NGC 5194/5. - The HII regions of NGC 5194 have been catalogued by Carranza, Crillon, and Monnet (1969) who also measured their velocities.
There appears to be a strong HII region south of the nucleus, which is marked by a cross on the drawing. The characteristic association of H II regions with the dust lanes and feather is clearly evident in NGC 5194. The brightest HII regions in the inner part of the spiral are found embedded in the darkest lanes or are associated with the feathers of dust which branch out from the primary lanes. A few fainter HII regions are found on the outer edges of the bright arms but also appear to be closely associated with heavy dust feathers. The blue 2.3-m photograph was taken during the conditions of poor seeing and the details of the dust shown on this sketch were based primarily on the superb Hubble atlas print of this galaxy.
The extension of the northern arm across NGC 5195 is shown on the following drawing and the photographs confirm the impression stated by Sandage (1961) that NGC 5195 has dust internal to itself.
A comparison of the high-resolution radio continuum study of M51 (Mathewson, van der Kruit, and Brouw 1972) has been made with the sketch of the distribution of dust. The two radio spiral arms are essentially coincident with the primary dust lanes.
If the distance modulus to M51 is 29.9 (van der Kruit 1973), the dust lanes are about 100 pc in width; the bright arms are about 1 kpc in thickness.