0 v 1 1 9 M ay 1 99 7 UV Interstellar Absorption Lines towards the Starburst Dwarf Galaxy NGC

Archival Goddard High Resolution Spectrograph low-resolution spectra of and a velocity resolution ∼ 100 km s −1 , have been used to derive the velocity structure and equivalent widths of the absorption lines of Si ii λ1190.42, λ1608.45Å in this sightline. Three relatively narrow absorption components are seen at LSR velocities –20 km s −1 , 260 km s −1 and 540 km s −1. Arguments are presented to show these absorption features are interstellar rather than stellar in origin based on a comparison with the C iii λ1175.7 ˚ A absorption feature. We identify the –20 km s −1 component with Milky Way disk/halo gas and the 260 km s −1 component with an isolated high-velocity cloud HVC 487. This small HVC is located ∼ 10 • from the H i gas which envelops the Magellanic Clouds and the Magellanic Stream (MS). The (Si/H) ratio for this HVC is > 0.6 (Si/H) ⊙ which together with velocity agreement, suggests association with the Magellanic Cloud and MS gas. H α emission line kinematics of NGC 1705 show the presence of a kpc-scale expanding supershell of ionized gas centered on the central nucleus with a blue-shifted emission component at 540 km s −1 (Meurer et al. 1992). We identify the 540 km s −1 absorption component seen in the GHRS spectra with the front side of this expanding, ionized supershell. The most striking feature of this component is strong Si ii and Al ii absorption but weak Fe ii λ1608Å absorption. The low Fe ii column density derived is most likely intrinsic since it cannot be accounted for by ionization corrections or dust depletion. Due to their shallow gravitational potential wells, dwarf galaxies have small gravitational binding energies and are vulnerable to large mass losses – 3 – from strong winds driven by the supernovae from the first generation of stars. Galactic winds from dwarf galaxies occur at timescales < 10 8 years, which is less than the timescale required to produce Type Ia supernovae. Consistent with our observations, shells produced by galactic winds are expected to be enriched with Type II supernova products like Si, Al, Mg and should be deficient in the products of Type Ia supernovae, like Fe and iron-peak elements.