Estimating the masses of galaxy groups - Alternatives to the virial theorem

Abstract

The authors present three alternatives to the virial theorem for estimating the masses of groups of galaxies. The projected mass estimator uses the mean value of V<SUB>i</SUB><SUP>2</SUP>|R<SUB>i</SUB>|, where V<SUB>i</SUB> and R<SUB>i</SUB>are the radial velocity and projected separation of galaxy i from the group center. The other two methods rely on the average and median of (V<SUB>i</SUB>-V<SUB>j</SUB>)<SUP>2</SUP>|R<SUB>i</SUB>-R<SUB>j</SUB>| over all pairs of galaxies in the group. These three estimators and the virial theorem estimator are tested using a series of N-body simulations and Monte Carlo realizations of Michie models. The authors use the mass estimators to calculate the masses and mass-to-light ratios of nearby groups catalogued by Huchra and Geller. The results confirm their work, implying that there is a large amount of dark mass in groups on scales of ≡700 kpc.