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. The spectra have been extracted with an optimal algorithm and co-added to produce spectra with the best possible signal-to-noise ratio. Our data support the picture proposed earlier on the basis of UV spectra from the Orbiting Astronomical Observatory and from the Astronomical Netherlands Satellite that spiral galaxies of later Hubble class have higher flux at the shortest UV wavelengths than do spiral galaxies of earlier Hubble class. A comparison of the UV spectra of low-ionization nuclear 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 featureless blue continuum plus emission lines, with the UV spectrum of the host galaxy apparent only at the longest UV wavelengths. The starbursting, 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 lack of an apparent dust feature (e.g., the 2200 angstrom bump) implies either 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.