The Baryonic and Dark Matter Distributions in Abell 401

We combine spatially resolved ASCA temperature data with ROSAT imaging data to constrain the total mass distribution in the cluster A401, assuming that the cluster is in hydrostatic equilibrium, but without the assumption of gas isothermality. We obtain a total mass within the X-ray core (290 h−1 50 kpc) of 1.2 −0.5×10 h−1 50 M at the 90 % confidence level, 1.3 times larger than the isothermal estimate. The total mass within r500 (1.7 h−1 50 Mpc) is M500 = 0.9 +0.3 −0.2× 10 h−1 50 M at 90 % confidence, in agreement with the optical virial mass estimate, and 1.2 times smaller than the isothermal estimate. Our M500 value is 1.7 times smaller than that estimated using the mass-temperature scaling law predicted by simulations. The best fit dark matter density profile scales as r−3.1 at large radii, which is consistent with the Navarro, Frenk & White (NFW) “universal profile” as well as the King profile of the galaxy density in A401. From the imaging data, the gas density profile is shallower than the dark matter profile, scaling as r−2.1 at large radii, leading to a monotonically increasing gas mass fraction with radius. Within r500 the gas mass fraction reaches a value of fgas = 0.21 −0.05 h −3/2 50 (90 % confidence errors). Assuming that fgas (plus an estimate of the stellar mass) is the universal value of the baryon fraction, we estimate the 90 % confidence upper limit of the cosmological matter density to be Ωm < 0.31, in conflict with an Einstein-deSitter universe. Even though the NFW dark matter density profile is statistically consistent with the temperature data, its central temperature cusp would lead to convective instability at the center, because the gas density does not have a corresponding peak. One way to reconcile a cusp-shaped total mass profile with the observed gas density profile, regardless of the temperature data, is to introduce a significant non-thermal pressure in the center. Such a pressure must satisfy the hydrostatic equilibrium condition without inducing turbulence. Alternately, significant mass drop-out from the cooling flow would make the temperature less peaked, and the NFW profile acceptable. However, the quality of data is not adequate to test this possibility. Subject headings: cosmology: observations – dark matter – galaxies: clusters: individual (A401) – intergalactic medium – X-rays: galaxies