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V. Hradecky (Caltech), C. Jones, R.H. Donnelly (Harvard-Smithsonian CfA), S.G. Djorgovski, R.R. Gal, S.C. Odewahn (Caltech)
We use galaxy catalogs resulting from the digitization and automated processing of the second generation Palomar Sky Survey plates calibrated with CCD images in Gunn-Thuan \emph{g}, \emph{r}, and \emph{i} bands, \emph{ASCA} spatially resolved spectroscopic X-ray observations and \emph{ROSAT} PSPC images to constrain total and gas mass-to-light ratios, gas mass fraction, baryon mass fraction and the ratio of total to luminous mass for a sample of 8 nearby relaxed galaxy groups and clusters. They are A262, A426, A478, A1795, A2052, A2063, A2199 and MKW4s.
Under the assumption of hydrostatic equilibrium and spherical symmetry, we can measure the total masses of clusters from their intracluster gas temperature and density profiles. Previous cluster mass estimates assumed the gas is isothermal, however \emph{ASCA} spatially resolved spectra show the gas temperature decreases with increasing distance from the center. By comparison, the isothermal assumption results in an underestimate of the total mass at small radii, and an overestimate at large cluster radii. The discrepancy between mass-to-light ratios obtained using virial mass estimates and X-ray mass estimates is thus increased. We argue that the virial mass estimate is a less reliable one, subject to systematic errors due to limited cluster volumes sampled and possible galaxy velocity anisotropies.
We obtained luminosity functions for the clusters in our sample using the digitized Palomar Sky Survey data calibrated with CCD images. To correct for contamination by background and foreground galaxies, we use color information to reject obvious non-members and subtract background galaxies statistically. We then estimate the total cluster luminosity using Schechter function fits to the galaxy catalogs.
The measured mass-to-light ratios are ~100~{\rm M}\odot/{\rm L}\odot, assuming {\rm H}0=50~{\rm km}~{\rm s}-1~{\rm Mpc}-1. This, along with a high baryonic fraction, are indicative of a low density universe with \Omega~0.2.
V.H. was partially supported by the Caltech SURF fellowship.
The author(s) of this abstract have provided an email address for comments about the abstract: vit@caltech.edu