First submission -540248356 Assigned ID: 486 ----- Attempting Submission 1 (Fri Apr 11 14:09:15 GMT 2014) ----- The intensity and spectral shape of the cosmic ionizing UV background radiation is important for understanding the structure, formation, evolution and physical state of the low-density intergalactic medium (IGM). The UV background radiation is produced by the integrated radiation from QSOs and galaxies, which is then reprocessed by the clumpy IGM. Despite its importance, the UV background is still poorly known both observationally and theoretically, especially at low redshifts. We propose to constrain the spectral shape of the ionizing UV background radiation at z ~ 0 in the energy range 1 < E < 6 ryd using 16 optically thin metal absorbers which are in photoionization equilibrium with the extragalactic UV background. The selected absorbers have C III and C IV detections often accompanied by Si II, Si III and Si IV, which are clearly associated with each other and with H I at the same redshift. Comparing observations to metal column density predictions based on the photoionization code CLOUDY, we propose the first study of the spectral shape, fluctuation and dominant ionizing source of the UV background radiation at z ~ 0. QUASAR ABSORPTION LINES AND IGM Default Medium false true false false false SUPPORTED