| Abstract : | The reionization of He II marks a major change in the thermal and ionization history of the intergalactic medium (IGM). The reionization process can be probed using ionic column density ratios of metals with ionization potentials close to that of He II. Using the best available model for extragalactic ultraviolet (UV) background, we perform a suite of photoionization models using the spectral synthesis routine CLOUDY to track the changes to ionic column density ratios of metals as a function of redshift for the physical conditions that prevail in the IGM. We find that the column density ratio involving C II, C IV, Si II, and Si IV are most suitable candidates for such an analysis because they not only are highly sensitive to changes in the UV background but also have significant redshift path lengths in high-redshift quasar surveys. Motivated by this we search these ions in a VLT/UVES survey of quasars that contains very high resolution 467 quasar spectra. We identified around 1228 absorption systems belonging to these four ions and calculated column density ratios for all combinations. We aim to compare these observational results with the predicted variation in column density ratios to better constrain the duration of He II reionization and the spectrum of the He II ionizing UV background. |
