Multicolor photoelectric photometry of bright extragalactic systems

Abstract

57 galaxies and 52 stars have been observed in four colors using a blue sensitive phototube. The response bands of the photometric system have effective reciprocal wave lengths of (micron units) 2.66, 2.39, 2.07, and 1.68. In addition to the blue measurements 8 galaxies and 19 stars have been observed in four colors using a red sensitive photocell. The effective reciprocal wave lengths of the red response bands are 1.55, 1.26, 1.14, and 1.00. From one to five different aperture sizes have been used in the photometry of each galaxy, with an average of three per galaxy. The entire eight color photometric system has been calibrated to place color index measurements on an absolute energy basis. Correlations between nuclear color index and inclination, spectral class, and nebular type indicate that most of the observed galaxies fall into fairly distinct groups. Radial color variations are also found to correlate with group membership. The dominant characteristics of the groups are as follows: Spectral type A, spectral type F, spectral type FG, nebular type SA, nebular type E, nebular type SO, nebular type SB. The correlation between nuclear color and inclination (axis ratio) is found to show a distinct separation of SA, SB, and elliptical galaxies, probably due in part to differences in internal absorption. The correlation of nuclear color and spectral type shows that FG galaxies, while later in spectral class than F galaxies, have nuclei bluer than those in F galaxies. A possible explanation is offered which interprets the inversion in terms of the relative numbers of normal and low metal abundance stars in each type of galaxy. Radial color investigation reveal the existence of blue nuclei in three galaxies. Synthetic models are given for five distinct types of extragalactic nuclei. K nuclei can be represented with a mixture of old population I and II stars. F, FG, and probably AF nuclei can be produced by adding various numbers of young blue stars to K galaxies. A type galaxies appear to be completely dominated by young stars.