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 super star cluster `1' in the nearby dwarf starburst galaxy NGC 1705, using the Goddard High Resolution Spectrograph on the Hubble Space Telescope. The exceptionally high UV flux and surface brightness of NGC 1705-1 makes it ideal for studying how UV spectral diagnostics can be used 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 absorption lines in the spectrum of NGC 1705-1 are interstellar rather than stellar. This highlights the difficulty in using the UV spectra of galaxies to deduce their stellar content. The second is that we are nevertheless able to cleanly detect the signature of the stellar population in NGC 1705-1 in the form of photospheric absorption lines from excited transitions. We deduce an age of about 10 Myr for NGC 1705-1, so it is a `postburst' in which a significant population of O stars is no longer present. The third is that the strong resonance absorption lines are blueshifted by about 80 \kms\ relative to the stellar photospheric lines, implying that the former arise in gas in the front part of the expanding superbubble seen in Hα . Thus, the interstellar absorption lines are telling us a great deal about the hydrodynamical consequences of a starburst on the interstellar medium, but provide little direct information about the gravitational potential or mass of the galaxy. These results have potentially important implications for the interpretation of rest-frame UV spectra of high-redshift galaxies.