  The contribution to the ultraviolet background (UVB) from thermal emission
due to gas shock heated during cosmic structure formation is assessed with an
updated version of Press-Schechter (Sheth & Tormen 1999) formalism. The
calculation is consistent with empirical estimates based on the observed
properties of galaxies and the observed cosmic star formation history. The bulk
of the radiation turns out to be produced by objects in the mass range 10^{11-
13} M_solar, i.e. large galaxies and small groups. When compared to more
conventional components (QSOs, stellar) it is found that near 1 Ry, thermal
emission is comparable to stellar contributions and amounts to about 10 %, 20 %
and 35 % of the total flux at redshifts of 3, 4.5 and higher, respectively;
more importantly, near the ionization threshold for HeII, the thermal
contribution is comparable to the QSO intensity already at redshift ~3 and
dominates at redshifts above 4. In addition, thermal photons alone are enough
to produce and sustain HeII reionization already at z~6. Finally, the observed
Gunn-Peterson effect at high redshifts (3<z<6) constrains the escape fraction
of ionizing photons from galaxies to less than a few percent.
