X-Ray Constraints on the Intrinsic Shapes and Baryon Fractions of Five Abell Clusters
Abstract
We analyzed ROSAT PSPC images of the bright, nearby (z < 0.1) galaxy clusters 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 recent work of Buote & Canizares and Buote). Following Buote & Tsai we probed the aggregate structure of the clusters on scales of approximately 1.5 h80-1 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 1996
- DOI:
- 10.1086/176753
- arXiv:
- arXiv:astro-ph/9504049
- Bibcode:
- 1996ApJ...457..565B
- Keywords:
-
- COSMOLOGY: DARK MATTER;
- GALAXIES: CLUSTERS: GENERAL;
- X-RAYS: GALAXIES;
- Astrophysics
- E-Print:
- 29 pages (2 figures), PostScript, to appear in ApJ on February 1, 1996. No changes to the text -- the publication date given before was incorrect