Luca Zappacosta

A Chandra View of Dark Matter in Early-Type Galaxies

We present a Chandra study of mass profiles in seven elliptical galaxies, of which three have galaxy-scale and four have group-scale halos, demarcated at 1013 M☉. These represent the best available data for nearby objects with comparable X-ray luminosities. We measure approximately flat mass-to-light (M/L) profiles within an optical half-light radius (Reff), rising by an order of magnitude at ~10 Reff, 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 Mvir 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/LK 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 Mvir 1013 M☉.

The X-Ray Concentration-Virial Mass Relation

We present the concentration (c)-virial mass (M) relation of 39 galaxy systems ranging in mass from individual early-type galaxies up to the most massive galaxy clusters, (0.06-20) × 1014 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 σ) 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 ΛCDM model (Ωm = 0.3, σ8 = 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 σ8 to vary in the concordance model, the c-M relation requires 0.76 99% confidence) both open CDM models and flat CDM models with Ωm ≈ 1. This result provides novel evidence for a flat, low-Ωm universe with dark energy using observations only in the local (z 1) universe. Possible systematic errors in the X-ray mass measurements of a magnitude ≈10% suggested by CDM simulations do not change our conclusions.

Probing the Dark Matter and Gas Fraction in Relaxed Galaxy Groups with X-Ray Observations from Chandra and XMM-Newton

We present radial mass profiles within ~0.3rvir for 16 relaxed galaxy groups—poor clusters (kT range 1-3 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/LK) 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/LK. In contrast to previous results for massive clusters, we find that the NFW concentration parameter (cvir) for groups decreases with increasing Mvir and is inconsistent with no variation at the 3 σ level. The normalization and slope of the cvir-Mvir relation are consistent with the standard ΛCDM cosmological model with σ8 = 0.9 (considering a 10% bias for early forming systems). The small intrinsic scatter measured about the cvir-Mvir relation implies that the groups represent preferentially relaxed, early forming systems. The mean gas fraction (f = 0.05 ± 0.01) of the groups measured within an overdensity Δ = 2500 is lower than for hot, massive clusters, but the fractional scatter (σf/f = 0.2) for groups is larger, implying a greater impact of feedback processes on groups, as expected.

The Absence of Adiabatic Contraction of the Radial Dark Matter Profile in the Galaxy Cluster A2589

We present an X-ray analysis of the radial mass profile of the radio-quiet galaxy cluster A2589 between 0.015 and 0.25rvir, using an XMM-Newton observation. Except for a ≈16 kpc shift of the X-ray center of the R = 45-60 kpc annulus, A2589 possesses a remarkably symmetrical X-ray image and is therefore an exceptional candidate for precision studies of its mass profile by applying hydrostatic equilibrium. The total gravitating matter profile is well described by the NFW model with cvir = 6.1 ± 0.3 and Mvir = 3.3 ± 0.3 × 1014 M☉ (rvir = 1.74 ± 0.05 Mpc), in excellent agreement with ΛCDM. When the mass of the hot intracluster medium is subtracted from the gravitating matter profile, the NFW model fitted to the resulting dark matter (DM) profile produces essentially the same result. However, when accounting for the stellar mass (M*) of the cD galaxy, the NFW fit to the DM profile substantially degrades in the central r ~ 50 kpc for reasonable values of M*/LV. Modifying the NFW DM halo by adiabatic contraction arising from the early condensation of stellar baryons in the cD galaxy further degrades the fit. The fit is improved substantially with a Sersic-like model recently suggested by high-resolution N-body simulations but with an inverse Sersic index, α ~ 0.5, that is a factor of ~3 higher than predicted. We argue that neither random turbulent motions nor magnetic fields can provide sufficient nonthermal pressure support to reconcile the XMM-Newton mass profile with adiabatic contraction of a CDM halo, assuming reasonable values of M*/LV. Our results support the scenario in which, at least for galaxy clusters, processes during halo formation counteract adiabatic contraction so that the total gravitating mass in the core approximately follows the NFW profile.

Large scatter in X-ray emission among elliptical galaxies : Correlations with mass of host HALO

Optically similar elliptical galaxies have an enormous range of X-ray luminosities. We show that this range can be attributed to large variations in the dark halo mass Mvir determined from X-ray observations. The K-band luminosity of ellipticals varies with virial mass, LK ∝ M, but with considerable scatter, probably due to the stochastic incidence of massive satellite galaxies that merge by dynamical friction to form group-centered ellipticals. Both the observed X-ray luminosity LX ∝ M and LX/LK ∝ M are sufficiently sensitive to the virial mass to explain the wide variation observed in LX among galaxies of similar LK. The central galaxy supernova energy per particle of diffuse gas increases dramatically with decreasing virial mass, and elliptical galaxies with the lowest X-ray luminosities (and Mvir) are easily explained by supernova-driven outflows.