Abstract
Quasars are among the most luminous objects in the Universe, and allow us to study the ISM in their host galaxies at very high redshift, up to z > 6 (Fan et al. 2001, 2003, 2004). The spectra of high redshift quasars contain important information about the enrichment history of the gas and, therefore, provide constraints on the star formation history in their host galaxies (Hamann & Ferland 1999). We have investigated the metallicity of the Broad Line Region (BLR) in a sample of quasars, at 4< z <6.4, by using a new diagnostic tool involving the line ratio of Si IV 1398+ O IV 1402 to C IV 1549 (Nagao et al. 2006).
