KINEMATICS OF THE INTERSTELLAR-MEDIUM IN NGC-1705 - IMPLICATIONS FOR DISTANT STAR-FORMING GALAXIES

We report the results of space-ultraviolet (UV) spectroscopy of the su per star cluster ''1'' in the nearby dwarf starburst galaxy NGC 1705, using the Goddard High Resolution Spectrograph on the Hobble Space Tel escope. The exceptionally high UV flux and surface brightness of NGC 1 705-1 makes it ideal for studying how UV spectral diagnostics can be u sed to infer the properties of the stellar and interstellar components of starbursts at low and high redshift. There are three main results of our analysis of these data. The first is that most of the strong ab sorption lines in the spectrum of NGC 1705-1 are interstellar rather t han stellar. This highlights the difficulty in using the UV spectra of galaxies to deduce their stellar content. The second is that we are n evertheless able to cleanly detect the signature of the stellar popula tion in NGC 1705-1 in the form of photospheric absorption lines from e xcited transitions. We deduce an age of about 10 Myr for NGC 1705-1, s o it is a ''postburst'' in which a significant population of O stars i s no longer present. The third is that the strong resonance absorption lines are blueshifted by about 80 km s(-1) relative to the stellar ph otospheric lines, implying that the former arise in gas in the front p art of the expanding superbubble seen in H alpha. Thus, the interstell ar absorption lines are telling us a great deal about the hydrodynamic al consequences of a starburst on the interstellar medium, but provide little direct information about the gravitational potential or mass o f the galaxy. These results have potentially important implications fo r the interpretation of rest-frame UV spectra of high-redshift galaxie s. (C) 1997 American Astronomical Society.