An Ionizing Ultraviolet Background Dominated by Massive Stars

We discuss the implications of a stellar-dominated UV background at high redshifts for the star formation history of Lyman break galaxies (LBGs) and the thermal and ionization state of the intergalactic medium (IGM). The composite spectrum of 29 LBGs evaluated by Steidel et al. at (z) = 3.4 can be well fitted by a stellar population with ongoing star formation, a Salpeter initial mass function, modest or negligible dust reddening, and no intrinsic H I photoelectric absorption. Fading starbursts in which star formation has ceased for 107 yr or more cannot reproduce the observed flux shortward of 1 ryd. The small H I optical depth in LBGs suggests that the neutral gas from which stars form is most likely contained in compact clouds of neutral gas with small covering factor. The escape fraction of H-ionizing photons must be close to 100% for the observed sample of LBGs. The spectrum of ionizing photons produced by a stellar population with ongoing star formation is similar to that of QSOs between 1 and 3 ryd but becomes softer between 3 and 4 ryd and drops sharply shortward of 4 ryd. A galaxy-dominated UV background appears inconsistent with the observed He II/H I opacity ratio at z = 2.4 but might be able to explain the Si IV/C IV abundances measured at z > 3 in QSO absorption spectra. A scenario may be emerging where star-forming galaxies reionize intergalactic hydrogen at z > 6 and dominate the 1 ryd metagalactic flux at z > 3, with quasi-stellar sources taking over at lower redshifts. If the large amplitude of the H-ionizing flux estimated by Steidel et al. is correct, hydrodynamical simulations of structure formation in the IGM within the cold dark matter paradigm require a baryon density (to explain the observed opacity of the Lyα forest in QSO absorption spectra) that is similar to or larger than that favored by recent cosmic microwave background experiments and is inconsistent with standard nucleosynthesis values.