Stellar abundances and molecular hydrogen in high-redshift galaxies: The far-ultraviolet view

Abstract

FUSE spectra of star-forming regions in nearby galaxies are compared to composite spectra of Lyman break galaxies (LBGs), binned by strength of Ly alpha emission and by mid-UV luminosity. Several far-UV spectral features, including lines dominated by stellar wind and by photospheric components, are very sensitive to stellar abundances. Their measurement in LBGs is compromised by the strong interstellar absorption features, allowing in some cases only upper limits to be determined. The derived C and N abundances in the LBGs are no higher than half solar (scaled to oxygen abundance for comparison with emission-line analyses), independent of the strength of Ly alpha emission. P v absorption indicates abundances as low as 0.1 solar, with an upper limit near 0.4 solar in the reddest and weakest emission galaxies. Unresolved interstellar absorption components would further lower the derived abundances. Trends of line strength and derived abundances are stronger with mid-UV luminosity than with Ly alpha strength. H-2 absorption in the Lyman and Werner bands is very weak in the LBGs. Template H-2 absorption spectra convolved to the appropriate resolution show that strict upper limits N(H-2) < 10(18) cm-(2) apply in all cases, with more stringent values appropriate for the stronger emission composites and for mixes of H-2 level populations like those on Milky Way sight lines. Since the UV-bright regions are likely to be widespread in these galaxies, these results rule out massive diffuse reservoirs of primordial H2 and suggest that the dust-to-gas ratio is already fairly large at z approximate to 3.