Is there primordial gas in IZw 18 ?
Abstract
The dwarf emission line galaxy IZw 18 has one of the lowest known O/H abundance 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) = 10^22^ H atoms cm^-2^). We have obtained spectra of the northwest emission region with the GHRS on the Hubble Space Telescope in a spectral range containing OI λ1302A and SiIIλ1304A and another range around Lyα. The GHRS spectrum combined with a lower resolution IUE spectrum shows that the HI column density in front of the northwest emission patch is N(HI) = 3.5 10^21^ cm^-2^. Accordingly, the absorption spectrum of IZw 18 would be classified as being a "damped Lyman α system. No Lyman α 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 I 8. The abundances are very uncertain but the best values are O/H = 7.9 10^-7^ and Si/H = 7.5 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 HII gas. Accordingly, we conclude that the O/H abundance in the HI region is a factor of about 20 below that in the HII region and that most of the heavy elements in the HII region have been produced in the present burst of star formation. Moreover, no more than one burst, analog to the present one, is required to produce most of the metals dispersed in the HI. We show that the dust to gas ratio in the neutral gas of IZw 18 is low and that it is consistent with the low heavy element abundance. IZw 18 thus has the lowest known heavy element abundance for low redshift interstellar gas; only certain QSO absorption systems at high redshifts have comparably low abundances. Up to now, no material with proven zero abundance has been found in the Universe. This could suggest some early nucleosynthesis of heavy elements (the hypothetical Population III). Our new results strengthen the importance of IZw 18 to determine the primordial He abundance. The main remaining uncertainty lies in the determination of He/H in IZw 18 itself. We also detected OI and SiII absorption at a velocity of -160 km/s due to a Galactic high velocity cloud (No. 117 of Hulsboch and Wakker). 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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- February 1994
- Bibcode:
- 1994A&A...282..709K
- Keywords:
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- 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;
- Astronomy