The ultraviolet spectra of the nuclei of spiral galaxies. I - NGC 4594, 3031, 5194 and 4258

Abstract

Absolute spectrophotometry is presented from 1250 A to 4 microns within the same aperture in the nuclei of four spiral galaxies. The light distributions within the apertures are often irregular but no positive evidence for internal reddening has been found, except possibly for NGC 3031. The absence of any reddening features could still be consistent with some dust if the extinction curve were not the same as in the Galaxy. The populations of stars of different types have been determined via a simple synthesis technique. The number of hot stars is adequate to photoionize the gas and explain the Balmer emission spectrum. It is argued that ultraviolet spectral features indicate these stars are young main-sequence objects. The mass functions determined from the models are not inconsistent with those seen in the solar neighbourhood. Finally, the application of nuclear energy distributions for K-corrections used with faint galaxy data are briefly discussed.