Abundances in the Neutral Medium of the Blue Compact Galaxy IZW 18

Details Abundances in the Neutral Medium of the Blue Compact Galaxy IZW 18 Abundances in the Neutral Medium of the Blue Compact Galaxy IZW 18

Jul 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6710l&link_type=abstract

HST Proposal ID #6710

Computer Science

Hst Proposal Id #6710 Galaxies &Amp, Clusters

Scientific paper

In a previous observation with the GHRS we obtained a very lowvalue (1/1000 of solar) of the abundance of oxygen in theneutral medium surrounding the ionH2 regions of the bluecompact galaxy IZw 18. This low value has profoundconsequences on the evolution of blue compact galaxies, on themixing processes in such objects, and on the possibleexistence of the Population III. Unfortunately, thisabundance determination is of very low accuracy due to thesaturation of the ionO1 line. We now propose using the GHRSwith the G140M grating at full resolution to observe ionS2absorption lines at 1251, 1254 and 1260 Angstrom arising fromthe gas in front of the ionizing stars of the ionH2 region.The relatively low column density of ionS2, the broad range ofoscillator strengths in the three lines, and the goodwavelength-resolving power of G140M, will allow us to obtainan accurate value of the column density of ionS2 and of thevelocity dispersion. These measurements will enable us todirectly test whether the metallicity of the absorbing gas isas low as previously deduced. The sightline toward IZw18 also passes through a Milky Way High Velocity H I Cloud(HVC); the data we propose to obtain will enable us todetermine a limit to the metallicity of the gas in this HVC, ameasurement not yet made for any of these types of clouds.

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