Ultraviolet Background Radiation from Cosmic Structure Formation

We calculate the contribution to the ultraviolet background (UVB) from thermal emission from gas shock heated by cosmic structure formation. Our main calculation is based on an updated version of Press-Schechter theory. It is consistent with a more empirical estimate based on the observed properties of galaxies and the observed cosmic star formation history. Thermal UVB emission is characterized by a hard spectrum extending well beyond 4 Ry. The bulk of the radiation is produced by objects in the mass range 10^11-13 M_solar, i.e. large galaxies and small groups. We compute a composite UVB spectrum due to QSO, stellar and thermal components. The ratio of the UVB intensities at the H and He Lyman limits increases from 60 at z=2 to more than 300 at z=6. A comparison of the resulting photoionization rates to the observed Gunn-Peterson effect at high redshifts constrains the escape fraction of ionizing photons from galaxies to be less than a few percent. Near 1 Ry, thermal and stellar emission are comparable amounting to about 10 %, 20 % and 35 % of the total flux at redshifts of 3, 4.5 and higher, respectively. However, near the ionization threshold for HeII, the thermal contribution is much stronger. It is comparable to the QSO intensity already at redshift ~3 and dominates at redshifts above 4. Thermal photons alone are enough to produce and sustain HeII reionization already at z~6. We discuss the possible implications of our results for the thermal history of the intergalactic medium, in particular for HeII reionization.