Radial Velocities and Masses of Double Galaxies.

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Abstract
With a grating spectrograph at the prime focus of the 82-inch reflector, spectra have been obtained of 20 double galaxies at approximately 330 A/mm dispersion. For each exposure, the slit was placed across both members of a pair and gnided accurately so that two spectra appear side by side between the comparison spectra on each film. These films were measured both for the mean radial velocity of each pair, V, values of which range from 900 to 6700 km/sec, and for the difference in radial velocity between the two members of each pair, V, which ranges from 0 to 360 km/sec. Measurement was facilitated by the emission lines, Ha and [0 II] X 3727, which appear in many of the spectra. The errors are estimated to be about +100 km/sec in V and i30 km/sec in AV. Using the measured values of V, the velocity-distance relation, and the angnlar separations, the projected linear separation of each pair is derived. From this and the measured value of AV, and assuming circular orbits, a "double-angled mass function"is obtained for each pair. The mean of these mass functions yields as a mean mass for the components of double nebulae, the value 8 X 1010 solar masses with an estimated observational error less than +40 per cent. Although the observations do not yet permit an accurate determination of the frequency distribution of masses among these nebulae, there is good evidence in these and other data that many individual masses are over twice the mean, and many others are one-tenth the mean or less. In fact, these limited data indicate a sharp division betweeii heavy-weight and light-weight galaxies of 1.5 X 10" and 0.5 X t0 solar masses, respectively.

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