Vol. 552
In section 3. Cosmology

The evolution of HI and CIV quasar absorption line systems at 1.9 < z < 3.2

This paper presents a comprehensive study of the C IV and Lyman alpha forest lines in a high signal-to-noise and high-resolution sample of 18 QSOs at intermediate redshift. A total of 3100 individual lines are measured. The authors derive differential number counts in a column density interval covering three orders of magnitude and show that there is a break between the metal-rich and metal-poor systems at log N_H ~ 15. The two systems show different cumulative distribution functions, and the combined result is shown to be produced by two distinct populations. The higher the neutral hydrogen column density, the greater the fraction of C IV absorbers within a velocity interval of 100 km/s, indicating that these arise from circumgalactic gas. The C IV poor absorbers appear to be intergalactic in origin.