A ROSAT determination of the mass of the central Virgo Cluster
Abstract
Mass determinations for clusters of galaxies from X-ray observations rely on the equation of hydrostatic equilibrium for the intracluster gas. Used directly, this equation involves the gradient of the gas pressure, rather than the pressure itself, so forcing the adoption of model-dependent assumptions. By integrating the equation of hydrostatic equilibrium we show how to obtain limits on the mass of a spherical cluster that do not involve the pressure gradient, avoiding the need for any model-dependent assumptions. With high-quality data this technique produces multiple upper and lower limits on the mass at many radii. We outline a method for combining multiple limits to give a single confidence range for the mass at each radius. ROSAT PSPC data for the Virgo Cluster for separate spectroscopic channels are deprojected to obtain spectra in shells. From these we construct temperature and density profiles for the gas in the central 48 arcmin of the cluster. The density and temperature profiles are used to obtain limits on the mass profile for the Virgo Cluster. The mass distribution is flatter in the vicinity of M87 than in the rest of the cluster, supporting the idea that the dark matter of the cluster and the mass in M87 form distinct components. The cluster component must then have a flat core in the vicinity of M87. The most likely mass profile is well fitted by two (approximate) isothermal mass distributions. One, representing the mass of M87, has a mass per unit length of 3.6x10^10 M_solar kpc^-1, and the other, representing the dark matter of the cluster, has a mass per unit length of 12.4x10^10 M_solar kpc^-1 and a core radius of 56 kpc (for a Virgo distance of 20 Mpc). Our results agree well with optical determinations of the mass profile.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- June 1995
- DOI:
- 10.1093/mnras/274.4.1093
- Bibcode:
- 1995MNRAS.274.1093N
- Keywords:
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- GALAXIES: CLUSTERS: INDIVIDUAL: VIRGO;
- GALAXIES: FUNDAMENTAL PARAMETERS;
- GALAXIES: INDIVIDUAL: M87;
- INTERGALACTIC MEDIUM;
- DARK MATTER;
- X-RAYS: GENERAL