X-RAY CONSTRAINTS ON THE INTRINSIC SHAPES AND BARYON FRACTIONS OF 5 ABELL CLUSTERS

We analyzed ROSAT PSPC images of the bright, nearby (z < 0.1) galaxy c lusters A401, A1656 (Coma), A2029, A2199, and A2256 to constrain their intrinsic shapes and baryon fractions; the intrinsic shapes of these clusters were analyzed previously by us using Einstein data (see the r ecent work of Buote & Canizares and Buote). Following Buote & Tsai we probed the aggregate structure of the clusters on scales of approximat ely 1.5 h(80)(-1) Mpc to reduce effects of possible substructure on sm aller scales (less than or equal to a few hundred kiloparsecs). The el lipticities of the X-ray isophotes are typically epsilon approximate t o 0.15-0.25 and display negative radial gradients highly significant a t the 95% confidence limit for all the clusters except Coma. By assumi ng hydrostatic equilibrium and a variety of mass models, we obtain ell ipticities epsilon(mass) approximate to 0.40-0.55 for isothermal model s of the total gravitating matter of the clusters; the epsilon(mass) c onstraints change by less than 10% upon consideration of the small tem perature gradients shown by ASCA(Astro-D) to be typical for rich clust ers; the observed X-ray ellipticity gradients require that mass models with constant ellipticity have steep density profiles, rho proportion al to r(-4). Estimates of the gas masses are very insensitive to the e llipticities of the X-ray isophotes. The clusters in our sample have i ncreasing fractions of gas mass to total mass with radius and have M(g as)/M(tot) = (4%-11%)h(80)(-3/2) within a radius 1.5 h(80)(-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, t he ellipticities of the dark matter distributions are essentially iden tical to epsilon(mass) and are consistent with the shapes of dark halo s predicted by N-body simulations and the shapes of the galaxy isoplet hs in the clusters in contrast to our previous conclusions using Einst ein data.