocity dispersions, 720 and 560 km/s respectively, may indicate
however that each group is at a different stage of transition to a cluster. We
aim to populate this important transition regime with additional results from
ongoing observations of the remaining lensing groups in our catalog.Comment: ApJ in press; 29 pages, including 10 figures and 7 tables. Latex with
emulateapj v03/07/0
Human Cep192 Is Required for Mitotic Centrosome and Spindle Assembly
SummaryAs cells enter mitosis, centrosomes dramatically increase in size and ability to nucleate microtubules. This process, termed centrosome maturation, is driven by the accumulation and activation of γ-tubulin and other proteins that form the pericentriolar material on centrosomes during G2/prophase. Here, we show that the human centrosomal protein, Cep192 (centrosomal protein of 192 kDa), is an essential component of the maturation machinery. Specifically, we have found that siRNA depletion of Cep192 results in a complete loss of functional centrosomes in mitotic but not interphase cells. In mitotic cells lacking Cep192, microtubules become organized around chromosomes but rarely acquire stable bipolar configurations. These cells contain normal numbers of centrioles but cannot assemble γ-tubulin, pericentrin, or other pericentriolar proteins into an organized PCM. Alternatively, overexpression of Cep192 results in the formation of multiple, extracentriolar foci of γ-tubulin and pericentrin. Together, our findings support the hypothesis that Cep192 stimulates the formation of the scaffolding upon which γ-tubulin ring complexes and other proteins involved in microtubule nucleation and spindle assembly become functional during mitosis
4098 galaxy clusters to z~0.6 in the Sloan Digital Sky Survey equatorial Stripe 82
We present a catalogue of 4098 photometrically selected galaxy clusters with
a median redshift = 0.32 in the 270 square degree 'Stripe 82' region of the
Sloan Digital Sky Survey (SDSS), covering the celestial equator in the Southern
Galactic Cap (-50 < RA < 59 deg, |Dec| < 1.25 deg). Owing to the multi-epoch
SDSS coverage of this region, the ugriz photometry is ~2 magnitudes deeper than
single scans within the main SDSS footprint. We exploit this to detect clusters
of galaxies using an algorithm that searches for statistically significant
overdensities of galaxies in a Voronoi tessellation of the projected sky. 32%
of the clusters have at least one member with a spectroscopic redshift from
existing public data (SDSS Data Release 7, 2SLAQ & WiggleZ), and the remainder
have a robust photometric redshift (accurate to ~5-9% at the median redshift of
the sample). The weighted average of the member galaxies' redshifts provides a
reasonably accurate estimate of the cluster redshift. The cluster catalogue is
publicly available for exploitation by the community to pursue a range of
science objectives. In addition to the cluster catalogue, we provide a linked
catalogue of 18,295 V<21 mag quasar sight-lines with impact parameters within
<3 Mpc of the cluster cores selected from the catalogue of Veron et al. (2010).
The background quasars cover 0.25 < z < 2, where MgII absorption-line systems
associated with the clusters are detectable in optical spectra.Comment: 10 pages, 2 figures, accepted for publication in MNRAS. Catalogues
available at http://www.physics.mcgill.ca/~jimgeach/stripe8
The galaxy populations from the centers to the infall regions in z~0.25 clusters
We conducted a panoramic spectroscopic campaign with MOSCA at the Calar Alto
observatory. We acquired spectra of more than 500 objects. Approximately 150 of
these spectra were of galaxies that are members of six different clusters,
which differ in intrinsic X-ray luminosity. The wavelength range allows us to
quantify the star formation activity by using the OII and the Halpha lines.
This activity is examined in terms of the large-scale environment expressed by
the clustercentric distance of the galaxies as well as on local scales given by
the spatial galaxy densities. A global suppression of star-formation is
detected in the outskirts of clusters, at about 3Rvir. Galaxies with ongoing
star-formation have similar activity, regardless of the environment. Therefore,
the decline of the star-formation activity inside the investigated clusters is
driven mainly by the significant change in the fraction of active versus
passive populations. This suggests that the suppression of the star-formation
activity occurs on short timescales. We detect a significant population of red
star-forming galaxies whose colors are consistent with the red-sequence of
passive galaxies. They appear to be in an intermediate evolutionary stage
between active and passive types. Since a suppression of star-formation
activity is measured at large clustercentric distances and low projected
densities, purely cluster-specific phenomena cannot fully explain the observed
trends. Therefore, as suggested by other studies, group preprocessing may play
an important role in transforming galaxies before they enter into the cluster
environment. Since models predict that a significant fraction of galaxies
observed in the outskirts may have already transversed through the cluster
center, the effects of ram-pressure stripping cannot be neglected. (ABRIDGED)Comment: Revised version. Astronomy and Astrophysics in press. Important typo
correcte
Cosmological Constraints from the SDSS maxBCG Cluster Catalog
We use the abundance and weak lensing mass measurements of the SDSS maxBCG
cluster catalog to simultaneously constrain cosmology and the richness--mass
relation of the clusters. Assuming a flat \LambdaCDM cosmology, we find
\sigma_8(\Omega_m/0.25)^{0.41} = 0.832\pm 0.033 after marginalization over all
systematics. In common with previous studies, our error budget is dominated by
systematic uncertainties, the primary two being the absolute mass scale of the
weak lensing masses of the maxBCG clusters, and uncertainty in the scatter of
the richness--mass relation. Our constraints are fully consistent with the WMAP
five-year data, and in a joint analysis we find \sigma_8=0.807\pm 0.020 and
\Omega_m=0.265\pm 0.016, an improvement of nearly a factor of two relative to
WMAP5 alone. Our results are also in excellent agreement with and comparable in
precision to the latest cosmological constraints from X-ray cluster abundances.
The remarkable consistency among these results demonstrates that cluster
abundance constraints are not only tight but also robust, and highlight the
power of optically-selected cluster samples to produce precision constraints on
cosmological parameters.Comment: comments welcom
ROSAT PSPC observations of 36 high-luminosity clusters of galaxies: constraints on the gas fraction
We present a detailed and homogeneous analysis of the ROSAT PSPC surface
brightness profiles of 36 clusters of galaxies with high X-ray luminosity (L_X
> 10^{45} erg s^{-1}) and redshifts between 0.05 and 0.44. Using recent ASCA
estimates of the temperature of the gas for most of the clusters in the sample,
we apply both the deprojection technique and model fitting to the surface
brightness profiles to constrain the gas and dark matter distributions under
the assumption that the gas is both isothermal and hydrostatic.
Applying robust estimators, we find that the gas fraction within r_{500} of
the clusters in our sample has a distribution centred on f_gas(r_{500}) = 0.168
h_{50}^{-1.5}. The gas fraction ranges from 0.101 to 0.245 at the 95 per cent
confidence level. The values of f_gas show highly significant variations
between individual clusters, which may be explained if the dark matter has a
significant baryonic component. Within a cluster, the average radial dependence
of the gas mass fraction increases outward as r^s, with s~0.20. Combining these
results with those of primordial nucleosynthesis calculations and the current
estimate of H_0, the above central location implies \Omega_{0, m} < 0.56 at the
95 per cent confidence level. This upper limit decreases to 0.34 if we take the
highest significant estimates for f_gas.
A significant decrease in cluster gas fraction with redshift from the local
value, f_{gas, 0}, of 0.21, found assuming \Omega_{0, m} =1, is also reduced if
\Omega_{0, m} is low.Comment: 17 pages, 15 figures, MNRAS in press. Also available at
http://xalph3.ast.cam.ac.uk/~settori/paper.htm
New Constraints on the Complex Mass Substructure in Abell 1689 from Gravitational Flexion
In a recent publication, the flexion aperture mass statistic was found to
provide a robust and effective method by which substructure in galaxy clusters
might be mapped. Moreover, we suggested that the masses and mass profile of
structures might be constrained using this method. In this paper, we apply the
flexion aperture mass technique to HST ACS images of Abell 1689. We demonstrate
that the flexion aperture mass statistic is sensitive to small-scale structures
in the central region of the cluster. While the central potential is not
constrained by our method, due largely to missing data in the central
0.5 of the cluster, we are able to place constraints on the masses and
mass profiles of prominent substructures. We identify 4 separate mass peaks,
and use the peak aperture mass signal and zero signal radius in each case to
constrain the masses and mass profiles of these substructures. The three most
massive peaks exhibit complex small-scale structure, and the masses indicated
by the flexion aperture mass statistic suggest that these three peaks represent
the dominant substructure component of the cluster (). Their complex structure indicates that the cluster --
far from being relaxed -- may have recently undergone a merger. The smaller,
subsidiary peak is located coincident with a group of galaxies within the
cluster, with mass . These results are in
excellent agreement with previous substructure studies of this cluster.Comment: 18 pages, 10 figures, MNRAS accepted (7 Dec 2010
NoSOCS in SDSS. II. Mass Calibration of Low Redshift Galaxy Clusters with Optical and X-ray Properties
We use SDSS data to investigate the scaling relations of 127 NoSOCS and 56
CIRS galaxy clusters at low redshift (). We show that richness and
both optical and X-ray luminosities are reliable mass proxies. The scatter in
mass at fixed observable is 40%, depending on the aperture, sample and
observable considered. For example, for the massive CIRS systems
= 0.33 0.05 and = 0.48
0.06. For the full sample = 0.43 0.03 and
= 0.56 0.06. We estimate substructure using two and
three dimensional optical data, verifying that substructure has no significant
effect on the cluster scaling relations (intercepts and slopes), independent of
which substructure test we use. For a subset of twenty-one clusters, we
estimate masses from the M-T relation using temperature measures from BAX.
The scaling relations derived from the optical and X-ray masses are indeed very
similar, indicating that our method consistently estimates the cluster mass and
yields equivalent results regardless of the wavelength from which we measure
mass. For massive systems, we represent the mass-richness relation by a
function with the form , with
M being expressed in units of 10 M. Using the virial
mass, for CIRS clusters, we find A = (1.39 0.07) and B = (1.00
0.11). The relations based on the virial mass have a scatter of
= 0.37 0.05, while = 0.77
0.22 for the caustic mass and = 0.34 0.08
for the temperature based mass (abridged).Comment: 27 pages, 22 figures, 12 tables, Accepted to MNRA
Constraining the cosmological parameters with the gas mass fraction in local and z>0.7 Galaxy Clusters
We present a study of the baryonic fraction in galaxy clusters aimed at
constraining the cosmological parameters Omega_m, Omega_Lambda and the ratio
between the pressure and density of the ``dark'' energy, w. We use results on
the gravitating mass profiles of a sample of nearby galaxy clusters observed
with the BeppoSAX X-ray satellite (Ettori, De Grandi, Molendi 2002) to set
constraints on the dynamical estimate of Omega_m. We then analyze Chandra
observations of a sample of eight distant clusters with redshift in the range
0.72 and 1.27 and evaluate the geometrical limits on the cosmological
parameters Omega_m, Omega_Lambda and w by requiring that the gas fraction
remains constant with respect to the look-back time. By combining these two
independent probability distributions and using a priori distributions on both
Omega_b and H0 peaked around primordial nucleosynthesis and HST-Key Project
results respectively, we obtain that, at 95.4 per cent level of confidence, (i)
w < -0.49, (ii) Omega_m = 0.34^+0.11_-0.05, Omega_Lambda = 1.30^+0.44_-1.09 for
w=-1 (corresponding to the case for a cosmological constant), and (iii) Omega_m
= 1-Omega_Lambda = 0.33^+0.07_-0.05 for a flat Universe. These results are in
excellent agreement with the cosmic concordance scenario which combines
constraints from the power spectrum of the Cosmic Microwave Background, the
galaxy and cluster distribution, the evolution of the X-ray properties of
galaxy clusters and the magnitude-redshift relation for distant type Ia
supernovae. By combining our results with the latter method we further
constrain Omega_Lambda =0.94^+0.28_-0.32 and w < -0.89 at the 2 sigma level.Comment: 12 pages, A&A in press. Equation 7 has been correcte
The cluster gas mass fraction as a cosmological probe: a revised study
(Abriged) We present the analysis of the baryonic content of 52 X-ray
luminous galaxy clusters observed with Chandra in the redshift range 0.3-1.273.
We use the deprojected X-ray surface brightness profiles and the measured
values of the gas temperature to recover the gas and total mass profiles. By
assuming that galaxy clusters are representative of the cosmic baryon budget,
the distribution of the cluster baryon fraction in the hottest (T> 4 keV)
systems as a function of redshift is used to constrain the cosmological
parameters. We discuss how our constraints are affected by several systematics,
namely the isothermality, the assumed baryon fraction in stars, the depletion
parameter and the sample selection. By using only the cluster baryon fraction
as a proxy for the cosmological parameters, we obtain that Omega is very well
constrained at the value of 0.35 with a relative statistical uncertainty of 11%
(1 sigma level; w=-1) and a further systematic error of about (-6,+7)%. On the
other hand, constraints on Lambda (without the prior of flat geometry) and w
(using the prior of flat geometry) are definitely weaker due to the presence of
larger statistical and systematic uncertainties (of the order of 40 per cent on
Lambda and larger than 50 per cent on w). If the WMAP 5-year best-fit results
are assumed to fix the cosmological parameters, we limit the contributions
expected from non-thermal pressure support and ICM clumpiness to be lower than
about 10 per cent, leaving also room to accommodate baryons not accounted for
either in the X-ray emitting plasma or in stars of the order of 18 per cent of
the total cluster baryon budget.Comment: A&A in press. Accepted on March 28, 2009. Revised to match version in
prin
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