The shape of the ionizing UV background at z ∼ 3.7 from the metal absorption systems of Q 0000-2619

Spectra of the z_em_=4.12 quasar Q 0000-2619 have been obtained in the range λλ= 4880-8115Å with a resolution of 13km/s and signal-to-noise ratio of s/n=15-60 per resolution element. The list of the identified absorption lines is given together with their fitted column densities and Doppler widths. The mode of the distribution of the Doppler parameters for the Lyα lines is =~25km/s. The fraction of lines with 10<20km/s is 17%. The Doppler values derived from uncontaminated Lyβ lines are smaller than those obtained from the corresponding Lyα lines, indicating the contribution of non saturated, non resolved components in the Lyα profiles. The integrated UV background estimated from the proximity effect is found to be J~7x10^-22^erg/s/cm^+2^/Hz/sr. This value is consistent with previous estimates obtained at a lower z, implying no appreciable redshift evolution of the UVB up to z=4. 13 metal systems are identified, five of which previously unknown. The analysis of the associated metal systems suggests abundances generally below the solar value with an average [C/H]~-0.5. This value is about one order of magnitude higher than that found in intervening systems at about the same redshift. The analysis of the intervening metal line systems has revealed in particular the presence of three optically thin systems with log N_HI_~15 showing associated CIV and SiIV absorptions. In order to make the observed column densities consistent with [Si/C] ratios lower than 10 times the solar value, it is necessary to assume a large jump in the spectrum of the ionizing UV background beyond the HeII edge (J_912_/J_228_>1000). This result, if confirmed in other spectra at the same redshift is suggestive of a possible dominance of a stellar ionizing emissivity over the declining quasar one at z>3.