Constraints for the Shape of the Ultraviolet Background and Z=2

Details Constraints for the Shape of the Ultraviolet Background and Z=2 Constraints for the Shape of the Ultraviolet Background and Z=2

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...274l...5v&link_type=abstract

Astronomy and Astrophysics, Vol. 274, NO. 1/JUL(I), P. L5, 1993

Astronomy and Astrophysics

Astrophysics

27

Scientific paper

Spectroscopic UV observations with the Hubble Space Telescope of the QSO HS 1700+6416 (z = 2.72) (Reimers et al. 1992,1993) offer for the first time the possibility to measure column densities of C, N and O simultaneously in several ionization stages (O III to O VI, C III and C IV, N II to N V). Photoionization models have been computed and fitted to observed column density ratios of three heavy-element systems (z = 1.8465, 2.1678, 2.433) in order to constrain the shape and intensity of the ionizing background radiation field. An optimal fit to observed ion column densities for each of the three redshift systems is obtained for an ionizing power law continuum with α = -0.6 (f_v_ ~ valpha^), while -0.1 >= α >= -1.5 is still consistent with the data. All three systems require the same radiation field which favours a universal background radiation field as origin of the ionization. The systems z = 1.8465 and 2.1678 exclude a He II 228A break in the ionizing radiation field by more than a factor of 5, inconsistent with published AGN based background radiation fields (e.g. Madau 1992). Harder QSO spectra (α >= -1) up to 100 eV and less Lyα cloud absorption could save the AGN based model. Models in which the UV background is dominated by young, starforming galaxies are ruled out by our observations because they typically show a strong He II break (Miralda-Escude & Ostriker 1990). Consequences for the possible origin of the UV background are briefly discussed. We find abundances [C/H = -1.7, -2.3, -1.9, [O/C] = 0.6, 0.9, 0.8, [N/C] = 0.4, 0.8, 0.8 for z = 1.8465, 2.1678 and 2.433, respectively. The probable origin of the LLSs is Halo gas of early galaxies.

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