3,442 research outputs found
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
Robust statistical methods for automated outlier detection
The computational challenge of automating outlier, or blunder point, detection in radio metric data requires the use of nonstandard statistical methods because the outliers have a deleterious effect on standard least squares methods. The particular nonstandard methods most applicable to the task are the robust statistical techniques that have undergone intense development since the 1960s. These new methods are by design more resistant to the effects of outliers than standard methods. Because the topic may be unfamiliar, a brief introduction to the philosophy and methods of robust statistics is presented. Then the application of these methods to the automated outlier detection problem is detailed for some specific examples encountered in practice
Dark Matter in the Galaxy Cluster CL J1226+3332 at Z=0.89
We present a weak-lensing analysis of the galaxy cluster CL J1226+3332 at
z=0.89 using Hubble Space Telescope Advanced Camera for Surveys images. The
cluster is the hottest (>10 keV), most X-ray luminous system at z>0.6 known to
date. The relaxed X-ray morphology, as well as its high temperature, is unusual
at such a high redshift. Our mass reconstruction shows that on a large scale
the dark matter distribution is consistent with a relaxed system with no
significant substructures. However, on a small scale the cluster core is
resolved into two mass clumps highly correlated with the cluster galaxy
distribution. The dominant mass clump lies close to the brightest cluster
galaxy whereas the other less massive clump is located ~40" (~310 kpc) to the
southwest. Although this secondary mass clump does not show an excess in the
X-ray surface brightness, the gas temperature of the region is much higher
(12~18 keV) than those of the rest. We propose a scenario in which the less
massive system has already passed through the main cluster and the X-ray gas
has been stripped during this passage. The elongation of the X-ray peak toward
the southwestern mass clump is also supportive of this possibility. We measure
significant tangential shears out to the field boundary (~1.5 Mpc), which are
well described by an Navarro-Frenk-White profile with a concentration parameter
of c200=2.7+-0.3 and a scale length of rs=78"+-19" (~600 kpc) with
chi^2/d.o.f=1.11. Within the spherical volume r200=1.6 Mpc, the total mass of
the cluster becomes M(r<r200)=(1.4+-0.2) x 10^15 solar mass. Our weak-lensing
analysis confirms that CL1226+3332 is indeed the most massive cluster known to
date at z>0.6.Comment: Accepted for publication in Ap
Tracing the Peculiar Dark Matter Structure in the Galaxy Cluster CL 0024+17 with Intracluster Stars and Gas
ICL is believed to originate from the stars stripped from cluster galaxies.
They are no longer gravitationally bound to individual galaxies, but to the
cluster, and their smooth distribution potentially makes them serve as much
denser tracers of the cluster dark matter than the sparsely distributed cluster
galaxies. We present our study of the ICL in Cl 0024+17 using both ACS and
Subaru data, where we previously reported discovery of a ringlike dark matter
structure with gravitational lensing. The ACS images provide much lower sky
levels than ground data, and enable us to measure relative variation of surface
brightness reliably. This analysis is repeated with the Subaru images to
examine if consistent features are recovered despite different reduction scheme
and instrumental characteristics. We find that the ICL profile clearly
resembles the peculiar mass profile, which stops decreasing at r~50" (~265 kpc)
and slowly increases until it turns over at r~75" (~397 kpc). This feature is
seen in both ACS and Subaru images for nearly all available passband images
while the features are stronger in red filters. The consistency across
different filters and instruments strongly rules out the possibility that the
feature might come from any residual, uncorrected calibration errors. In
addition, our re-analysis of the cluster X-ray data shows that the peculiar
mass structure is also indicated by a non-negligible bump in the intracluster
gas profile when the geometric center of the dark matter ring, not the peak of
the X-ray emission, is chosen as the center of the radial bin. The location of
the gas ring is closer to the center by ~15" (~80 kpc), raising an interesting
possibility that the ring-like structure is expanding and the gas ring is
lagging behind perhaps because of the ram pressure if both features in mass and
gas share the same dynamical origin.Comment: Accepted to ApJ for publicatio
A Model of Hierarchical Professionals: Cooperation and Conflict between Anesthesiologists and CRNAs
This paper examines a labor market with two professional groups both cooperating and directly competing with each other: certified registered nurse anesthetists (CRNAs) and anesthesiologists (MDAs). We develop a model where the supply of MDAs endogenously determines (1) the earnings of CRNAs and MDAs, and (2) the extent of supervision of CRNAs by MDAs. We also analyze how MDAs may lobby to limit the scope of practice of CRNAs, and the resulting market equilibrium. Our theoretical model can be applied to the analysis of relationships between other hierarchical professionals with overlapping responsibilities, such as nurse practitioners and primary care physicians.
Hubble Space Telescope Weak-lensing Study of the Galaxy Cluster XMMU J2235.3-2557 at z=1.4: A Surprisingly Massive Galaxy Cluster when the Universe is One-third of its Current Age
We present a weak-lensing analysis of the z=1.4 galaxy cluster XMMU
J2235.3-2557, based on deep Advanced Camera for Surveys images. Despite the
observational challenge set by the high redshift of the lens, we detect a
substantial lensing signal at the >~ 8 sigma level. This clear detection is
enabled in part by the high mass of the cluster, which is verified by our both
parametric and non-parametric estimation of the cluster mass. Assuming that the
cluster follows a Navarro-Frenk-White mass profile, we estimate that the
projected mass of the cluster within r=1 Mpc is (8.5+-1.7) x 10^14 solar mass,
where the error bar includes the statistical uncertainty of the shear profile,
the effect of possible interloping background structures, the scatter in
concentration parameter, and the error in our estimation of the mean redshift
of the background galaxies. The high X-ray temperature 8.6_{-1.2}^{+1.3} keV of
the cluster recently measured with Chandra is consistent with this high lensing
mass. When we adopt the 1-sigma lower limit as a mass threshold and use the
cosmological parameters favored by the Wilkinson Microwave Anisotropy Probe
5-year (WMAP5) result, the expected number of similarly massive clusters at z
>~ 1.4 in the 11 square degree survey is N ~ 0.005. Therefore, the discovery of
the cluster within the survey volume is a rare event with a probability < 1%,
and may open new scenarios in our current understanding of cluster formation
within the standard cosmological model.Comment: Accepted to ApJ for publication. 40 pages and 14 figure
MC: Multi-wavelength and dynamical analysis of the merging galaxy cluster ZwCl 0008.8+5215: An older and less massive Bullet Cluster
We analyze a rich dataset including Subaru/SuprimeCam, HST/ACS and WFC3,
Keck/DEIMOS, Chandra/ACIS-I, and JVLA/C and D array for the merging galaxy
cluster ZwCl 0008.8+5215. With a joint Subaru/HST weak gravitational lensing
analysis, we identify two dominant subclusters and estimate the masses to be
M
and 1.2 M. We estimate the
projected separation between the two subclusters to be
924 kpc. We perform a clustering analysis on
confirmed cluster member galaxies and estimate the line of sight velocity
difference between the two subclusters to be 92164 km s. We
further motivate, discuss, and analyze the merger scenario through an analysis
of the 42 ks of Chandra/ACIS-I and JVLA/C and D polarization data. The X-ray
surface brightness profile reveals a remnant core reminiscent of the Bullet
Cluster. The X-ray luminosity in the 0.5-7.0 keV band is
1.70.110 erg s and the X-ray
temperature is 4.900.13 keV. The radio relics are polarized up to 40.
We implement a Monte Carlo dynamical analysis and estimate the merger velocity
at pericenter to be 1800 km s. ZwCl
0008.8+5215 is a low-mass version of the Bullet Cluster and therefore may prove
useful in testing alternative models of dark matter. We do not find significant
offsets between dark matter and galaxies, as the uncertainties are large with
the current lensing data. Furthermore, in the east, the BCG is offset from
other luminous cluster galaxies, which poses a puzzle for defining dark matter
-- galaxy offsets.Comment: 22 pages, 19 figures, accepted for publication in the Astrophysical
Journal on March 13, 201
Aggregation in mixing tanks - the role of inter-particle forces
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