X-Ray Constraints on the Intrinsic Shapes and Baryon Fractions of Five Abell Clusters

We analyzed ROSAT PSPC images of the bright, nearby (z 80-1 Mpc to reduce effects of possible substructure on smaller scales (less than or equal to a few hundred kiloparsecs). The ellipticities of the X-ray isophotes are typically E 0.15-0.25 and display negative radial gradients highly significant at the 95% confidence limit for all the clusters except Coma. By assuming hydrostatic equilibrium and a variety of mass models, we obtain ellipticities ɛmass ≈ 0.40-0.55 for isothermal models of the total gravitating matter of the clusters; the ɛmass constraints change by less than 10% upon consideration of the small temperature gradients shown by ASCA(Astro-D) to be typical for rich clusters; the observed X-ray ellipticity gradients require that mass models with constant ellipticity have steep density profiles, ρ ∝ r-4. Estimates of the gas masses are very insensitive to the ellipticities of the X-ray isophotes The clusters in our sample have increasing fractions of gas mass to total mass with radius and have Mgas/Mtot = (4%-11%)h80-3/2 within a radius 1.5 h80-1 Mpc, in excellent agreement with the results of White & Fabian and the baryon catastrophe highlighted in the recent work of White et al. Finally, the ellipticities of the dark matter distributions are essentially identical to ɛmass and are consistent with the shapes of dark halos predicted by N-body simulations and the shapes of the galaxy isopleths in the clusters in contrast to our previous conclusions using Einstein data.