Structure of galaxy groups and clusters and measurement of their masses

We report the results of measurement and comparison of masses for a sample of 29 groups and clusters of galaxies (z < 0.1). We use the SDSSDR7 archive data to determine dynamical masses from the one-dimensional dispersion of line-of-sight velocities for virialized regions of radii R200 and Re. Our method for determination of effective radii of galaxy systems fromthe cumulative distribution of the number of galaxies depending on squared clustercentric distance allowed us to estimate masses M1/2 (within Re), which are related to the masses contained inside R200: M200 ~ 1.65M1/2. A comparison of the inferred dynamic masses and the hydrostatic masses determined from the radiation of hot gas in galaxy groups and clusters (based on published data) led us to conclude that the inferred masses for the main sample of 21 groups and clusters agree to within 12%. These systems also obey the relation MX,200 ~ 1.65M1/2. For the remaining eight systems, which are mostly located in the Hercules supercluster, the discrepancy between the hydrostatic and the dynamic masses amounts to 2σ. This discrepancy is most likely due to the incompleteness of the formation processes of these clusters via hierarchical merger in the region of the rich Hercules supercluster.