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 ⊙ .

We show that the canonical oscillation-based (non-resonant) production of sterile neutrino dark matter is inconsistent at > 99% confidence with observations of galaxies in the Local Group. We set lower limits on the non-resonant sterile neutrino mass of 2.5 keV (equivalent to 0.7 keV thermal mass) using phase-space densities derived for dwarf satellite galaxies of the Milky Way, as well as limits of 8.8 keV (equivalent to 1.8 keV thermal mass) based on subhalo counts of N -body simulations of M 31 analogues. Combined with improved upper mass limits derived from significantly deeper X-ray data of M 31 with full consideration for background variations, we show that there remains little room for non-resonant production if sterile neutrinos are to explain 100% of the dark matter abundance. Resonant and non-oscillation sterile neutrino production remain viable mechanisms for generating sufficient dark matter sterile neutrinos.

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.

We present the first in a series of papers studying with Chandra the X-ray properties of a sample of 28 early-type galaxies which span ∼3 orders of magnitude in X-ray luminosity (L X ). We report emission-weighted Fe abundance (Z Fe ) constraints and, for many of the galaxies, abundance constraints for key elements such as O, Ne, Mg, Si, S and Ni. We find no evidence of the very subsolar Z Fe historically reported, confirming a trend in recent X-ray observations of bright galaxies and groups, nor do we find any correlation between Z Fe and luminosity. Except in one case we do not find evidence for a multi-phase interstellar medium (ISM), indicating that multi-temperature fits required in previous ASCA analysis arose due to the strong temperature gradients which we are able to resolve with Chandra. We compare the stellar Z Fe , estimated from simple stellar population model fits, to that of the hot gas. Excepting one possible outlier we find no evidence that the gas is substantially more metal-poor than the stars and, in a few systems, Z Fe is higher in the ISM. In general, however, the two components exhibit similar metallicities, which is inconsistent with both galactic wind models and recent hierarchical chemical enrichment simulations. Adopting standard SNIa and SNII metal yields our abundance ratio constraints imply 66±11% of the Fe within the ISM was produced in SNIa, which is remarkably similar to the Solar neighbourhood, and implies similar enrichment histories for the cold ISM in a spiral and the hot ISM in elliptical galaxies. Although these values are sensitive to the considerable systematic uncertainty in the supernova yields, they are also in very good agreement with observations of more massive systems. These results indicate a remarkable degree of homology in the enrichment process operating from cluster scales to low-to-intermediate L X galaxies. In addition the data uniformly exhibit the low Z O /Z Mg abundance ratios which have been reported in the centres of clusters, groups and some galaxies. This is inconsistent with the standard calculations of metal production in SNII and may indicate an additional source of α-element enrichment, such as Population III hypernovae.