Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...282..709k&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 282, no. 3, p. 709-716
Astronomy and Astrophysics
Astronomy
107
Abundance, Dwarf Galaxies, H I Regions, Interstellar Gas, Lyman Spectra, Spaceborne Astronomy, Ultraviolet Astronomy, Absorption Spectra, Astronomical Spectroscopy, Hubble Space Telescope, Iue, Lyman Alpha Radiation, Spectrographs
Scientific paper
The dwarf emission line galaxy IZw 18 has one of the lowest known O/H abundances ratios (O/H=1.75 10-5) deduced from its emission lines. Its two main emission patches are associated with two extended HI clouds with very high column densities (N(HI)=1022 H atoms/sq cm). We have obtained spectra of the NW emission region with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope in a spectral range containing OI lambda(1302)A and SiII lambda(1304)A and another range around Lyman-alpha. The GHRS spectrum combined with a lower resolution IUE spectrum shows that the HI column density in front of the NW emission patch is N(HI)=3.5 x 1021/sq cm. Accordingly, the absorption spectrum of IZw 18 would be classified as being a 'damped Lyman-alpha' system. No Lyman-alpha emission is detected from IZW 18 due to multiple scattering in the HI gas. Absorption lines due to OI and SiII were detected at the redshift of IZw 18. The abundances are very uncertain but the best values are O/H=7.9 x 10-7 and Si/H=7.5 x 10-8. We show that most of the observed OI is produced in the HI gas in IZw 18 and very little in the transition region of the HI gas. Our new results strengthen the importance of IZw 18 determine the primordial He abundance. We also detected OI and SiII absorption at a velocity of -160 km/s due to a Galactic high velocity cloud. We derive abundance ratios O/H and Si/H which are about 1/10 that in the local interstellar gas, illustrating that the high velocity clouds are not composed of primordial material.
Kunth Daniel
Lequeux James
Sargent Wallace L. W.
Viallefond Francois
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