AN ATLAS OF ULTRAVIOLET-SPECTRA OF STAR-FORMING GALAXIES

An atlas of ultraviolet spectra of the central regions of 143 spiral, irregular, blue compact, Seyfert 2, and starburst galaxies is produced by combining 387 low-resolution spectra from the IUE data archives. T he spectra have been extracted with an optimal algorithm and co-added to produce spectra with the best possible signal-to-noise ratio. Our d ata support the picture proposed earlier on the basis of UV spectra fr om the Orbiting Astronomical Observatory and from the Astronomical Net herlands Satellite that spiral galaxies of later Hubble class have hig her flux at the shortest UV wavelengths than do spiral galaxies of ear lier Hubble class. A comparison of the UV spectra of low-ionization nu clear emission-line region (LINER) galaxies with the spectra of normal spiral galaxies shows that the UV continuum of LINERs is dominated by the galaxy continuum. In contrast, Seyfert 2 galaxies show a featurel ess blue continuum plus emission lines, with the UV spectrum of the ho st galaxy apparent only at the longest UV wavelengths. The starburstin g, blue compact, and blue compact dwarf galaxies have UV continua that are flat or increasing toward short wavelengths, with a wide range in spectral index, going from F(lambda) is-proportional-to lambda0.26+/- 0.14 to F(lambda) is-proportional-to lambda-1.85+/-0.06. The spectral signature of dust with a wavelength dependence of the extinction such as is seen in the Milky Way is not apparent in these galaxies. The lac k of an apparent dust feature (e.g., the 2200 angstrom bump) implies e ither that the extinction law is different or that any dust present in the galaxies is in the form of clumps and does not contribute to the flux, owing to the very high optical depth.