1,427 research outputs found
The NGC 5846 Group: Dynamics and the Luminosity Function to M_R=-12
We conduct a photometric and spectroscopic survey of a 10 sq. deg. region
surrounding the nearby NGC 5846 group of galaxies, using the
Canada-France-Hawaii and Keck I telescopes to study the population of dwarf
galaxies as faint as M_R=-10. Candidates are identified on the basis of
quantitative surface brightness and qualitative morphological criteria.
Spectroscopic follow up and a spatial correlation analysis provide the basis
for affirming group memberships. Altogether, 324 candidates are identified and
83 have spectroscopic membership confirmation. We argue on statistical grounds
that a total 251 +/- 10 galaxies in our sample are group members. The
observations, together with archival Sloan Digital Sky Survey, ROSAT,
XMM-Newton, and ASCA data, suggest that the giant ellipticals NGC 5846 and NGC
5813 are the dominant components of subgroups separated by 600 kpc in
projection and embedded in a 1.6 Mpc diameter dynamically evolved halo. The
galaxy population is overwhelmingly early type. The group velocity dispersion
is 322 km/s, its virial mass is 8.4 x 10^13 M_sun, and M/L_R = 320 M_sun/L_sun.
The ratio of dwarfs to giants is large compared with other environments in the
Local Supercluster studied and, correspondingly, the luminosity function is
relatively steep, with a faint end Schechter function slope of \alpha_d = -1.3
+/- 0.1 (statistical) +/- 0.1 (systematic) at our completeness limit of M_R =
-12.Comment: 17 pages; accepted for publication in the Astronomical Journa
The Orbital Structure of Dark Matter Halos with Gas
With the success of the Chandra and XMM missions and the maturation of
gravitational lensing techniques, powerful constraints on the orbital structure
of cluster dark matter halos are possible. I show that the X-ray emissivity and
mass of a galaxy cluster uniquely specify the anisotropy and velocity
dispersion profiles of its dark matter halo. I consider hydrostatic as well as
cooling flow scenarios, and apply the formalism to the lensing cluster
CL0024+16 and the cooling flow cluster Abell 2199. In both cases, the model
predicts a parameter-free velocity dispersion profile that is consistent with
independent optical redshift surveys of the clusters.Comment: 17 pages, 12 figures; to appear in the Astrophysical Journa
XMM-Newton and Gemini Observations of Eight RASSCALS Galaxy Groups
We study the distribution of gas pressure and entropy in eight groups of
galaxies belonging to the ROSAT All-Sky Survey / Center for Astrophysics Loose
Systems (RASSCALS). We use archival and proprietary XMM-Newton observations,
supplementing the X-ray data with redshifts derived from the literature; we
also list 127 new redshifts measured with the Gemini North telescope. The
groups show remarkable self-similarity in their azimuthally averaged entropy
and temperature profiles. The entropy increases with radius; the behavior of
the entropy profiles is consistent with an increasing broken power law with
inner and outer slope 0.92+0.04-0.05 and 0.42+0.05-0.04 (68% confidence),
respectively. There is no evidence of a central, isentropic core, and the
entropy distribution in most of the groups is flatter at large radii than in
the inner region, challenging earlier reports as well as theoretical models
predicting large isentropic cores or asymptotic slopes of 1.1 at large radii.
The pressure profiles are consistent with a self-similar decreasing broken
power law in radius; the inner and outer slopes are -0.78+0.04-0.03 and
-1.7+0.1-0.3, respectively. The results suggest that the larger scatter in the
entropy distribution reflects the varied gasdynamical histories of the groups;
the regularity and self-similarity of the pressure profiles is a sign of a
similarity in the underlying dark matter distributions.Comment: Accepted for publication in the Astrophysical Journa
Joint Analysis of Cluster Observations: II. Chandra/XMM-Newton X-ray and Weak Lensing Scaling Relations for a Sample of 50 Rich Clusters of Galaxies
We present a study of multiwavelength X-ray and weak lensing scaling
relations for a sample of 50 clusters of galaxies. Our analysis combines
Chandra and XMM-Newton data using an energy-dependent cross-calibration. After
considering a number of scaling relations, we find that gas mass is the most
robust estimator of weak lensing mass, yielding 15 +/- 6% intrinsic scatter at
r500 (the pseudo-pressure YX has a consistent scatter of 22%+/-5%). The scatter
does not change when measured within a fixed physical radius of 1 Mpc. Clusters
with small BCG to X-ray peak offsets constitute a very regular population whose
members have the same gas mass fractions and whose even smaller <10% deviations
from regularity can be ascribed to line of sight geometrical effects alone.
Cool-core clusters, while a somewhat different population, also show the same
(<10%) scatter in the gas mass-lensing mass relation. There is a good
correlation and a hint of bimodality in the plane defined by BCG offset and
central entropy (or central cooling time). The pseudo-pressure YX does not
discriminate between the more relaxed and less relaxed populations, making it
perhaps the more even-handed mass proxy for surveys. Overall, hydrostatic
masses underestimate weak lensing masses by 10% on the average at r500; but
cool-core clusters are consistent with no bias, while non-cool-core clusters
have a large and constant 15-20% bias between r2500 and r500, in agreement with
N-body simulations incorporating unthermalized gas. For non-cool-core clusters,
the bias correlates well with BCG ellipticity. We also examine centroid shift
variance and and power ratios to quantify substructure; these quantities do not
correlate with residuals in the scaling relations. Individual clusters have for
the most part forgotten the source of their departures from self-similarity.Comment: Corrects an error in the X-ray luminosities (erratum
submitted)---none of the other results are affected. Go to
http://sfstar.sfsu.edu/jaco for an electronic fitter and updated quick data
download link
A Study of the Dark Core in A520 with Hubble Space Telescope: The Mystery Deepens
We present a Hubble Space Telescope/Wide Field Planetary Camera 2
weak-lensing study of A520, where a previous analysis of ground-based data
suggested the presence of a dark mass concentration. We map the complex mass
structure in much greater detail leveraging more than a factor of three
increase in the number density of source galaxies available for lensing
analysis. The "dark core" that is coincident with the X-ray gas peak, but not
with any stellar luminosity peak is now detected with more than 10 sigma
significance. The ~1.5 Mpc filamentary structure elongated in the NE-SW
direction is also clearly visible. Taken at face value, the comparison among
the centroids of dark matter, intracluster medium, and galaxy luminosity is at
odds with what has been observed in other merging clusters with a similar
geometric configuration. To date, the most remarkable counter-example might be
the Bullet Cluster, which shows a distinct bow-shock feature as in A520, but no
significant weak-lensing mass concentration around the X-ray gas. With the most
up-to-date data, we consider several possible explanations that might lead to
the detection of this peculiar feature in A520. However, we conclude that none
of these scenarios can be singled out yet as the definite explanation for this
puzzle.Comment: Published in ApJ. Figures are slightly degraded to meet the size
limi
Effect of gold nanoparticles on prostate dose distribution under Ir-192 internal and 18 MV external radiotherapy procedures using gel dosimetry and monte carlo method
Background: Gel polymers are considered as new dosimeters for determining radiotherapy dose distribution in three dimensions. Objective: The ability of a new formulation of MAGIC-f polymer gel was assessed by experimental measurement and Monte Carlo (MC) method for studying the effect of gold nanoparticles (GNPs) in prostate dose distributions under the internal Ir-192 and external 18MV radiotherapy practices. Method: A Plexiglas phantom was made representing human pelvis. The GNP shaving 15 nm in diameter and 0.1 mM concentration were synthesized using chemical reduction method. Then, a new formulation of MAGIC-f gel was synthesized. The fabricated gel was poured in the tubes located at the prostate (with and without the GNPs) and bladder locations of the phantom. The phantom was irradiated to an Ir-192 source and 18 MV beam of a Varian linac separately based on common radiotherapy procedures used for prostate cancer. After 24 hours, the irradiated gels were read using a Siemens 1.5 Tesla MRI scanner. The absolute doses at the reference points and isodose curves resulted from the experimental measurement of the gels and MC simulations following the internal and external radiotherapy practices were compared. Results: The mean absorbed doses measured with the gel in the presence of the GNPs in prostate were 15 and 8 higher than the corresponding values without the GNPs under the internal and external radiation therapies, respectively. MC simulations also indicated a dose increase of 14 and 7 due to presence of the GNPs, for the same experimental internal and external radiotherapy practices, respectively. Conclusion: There was a good agreement between the dose enhancement factors (DEFs) estimated with MC simulations and experiment gel measurements due to the GNPs. The results indicated that the polymer gel dosimetry method as developed and used in this study, can be recommended as a reliable method for investigating the DEF of GNPs in internal and external radiotherapy practices. © 2015 Shiraz University of Medical Sciences. All Rights Reserved
Meeting, Mating, and Cheating Online in Iran
Urban young Iranians have recently begun to use the blogosphere and cyberspace to construct new sexual and social discourses. This article describes how Iranian youth link indigenous discourses of sexuality to other discourses of sexuality and how they understand their practices as political and social statements. For these youths, the internet provides important avenues to circumvent restrictions on cultural consumption and production while allowing them to engage in “safe” sexual encounters. As such, it fuels the creation of social movements that may pose new challenges to the regime
Comparison of Coupled Flow-deformation and Drained Analyses for Road Embankments on CMC Improved Ground
© 2016 The Authors. Published by Elsevier B.V. The use of controlled modulus columns (CMC) is gaining increased popularity in the support of rail and road bridge approach embankments on soft soils. The further columns are driven into the competent firm soils, the further the design will rely on the inclusions to take the bulk of the vertical loads, as they become rigid inclusions. The advantage of this design approach is that it produces increased control over the settlement, but as a result the columns will attract greater loads, including bending moment and shear force in situations where non-uniform loading or ground conditions exist. The load on the composite soil-CMC is uniformly distributed by the upper layer of granular load transfer platform (LTP), which also includes a layer of reinforcement. Finite difference program FLAC3D has been used to numerically simulate an embankment on the improved ground with end-bearing CMC. A geosynthetic reinforcement layer has been simulated using the inbuilt FLAC3D geogrid element. In this paper, a comparison has been made between the drained and coupled flow-deformation analyses. The force in the reinforcement layer, in particular, has been compared for the two analysis approaches. It was found that according to the numerical simulation, the drained analysis provides lower estimates of the settlement, lateral displacement; and therefore, predicts less tension in the geosynthetic layer
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