No abstract

We present a Chandra study of mass profiles in 7 elliptical galaxies, of which 3 have galaxy-scale and 4 group-scale halos, demarcated at 10 13 M ⊙ . These represent the best available data for nearby objects with comparable X-ray luminosities. We measure ∼flat mass-to-light (M/L) profiles within an optical half-light radius (R eff ), rising by an order of magnitude at ∼10R eff , which confirms the presence of dark matter (DM). The data indicate hydrostatic equilibrium, which is also supported by agreement with studies of stellar kinematics in elliptical galaxies. The data are well-fitted by a model comprising an NFW DM profile and a baryonic component following the optical light. The distribution of DM halo concentration parameters (c) versus M vir agrees with ΛCDM predictions and our observations of bright groups. Concentrations are slightly higher than expected, which is most likely a selection effect. Omitting the stellar mass drastically increases c, possibly explaining large concentrations found by some past observers. The stellar M/L K agree with population synthesis models, assuming a Kroupa IMF. Allowing adiabatic compression (AC) of the DM halo by baryons made M/L more discrepant, casting some doubt on AC. Our best-fitting models imply total baryon fractions ∼0.04-0.09, consistent with models of galaxy formation incorporating strong feedback. The groups exhibit positive temperature gradients, consistent with the "Universal" profiles found in other groups and clusters, whereas the galaxies have negative gradients, suggesting a change in the evolutionary history of the systems around M vir ≃ 10 13 M ⊙ .

The search for diffuse non-thermal inverse Compton (IC) emission from galaxy clusters at hard X-ray energies has been undertaken with many instruments, with most detections being either of low significance or controversial. Because all prior telescopes sensitive at E > 10 keV do not focus light and have degree-scale fields of view, their backgrounds are both high and difficult to characterize. The associated uncertainties result in lower sensitivity to IC emission and a greater chance of false detection. In this work, we present 266 ks NuSTAR observations of the Bullet cluster, which is detected in the energy range 3-30 keV. NuSTAR's unprecedented hard X-ray focusing capability largely eliminates confusion between diffuse IC and point sources; however, at the highest energies, the background still dominates and must be well understood. To this end, we have developed a complete background model constructed of physically inspired components constrained by extragalactic survey field observations, the specific parameters of which are derived locally from data in non-source regions of target observations. Applying the background model to the Bullet cluster data, we find that the spectrum is well-but not perfectly-described as an isothermal plasma with kT = 14.2 ± 0.2 keV. To slightly improve the fit, a second temperature component is added, which appears to account for lower temperature emission from the cool core, pushing the primary component to kT ∼ 15.3 keV. We see no convincing need to invoke an IC component to describe the spectrum of the Bullet cluster, and instead argue that it is dominated at all energies by emission from purely thermal gas. The conservatively derived 90% upper limit on the IC flux of 1.1 × 10 −12 erg s −1 cm −2 (50-100 keV), implying a lower limit on B 0.2 μG, is barely consistent with detected fluxes previously reported. In addition to discussing the possible origin of this discrepancy, we remark on the potential implications of this analysis for the prospects for detecting IC in galaxy clusters in the future.

We present radial mass profiles within $0:3r vir for 16 relaxed galaxy groups-poor clusters (kT range 1Y3 keV ) selected for optimal mass constraints from the Chandra and XMM-Newton data archives. After accounting for the mass of hot gas, the resulting mass profiles are described well by a two-component model consisting of dark matter, represented by an NFW model, and stars from the central galaxy. The stellar component is required only for eight systems, for which reasonable stellar mass-to-light ratios (M/L K ) are obtained, assuming a Kroupa IMF. Modifying the NFW dark matter halo by adiabatic contraction does not improve the fit and yields systematically lower M /L K . In contrast to previous results for massive clusters, we find that the NFW concentration parameter (c vir ) for groups decreases with increasing M vir and is inconsistent with no variation at the 3 level. The normalization and slope of the c vir -M vir relation are consistent with the standard ÃCDM cosmological model with 8 ¼ 0:9 (considering a 10% bias for early forming systems). The small intrinsic scatter measured about the c vir -M vir relation implies that the groups represent preferentially relaxed, early forming systems. The mean gas fraction ( f ¼ 0:05 AE 0:01) of the groups measured within an overdensity Á ¼ 2500 is lower than for hot, massive clusters, but the fractional scatter ( f /f ¼ 0:2) for groups is larger, implying a greater impact of feedback processes on groups, as expected.

We present the concentration (c)Yvirial mass (M ) relation of 39 galaxy systems ranging in mass from individual early-type galaxies up to the most massive galaxy clusters, (0:06Y20) ; 10 14 M . We selected for analysis the most relaxed systems possessing the highest quality data currently available in the Chandra and XMM-Newton public data archives. A power-law model fitted to the X-ray c-M relation requires at high significance (6.6 ) that c decreases with increasing M, which is a general feature of CDM models. The median and scatter of the c-M relation produced by the flat, concordance ÃCDM model ( m ¼ 0:3, 8 ¼ 0:9) agrees with the X-ray data, provided that the sample is comprised of the most relaxed, early-forming systems, which is consistent with our selection criteria. When allowing only 8 to vary in the concordance model, the c-M relation requires 0:76 < 8 < 1:07 (99% confidence), assuming a 10% upward bias in the concentrations for early-forming systems. The tilted, low-8 model suggested by a new WMAP analysis is rejected at 99.99% confidence, but a model with the same tilt and normalization can be reconciled with the X-ray data by increasing the dark energy equation of state parameter to w % À0:8. When imposing the additional constraint of the tight relation between 8 and m from studies of cluster abundances, the X-ray c-M relation excludes (>99% confidence) both open CDM models and flat CDM models with m % 1. This result provides novel evidence for a flat, low-m universe with dark energy using observations only in the local (zT1) universe. Possible systematic errors in the X-ray mass measurements of a magnitude %10% suggested by CDM simulations do not change our conclusions.