An Ionizing Ultraviolet Background Dominated by Massive Stars

We discuss implications of a stellar-dominated UV background at highredshifts. The composite spectrum of 29 LBGs evaluated by Steidel etal (2000)at =3.4 can be well fit by a stellar population with ongoing star formation,a Salpeter initial mass function, modest or negligible dust reddening, and nointrinsic HI photoelectric absorption. Fading starbursts in which starformation has ceased for 10^7 yr or more cannot reproduce the observed fluxshortward of 1 Ryd.The escape fraction of H-ionizing photons must be close to100 percent for the observed sample of LBGs. The spectrum of ionizing photonsproduced by a stellar population with ongoing star formation is similar to thatof QSOs between 1 and 3 Ryd, becomes softer between 3 and 4 Ryd and dropssharply shortward of 4 Ryd. A galaxy-dominated UV background appearsinconsistent with the observed HeII/HI opacity ratio at z=2.4, but might beable to explain the SiIV/CIV abundances measured at z>3 in absorption spectra.A scenario may be emerging where star-forming galaxies reionized intergalactichydrogen at $z>6$ and dominate the 1 Ryd metagalactic flux at z>3, withquasi-stellar sources taking over at low redshifts. If the large amplitude ofthe H-ionizing flux estimated by Steidel etal is correct, hydrodynamicalsimulations of structure formation in the IGM require a baryon density (toexplain the observed Lyalpha opacity in absorption spectra) which is similar toor larger than that favoured by recent CMB experiments, and is inconsistentwith standard nucleosynthesis values.Comment: 4 pages, LaTeX, emulateapj, 4 postscript figures included; submitted to Ap