Ultraviolet photometry from the Orbiting Astronomical Observatory. XXVI. Energy distributions of seven early-type galaxies and the central bulge of M31.

Medium-broad-band photometry is presented for the integrated light of seven elliptical and 50 galaxies and the central bulge of M31 in terms of flux per unit wavelength interval received at the top of the atmosphere. The observations cover the region 1 A. The energy distribution of the Fornax dwarf galaxy is quite similar to that of the Galactic globular clusters having richly populated blue horizontal branches. The ultraviolet energy of the M3 1 bulge could arise from either the blue horizontal branch of an evolved population or from a young upper main sequence. Stars of the latter type, if present, might be detectable in ground-based ultraviolet photographs. Large scatter is found among the ultraviolet energy distributions of early-type galaxies. While most appear to decline monotonically, there are some cases where an increase in energy is observed at the shortest wavelengths. We examine implications of this scatter for the computation of realistic K-corrections to photometry of very distant elliptical galaxies. At the present epoch most early-type galaxies are much fainter at ultraviolet wavelengths than a typical late-type spiral galaxy. Since both galaxy types are found in at least some rich galaxy clusters, it may be (neglecting evolution) that at large redshifts a spiral galaxy will be observed as the brightest cluster member. Great caution must therefore be exercised in extending the redshift-magnitude diagram to large distances. Subject headings: cosmology - galaxies: individual - galaxies: photometry - ultraviolet: general