Spectral shape of the UV ionizing background and OVI absorbers at z ~ 1.5 towards HS0747+4259

Abstract

We report on high resolution spectra of the bright QSO HS0747+4259 (zem = 1.90, V = 15.8) observed to search for intermediate redshift OVI absorption systems. The spectra were obtained by means of the Space Telescope Imaging Spectrograph (STIS) at the Hubble Space Telescope (HST) and the High Resolution Echelle Spectrometer (HIRES) at the W. M. Keck telescope. We identify 16 OVI systems in the range 1.07 <= z <= 1.87. Among them, six systems with zabs = 1.46-1.8 exhibit a sufficient number of lines of different ionic transitions to estimate the shape of the ionizing radiation field in the range 1 Ryd < E < 10 Ryd. All recovered UV ionizing spectra are characterized by the enhanced intensity at E > 3 Ryd compared to the model spectrum of Haardt and Madau (1996). This is in line with the observational evidence of a deficiency of strong Ly-alpha absorbers with N(HI) > 10^{15} cm^{-2}, at z < 2. The UV background shows significant local variations: the spectral shape estimated at z = 1.59 differs from that obtained at z = 1.81 and 1.73. A possible cause of these variations is the presence of a QSO/AGN at z ~= 1.54-1.59 close to the line of sight. No features favoring the input of stellar radiation to the ionizing background are detected, limiting the escape fraction of the galactic UV photons to f_esc < 0.05.