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

MC2^2: 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$_{200}=\text{5.7}^{+\text{2.8}}_{-\text{1.8}}\times\text{10}^{\text{14}}\,\text{M}_{\odot}$ and 1.2$^{+\text{1.4}}_{-\text{0.6}}\times10^{14}$ M$_{\odot}$. We estimate the projected separation between the two subclusters to be 924$^{+\text{243}}_{-\text{206}}$ 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 92$\pm$164 km s$^{-\text{1}}$. We further motivate, discuss, and analyze the merger scenario through an analysis of the 42 ks of Chandra/ACIS-I and JVLA/C and D polarization data. The X-ray surface brightness profile reveals a remnant core reminiscent of the Bullet Cluster. The X-ray luminosity in the 0.5-7.0 keV band is 1.7$\pm$0.1$\times$10$^{\text{44}}$ erg s$^{-\text{1}}$ and the X-ray temperature is 4.90$\pm$0.13 keV. The radio relics are polarized up to 40$\%$. We implement a Monte Carlo dynamical analysis and estimate the merger velocity at pericenter to be 1800$^{+\text{400}}_{-\text{300}}$ km s$^{-\text{1}}$. 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

Peer reviewedPostprin