Chandra Observations of A2029: The Dark Matter Profile Down to below 0.01rvir in an Unusually Relaxed Cluster

We have used a high spatial resolution Chandra observation to examine the core mass distribution of the unusually regular cD cluster A2029. This bright, nearby system is especially well suited for analysis of its mass distribution under the assumption of hydrostatic equilibrium; it exhibits an undisturbed, symmetric X-ray morphology and a single-phase intracluster medium (ICM). From the deprojected temperature and density profiles, we estimate the total mass and the contributions of the gas and dark matter (DM) components from less than 3'' to ~3' (<4.4-260 h kpc, 0.001-0.1rvir). The gas density profile is not adequately described by a single-β model fit because of an increase in the density at the center (r < 17 h kpc, <12''), but it is well fitted by either a double-β model, or a "cusped" β model. The temperature data increase as a function of radius and are well fitted by a Bertschinger & Meiksin profile and may be approximated by a power-law T(r) ∝ r, with αT = 0.27 ± 0.01. Using the fitted profiles to obtain smooth functions of density and temperature, we employed the equation of hydrostatic equilibrium to compute the total enclosed mass as a function of radius. We report a total mass of 9.15 ± 0.25 × 1013 h M☉ within 260 h kpc, using the chosen parameterization of gas density and temperature. The mass profile is remarkably well described down to 0.002rvir by the Navarro, Frenk, & White (NFW) profile, or a Hernquist profile, over two decades of radius and three decades of mass. For the NFW model, we measure a scale radius rs = 540 ± 90 h kpc (≈0.2rvir) and concentration parameter c = 4.4 ± 0.9. The mass profile is also well approximated by a simple power-law fit [M(< r) ∝ r], with αm = 1.81 ± 0.04 (corresponding to a logarithmic density profile slope of -1.19 ± 0.04). The density profile is too shallow to be fitted with the profile described by Moore et al. The consistency with NFW down to less than 0.01rvir is incompatible with the flattened core DM profiles predicted for self-interacting DM (e.g., Spergel & Steinhardt) and thus contrasts with the strong deviations from cold dark matter (CDM) predictions observed in the rotation curves of low surface brightness galaxies and dwarf galaxies. This suggests that while CDM simulations may adequately describe objects of cluster mass, they do not currently properly account for the formation and evolution of smaller halos. Assuming that the cD dominates the optical cluster light within its effective radius (Re = 52′′,76 h kpc), we observe a total mass-to-light ratio M/LV ≈ 12 M☉/L☉ at r < 20 h kpc, rising rapidly to greater than 100 M☉/L☉ beyond 200 h kpc. The consistency with a single NFW mass component and the large M/L suggests the cluster is DM-dominated down to very small radii (≲0.005rvir). We observe the ICM gas mass to rise from 3% ± 1% of the total mass in the center to 13.9% ± 0.4% at the limit of our observations. This provides an upper limit to the current matter density of the universe, Ωm ≤ 0.29 ± 0.03 h.