On the Physical Characteristics of the Hydra Cluster of Nebulae

Abstract

A. Observational Data. - In a previous paper [1] counts of the brighter nebulae in the Hydra cluster were communicated. The distance of the Hydra cluster was estimated as 7.3 X 10^6 parsecs and the average apparent velocity of recession to be expected was given as v = 4100 km./sec. No observations of the red shift for nebulae in the Hydra cluster were available at the time. Dr. Hubble subsequently obtained a spectrum plate for NGC 3309 which nebula is a member of the Hydra cluster. He kindly informs me that the apparent velocity of recession for NGC 3309 is of the order of 3950 km./sec., a value which is in good agreement with our original estimate. B. The Radial Distribution of Nebulae in the Hydra Cluster. - As emphasized previously [1] the Hydra cluster exhibits spherical symmetry and is therefore suspected to have reached a statistically stationary state. In order to check this conclusion two additional tests are available. In the first place we may compare the observed radial distribution of nebulae in the Hydra cluster with the distribution derived by Emden [2] for the bounded isothermal gravitational gas sphere and secondly we can verify whether the ratio wr^2/p0 of the square of the dispersion in radial velocities of the cluster nebulae to the central density po of the cluster can be correctly determined from the observed structural length or structural index a of the cluster [3].

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