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The Astronomical Journal


Department of Physics and Astronomy


We describe observations of the nearby (cz = 11,487 km s-1) cluster of galaxies Abell 576 beyond the virial radius and into the infall region where galaxies are on their first or second pass through the cluster. Using 1057 redshifts, we use the infall pattern in redshift space to determine the mass profile of A576 to a radius of ~4 h-1 Mpc. This mass estimation technique makes no assumptions about the equilibrium state of the cluster. Within ~1 h-1 Mpc, the mass profile we derive exceeds that determined from X-ray observations by a factor of ~2.5. At ~2.5 h-1 Mpc, however, the mass profile agrees with virial mass estimates. Our mass profile is consistent with a Navarro, Frenk, & White or Hernquist profile, but it is inconsistent with an isothermal sphere. R-band images of a 3° × 3° region centered on the cluster allow an independent determination of the cluster light profile. We calculate the integrated mass-to-light ratio as a function of cluster radius; it decreases smoothly from the core to M/LR~300 h at ~4 h-1 Mpc. The differential M/LR profile decreases more steeply; we find δMLR ~ 100 h at ~4 h-1 Mpc, in good agreement with the mass-to-light ratios of individual galaxies. If the behavior of M/LR in A576 is general, Ωm 0.4 at 95% confidence. For a Hernquist model, the best-fit mass profiles differ from the observed surface number density of galaxies; the galaxies have a larger scale radius than the mass. This result is consistent with the centrally peaked M/LR profile. Similarly, the scale radius of the light profile is larger than that of the mass profile. We discuss some potential systematic effects; none can easily reconcile our results with a constant mass-to-light ratio.