Ultraviolet Interstellar Absorption Lines toward the Starburst Dwarf Galaxy NGC 1705^*

Archival Goddard High Resolution Spectrograph (GHRS) low-resolution spectra of NGC 1705, with wavelength ranges of 1170.3-1461.7 Å and 1453.5-1740.1 Å and a velocity resolution of ~120 kms^-1, have been used to derive the velocity structure and equivalent widths of the absorption lines of Si II λ1190.42, λ1260.42, λ1304.37, and λ1526.71, S II λ1253, Al II λ1670.79, and Fe II λ1608.45 in this sight line. Three relatively narrow absorption components are seen at LSR velocities of -20 kms^-1, 260 kms^-1, and 540 kms^-1. Arguments are presented to show that these absorption features are interstellar rather than stellar in origin on the basis of a comparison with the C III λ1175.7 absorption feature. We identify the -20 kms^-1 component with Milky Way disk and halo gas and the 260 kms^-1 component with an isolated high-velocity cloud, HVC 487. This small HVC is located ~10° from the H I gas that envelops the Magellanic Clouds and the Magellanic Stream (MS). The Si/H ratio for this HVC is greater than 0.6(Si/H)_☉, which with velocity agreement suggests association with the Magellanic Cloud and MS gas.

Hα emission line kinematics of NGC 1705 show the presence of a kiloparsec-scale expanding supershell of ionized gas centered on the central nucleus with a blueshifted emission component at 540 kms^-1. We identify the 540 kms^-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 λ1608 absorption. The low Fe II column density derived is most likely intrinsic since it cannot be accounted for by ionization corrections or dust depletion.

Because of their shallow gravitational potential wells, dwarf galaxies have small gravitational binding energies and are vulnerable to large mass losses from strong winds driven by the supernovae from the first generation of stars. Galactic winds from dwarf galaxies occur at timescales less than 10⁸ yr, 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, and Mg and should be deficient in the products of Type Ia supernovae, like Fe and iron-peak elements.