ChandraObservations of A2029: The Dark Matter Profile Down to below 0.01rvirin an Unusually Relaxed Cluster

We have used a high resolution Chandra observation to examine the core massdistribution of the unusually regular cD cluster Abell 2029. This system isespecially well-suited for analysis of its mass distribution under theassumption of hydrostatic equilibrium: it exhibits an undisturbed X-raymorphology and a single-phase ICM. We estimate the total mass, and thecontributions of the gas and dark matter components from <3 arcsec to ~3 arcmin(<4.4-260 kpc, 0.001-0.1 r_vir). The gas density is well fitted by either adouble beta-model, or a ``cusped'' beta-model. The temperature data areincreasing, and can be approximated by a power-law, with alpha_T = 0.27+/-0.01. Fitting smooth functions to density and T, we obtain a mass profile, which isremarkably well described down to 0.002 r_vir by the NFW model, for which wemeasure r_scale = 540+/-90 kpc (~0.2 r_vir) and c = 4.4 +/- 0.9. The massprofile is also approximated by a power-law fit with alpha_m = 1.81+/-0.04(corresponding to a logarithmic density slope of -1.19+/-0.04). The densityprofile is too shallow to be fitted with a Moore et al. model. The consistencywith NFW at <0.01 r_vir is incompatible with SIDM, and contrasts with theresults from rotation curves of LSB and dwarf galaxies. This suggests thatwhile CDM may adequately describe clusters, it does not currently accountproperly for small halos. Using the cD light, we observe a total M/L_V ~ 12 at r < 20 kpc, rising to >100 beyond 200 kpc. The consistency with an NFW halo model and the large M/Lsuggest the cluster is DM-dominated down to <~ 0.005 r_vir. We observe a gasmass fraction = 0.139+/-0.004 at the limit of our observations, finding anupper limit to Omega_m <= 0.29+/-0.03 (abridged).Comment: ApJ Accepted Version; to appear in 2003 March 20 Issue; 8 pages, emulateapj5, 3 figures (2 color