Six-Color Photometry of Stars. VI. The Colors of Extragalactic Nebulae.

Photoelectric measures in six colors have been made of eight bright extragalactic nebulae of types E to Sc. As was to be expected from their mixed stellar content, the nebulae show excess radiation in both the ultraviolet and the infrared over that of a star of matching color in the visual-photographic region. With a suitable combination of stars of early and late types, the color can be reproduced over the whole observed range, while the spectrum in the blue-violet would appear to be that of stars of an earlier type than the color class in the same region. There is no need to assume internal space absorption in the nebulae to account for the difference between spectral class and color class. With the 1P21 photomultiplier tube and a two-filter system, magnitudes and colors of E-type nebulae in clusters have been measured with the 100-inch reflector down to lflpg = 18.2. The nebulae are found to be redder in proportion to their velocity of recession until for the Boötes cluster at 70,000,000 parsecs with V = 38,900 km/sec the color excess is 0.52 mag. on the International system, of which 0.22 mag. can be explained by the red shift, and 0.30 mag. is unaccounted for. Following are the tentative con- clusions: 1. The reddening of the nebulae is proportional to the red shift. On the plausible assumption that the unexplained part of the excess is linear with distance, this result supports a linear velocity-distance rela- tion. 2. The excess reddening can be explained as selective absorption by internebular material of mean density ten to one hundred times the smoothed density of the luminous nebulae in all space. The con- siderable inward corrections to the previously assumed distances would, however, be difficult to reconcile with the nebular counts. 3. The excess reddening can be interpreted as an intrinsic change in E-type nebulae in the 2 X 108 years since the observed light left the Boötes cluster. Fading of red supergiants as their energy sources are consumed could provide a mechanism. 4. Both the excess reddening and the ultraviolet excess found in the six-color results will affect the luminosity correction to be applied to a faint nebula with a large red shift. Previous ConcluSions about the interpretation of the nebular counts are therefore open to revision