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Session 77 - Irregular and Star Burst Galaxies.
Oral session, Thursday, June 11

[77.04] Interstellar Absorption Lines in the Spectrum of the Starburst Galaxy NGC 1705

M. S. Sahu (NASA/GSFC)

I present the results of a GHRS archival study of the interstellar absorption lines in the line-of-sight to the H \sc i-rich, starburst dwarf galaxy NGC 1705 in the 1170 to 1740 Å\ range at \sim 120 \mboxkm\,s^-1 resolution. In an earlier study using the same data, Sahu amp; Blades (1997) identified the interstellar component at --20 \mboxkm\,s^-1 (LSR velocities) with Milky Way disk/halo gas and the 260 \mboxkm\,s^-1 component with a small, isolated high-velocity cloud HVC 487, located \sim 10^\circ from the H \sc i gas which envelops the Magellanic Clouds and the Magellanic Stream (MS). The 540 \mboxkm\,s^-1 absorption component seen in the GHRS spectra was identified with the front side of a kpc-scale expanding, ionized supershell centred on the central superstar cluster NGC 1705-1 and has a low Fe/Al abundance ratio.

I report a low N/Si abundance ratio for the 540 \mboxkm\,s^-1 component. Massive stars (> 10 M_ødot) evolve on time scales \leq 2 \times 10^7 years and return elements like Si into the ISM through Type II SNe while the main contributors to nitrogen are intermediate stars (1 to 10 M_ødot) through He shell burning. The N/Si ratio of the supershell component of NGC 1705 is a good indicator of the age of the dwarf galaxy: the N/Si is expected to decrease because of the increase in Si through Type II SNe due to the massive stars, until the onset of He shell burning in intermediate mass stars which occurs on time scales of 10^8 -- 10^9 years. The data point to nucleosythetic enrichment of the supershell component by the first generation of massive stars in NGC 1705-1 over a time scale of < 10^8 years. This time scale for the supershell component is consistent the age estimate of 10 -- 20 \times 10^6 years for NGC 1705-1 (Heckman and Leitherer, 1997) and with the low Fe/Al abundance reported by Sahu and Blades.

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