An ionizing ultraviolet background dominated by massive stars

We discuss the implications of a stellar-dominated UV background at high re dshifts for the star formation history of Lyman break galaxies (LBGs) and t he thermal and ionization state of the intergalactic medium (IGM). The comp osite spectrum of 29 LBGs evaluated by Steidel et al. at (z) = 3.4 can be w ell fitted by a stellar population with ongoing star formation, a Salpeter initial mass function, modest or negligible dust reddening, and no intrinsi c H I photoelectric absorption. Fading starbursts in which star formation h as ceased for 10(7) yr or more cannot reproduce the observed flux shortward of 1 ryd. The small H I optical depth in LBGs suggests that the neutral ga s from which stars form is most likely contained in compact clouds of neutr al gas with small covering factor. The escape fraction of H-ionizing photon s must be close to 100% for the observed sample of LBGs. The spectrum of io nizing 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 bac kground 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-formi ng galaxies reionize intergalactic hydrogen at z > 6 and dominate the 1 ryd metagalactic flux at z > 3, with quasi-stellar sources taking over at lowe r redshifts. If the large amplitude of the H-ionizing flux estimated by Ste idel et al, is correct, hydrodynamical simulations of structure formation i n the IGM within the cold dark matter paradigm require a baryon density (to explain the observed opacity of the Ly alpha forest in QSO absorption spec tra) that is similar to or larger than that favored by recent cosmic microw ave background experiments and is inconsistent with standard nucleosynthesi s values.