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 tha t 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(50)( -1) kpc) of 1.2(-0.5)(+0.1) x 10(14) h(50)(-1) M-circle dot at the 90% conf idence level, 1.3 times larger than the isothermal estimate. The total mass within r(500) (1.7 h(50)(-1) Mpc) is M-500 = 0.91(-0.2)(+0.3) x 10(15) h(5 0)(-1) M-circle dot at 90% confidence, in agreement with the optical virial mass estimate, and 1.2 times smaller than the isothermal estimate. Our M-5 00 value is 1.7 times smaller than that estimated using the mass-temperatur e scaling law predicted by simulations. The best-fit dark matter density pr ofile scales as r-(3.1) at large radii, which is consistent with the Navarr o, Frenk & White (NFW) "universal profile" as well as the King profile of t he galaxy density in A401. From the imaging data, the gas density profile i s shallower than the dark matter profile, scaling r(-2.1) at large radii, l eading to a monotonically increasing gas mass fraction with radius. Within r(500) asr the gas mass fraction reaches a value of f(gas) = 0.21(-0.05)(+0 .06) h(50)(-3/2) (90% confidence errors). Assuming that f(gas) (plus an est imate of the stellar mass) is the universal value of the baryon fraction, w e estimate the 90% confidence upper limit of the cosmological matter densit y to be Omega(m) < 0.31, in conflict with an Einstein-deSitter universe. Ev en though the NFW dark matter density profile is statistically consistent w ith the temperature data, its central temperature cusp would lead to convec tive instability at the center, because the gas density does not have a cor responding peak. One way to reconcile a cusp-shaped total mass profile with the observed gas density profile, regardless of the temperature data, is t o introduce a significant nonthermal pressure in the center. Such a pressur e must satisfy the hydrostatic equilibrium condition without inducing turbu lence. Alternately, significant mass drop-out from the cooling flow would m ake the temperature less peaked and the NFW profile acceptable. However, th e quality of data is not adequate to test this possibility.