THE O/C ABUNDANCE RATIO IN ABSORBING GAS CLOUDS AT HIGH REDSHIFT

THE intrinsic extreme ultraviolet (EUV) spectrum of quasars, in which spectral lines of He and CNO ions should appear, is in most cases strongly absorbed by the hydrogen Lyman continuum of intergalactic matter at cosmological distances. The bright quasar HS1700+6416 (at redshift z = 2.72), from the Hamburg survey1, is a fortuitous exception, the line of sight towards it being relatively transparent. Here we report detailed ultraviolet spectroscopic measurements of HS1700+6416, obtained with the Hubble Space Telescope. We observe a rich absorption line spectrum, in which many EUV resonance lines (including He I, O II to O V, and N III and N IV) are seen. The O/C abundance ratios inferred from the line-strengths are higher than Solar System values by factors of 3 to 5. This suggests that the absorbing material is halo gas of early galaxies, from which population II stars have yet to form, oxygen being a primary nucleosynthesis product from a first generation of massive stars.