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\begin{document}

\title{UV INTERSTELLAR ABSORPTION LINES TOWARDS THE STARBURST\\
~\\
DWARF GALAXY NGC~1705\/\thanks{Based on observations taken with 
the NASA/ESA {\it Hubble Space Telescope\/}, obtained at the 
Space Telescope Science Institute, which is operated by the 
Association of the Universities for Research in Astronomy
under contract NAS 5-26555.}
} 

\author{M.\ S.\ Sahu and J.\ Chris Blades\\
\\
Space Telescope Science Institute\\
\\
3700 San Martin Drive\\
\\
Baltimore, MD 21218\\
\\
msahu@stsci.edu, blades@stsci.edu}

\pub{ApJ Letters}
\recacc{3 March 1997}{13 May 1997}

\maketitle

\abstract{Archival Goddard High Resolution Spectrograph low-resolution 
spectra of NGC~1705, with wavelength ranges 1170.3 to 1461.7~\AA\ and 
1453.5 to 1740.1~\AA\ and a velocity resolution $\sim100$~\kms, have 
been used to derive the velocity structure and equivalent widths of 
the absorption lines of Si~{\sc ii} $\lambda$1190.42, 
$\lambda$1260.42, $\lambda$1304.37 and $\lambda$1526.71~\AA, S~{\sc ii} 
$\lambda$1253~\AA, Al~{\sc ii} $\lambda$1670.79~\AA\ and Fe~{\sc ii} 
$\lambda$1608.45~\AA\ in this sightline. Three relatively narrow 
absorption components are seen at LSR velocities $-20$~\kms, 260~\kms\ 
and 540~\kms. Arguments are presented to show these absorption features 
are interstellar rather than stellar in origin based on a comparison 
with the C~{\sc iii} $\lambda$1175.7~\AA\ absorption feature. 
We identify the $-20$~\kms\ component with Milky Way disk/halo gas and 
the 260~\kms\ component with an isolated high-velocity cloud HVC~487. 
This small HVC is located $\sim10^\circ$ from the H~{\sc i} gas which 
envelops the Magellanic Clouds and the Magellanic Stream (MS). The 
(Si/H) ratio for this HVC is $>0.6$~(Si/H)$_\odot$ which together with 
velocity agreement, suggests association with the Magellanic Cloud 
and MS gas.

H$_{\alpha}$ 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~\kms\  (Meurer 
\etal 1992).  We identify the 540~\kms\ absorption component seen 
in the {\it GHRS\/} spectra with the front side of this expanding, 
ionized supershell. The most striking feature of this component is 
strong Si~{\sc ii} and Al~{\sc ii} absorption but weak Fe~{\sc ii} 
$\lambda$1608~\AA\ absorption. The low Fe~{\sc 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 from strong winds driven by the supernovae from the 
first generation of stars. Galactic winds from dwarf galaxies occur 
at time-scales $<10^{8}$ years, which is less than the time-scale 
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.}