115530296
Mapping the dark trajectory of the Bullet Cluster
We model the electrons/positrons produced by dark matter annihilations in the colliding galaxy cluster system 1E0657-56 (the bullet cluster). These charged particles, confined by the Magnetic field, clearly trace the path of the bullet, which passes through the main cluster with a speed of 3000-5000 km/s. Adding the effect of dark matter substructure (subhalos) in each cluster we find the annihilation rate is enhancedgreatly and the density of positrons in the trail is similar to that within the bullet cluster. This opens the door to unambiguous detection of the annihilation signal through the SZ effect, at significant separation from the thermal electrons.
13839223
Systematic uncertainties in gravitational lensing models: a semi‐analytical study of PG1115+080
While the Hubble constant can be derived from observable time delays between images of lensed quasars, the result is often highly sensitive to assumptions and systematic uncertainties in the lensing model. Unlike most previous authors, we put minimal restrictions on the radial profile of the lens and allow for non-elliptical lens potentials. We explore these effects using a broad class of models with a lens potential which has an unrestricted radial profile but self-similar iso-potential contours defined by For these potentials, the lens equations can be solved semi-analytically. The axis ratio and position angle of the lens can be determined from the image positions of quadruple gravitational lensed systems directly, independent of the radial profile. We give simple equations for estimating the power-law slope of the lens density directly from the image positions and for estimating the time delay ratios. Our method greatly simplifies the numerics for fitting observations and is fast in exploring the model parameter space. As an illustration, we apply the model to PG1115+080. An entire one-parameter sequence of models fits the observations exactly. We show that the measured image positions and time delays do not uniquely determine the Hubble constant.
119278882
Mass discrepancy in galaxy clusters as a result of the offset between dark matter and baryon distributions
Recent studies of lensing clusters reveal that it might be fairly common for a galaxy cluster that the X-ray centre has an obvious offset from its gravitational centre which is measured by strong lensing. We argue that if these offsets exist, then X-rays and lensing are indeed measuring different regions of a cluster, and may thus naturally result in a discrepancy in the measured gravitational masses by the two different methods. Here we investigate theoretically the dynamical effects of such lensing–X-ray offsets and compare with observational data. We find that for typical values, the offset alone can give rise to a factor of 2 difference between the lensing and X-ray-determined masses for the core regions of a cluster, suggesting that such ‘offset effect’ may play an important role and should not be ignored in our dynamical measurements of clusters.
122064075
Stirring N-body systems 2: Avoiding Convergence
We study the evolution of the phase-space of collisionless N-body systems under repeated stirrings or perturbations, which has been shown to lead to a convergence towards a limited solution group, in accordance with Hansen et al. (2010), that is independent of the initial system and environmental conditions. We address, and dismiss, concerns that such convergent behaviour was due to collapse through radial orbit instability. We further examine the eects of radial velocity kicks in rigid potential and isotropic velocity kicks. There are no energy exchanges in these two recipes of kicks. We nd that these kicks do not lead to the attractor.
118866594
A consistent microlensing model for the Galactic bar
We compute a microlensing map for the Galactic bar. The predicted event rate and event duration distribution are consistent with the 55 events recently reported by the MACHO and OGLE collaborations. Most of the events are due to lensing by stars in the near end of the bar_ Lens mass functions with about 30-60 per cent of the lens mass as brown dwarfs are rejected at 20'-60' levels_ To make our model useful for other workers, we tabulate the optical depth of the bar and average event duration (scaled to I-M0 lenses) on a grid of galactic coordinates. The distance and the proper motions of the lens and the source are derived from a consistent dynamical model of the stellar bar, which has originally been built to fit data on the stellar light and stellar/gas kinematics of the bar. We explore several alternative models, and we find that our standard model best matches the observations.
121927669
Dynamical limits on galactic winds, halo MACHOs and intergalactic globular clusters
We argue that any violent galactic winds following the early epoch of starbursts will significantly weaken the potentials of galaxies, and leave lasting signatures such as a lowered dark halo density and preferentially radial/escaping orbits for halo tracers such as globular clusters. A galaxy is disintegrated if more than half of its dynamical mass is blown off. The presence of dense haloes in galaxies and the absence of intergalactic/escaping globulars should imply an upper limit on the amount of baryons lost in galactic winds of around 4 per cent of the total mass of the galaxy. This translates to limits on the baryons participating in the early starbursts and baryons locked in stellar remnants, such as white dwarfs. The numbers of halo white dwarfs claimed in recent proper motion searches and microlensing observations in the Galactic halo are too high to be consistent with our dynamical upper limits. Similar arguments also imply upper limits for the number of neutron stars and stellar black holes in galaxy haloes. Nevertheless, a milder outflow is desirable, especially in dwarf galaxies, both to lower their cold dark matter central density and to inject metals into the intergalactic medium.
118386819
Simulation of background reduction and Compton suppression in a low-background HPGe spectrometer at a surface laboratory
High-purity germanium (HPGe) detectors are well suited to analyse the radioactivity of samples. In order to reduce the environmental background for an ultra-low background HPGe spectrometer, low-activity lead and oxygen free copper are installed outside the probe to shield from gamma radiation, with an outer plastic scintillator to veto cosmic rays, and an anti-Compton detector to improve the peak-to-Compton ratio. Using Geant4 tools and taking into account a detailed description of the detector, we optimize the sizes of these detectors to reach the design requirements. A set of experimental data from an existing HPGe spectrometer was used to compare with the simulation. For the future low-background HPGe detector simulation, considering different thicknesses of BGO crystals and anti-coincidence efficiency, the simulation results show that the optimal BGO thickness is 5.5 cm, and the peak-to-Compton ratio of (40).K is raised to 1000 when the anti-coincidence efficiency is 0.85. In the background simulation, 15 cm oxygen-free copper plus 10 cm lead can reduce the environmental gamma rays to 0.0024 cps/100 cm(3) Ge (50 keV-2.8 MeV), which is about 10(-5) of the environmental background.