An ionizing UV background dominated by massive stars

Abstract

We discuss implications of a stellar-dominated UV background at high redshifts. The composite spectrum of 29 LBGs evaluated by Steidel etal (2000) at <z>=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 no intrinsic HI photoelectric absorption. Fading starbursts in which star formation has ceased for 10^7 yr or more cannot reproduce the observed flux shortward of 1 Ryd.The escape fraction of H-ionizing photons must be close to 100 percent 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, becomes softer between 3 and 4 Ryd and drops sharply shortward of 4 Ryd. A galaxy-dominated UV background appears inconsistent with the observed HeII/HI opacity ratio at z=2.4, but might be able to explain the SiIV/CIV abundances measured at z>3 in absorption spectra. A scenario may be emerging where star-forming galaxies reionized intergalactic hydrogen at $z>6$ and dominate the 1 Ryd metagalactic flux at z>3, with quasi-stellar sources taking over at low redshifts. If the large amplitude of the H-ionizing flux estimated by Steidel etal is correct, hydrodynamical simulations of structure formation in the IGM require a baryon density (to explain the observed Lyalpha opacity in absorption spectra) which is similar to or larger than that favoured by recent CMB experiments, and is inconsistent with standard nucleosynthesis values.