Early Star Formation Traced by the Highest Redshift Quasars

The iron abundance relative to ’¦Á-elements in the circumnuclear region of quasars is regarded as a clock of the star formation history and, more specifically, of the enrichment by Type Ia supernovae. We investigate the iron abundance in a sample of 22 quasars in the redshift range 3.06. We also detect a population of weak iron emitters at z~4.5, which are possibly hosted in systems that evolved more slowly. Alternatively, the trend of the Fe II/Mg II ratio at high redshift may reflect significantly different physical conditions of the circumnuclear gas in such high-redshift quasars.