Interstellar absorption lines in the spectrum of the starburst galaxy NGC 1705

Abstract

I present a GHRS archival study of the interstellar absorption lines in the line-of-sight to the HI-rich starburst dwarf galaxy NGC 1705, in the 1170 to 1740A range at ~ 120 km/s resolution. The absorption features arising due to photospheric lines are distinctly different from the interstellar lines. The interstellar lines consist of three relatively narrow components at LSR velocities of -20, 260 and 540 km/s and include absorption by neutral atoms, singly ionized atoms and atoms in higher ionization states. The analysis presented in this paper consists of (1) a list of all interstellar absorption features with > 3 sigma significance and their measured equivalent widths and (2) plots of the lines in the various atomic species together with the results of non-linear least square fit profiles to the observed data and (3) unpublished 21-cm maps from the Wakker & van Woerden survey showing the large-scale HI distribution in the region near the NGC 1705 sightline and high velocity cloud HVC 487. Further, I report weak N I 1200A triplet absorption for the supershell component, which in the absence of dust depletion and ionization corrections implies a low N abundance. A low N abundance for the supershell is consistent with an interpretation of nucleosynthetic enrichment by time-delayed "primary" nitrogen production, the age estimate of 10 - 20 x 10^6 years for the central superstar cluster NGC 1705-1 (Heckman and Leitherer, 1997) and the underabundance of Fe reported by Sahu and Blades (1997).