The rise of the C IV mass density at z<2.5

The cosmic evolution of the metal content of the intergalactic medium puts stringent constraints on the properties of galactic outflows and on the nature of UV background. In this paper, we present a new measure of the redshift evolution of the mass density of C IV, Omega_CIV, in the interval 1.5 < z < 4 based on a sample of more than 1500 C IV lines with column densities 10^{12} < N(CIV) < 10^{15} cm^{-2}. This sample more than doubles the absorption redshift path covered in the range z<2.5 by previous samples. The result shows a significant increase of Omega_CIV towards the lower redshifts at variance with the previously pictured constant behaviour.