2021MNRAS.506.1355S

Using the MEGAFLOW survey, which consists of a combination of MUSE and UVES observations of 22 quasar fields selected to contain strong Mg II absorbers, we measure the covering fractions of C IV and Mg II as a function of impact parameter b using a novel Bayesian logistic regression method on unbinned data, appropriate for small samples. We also analyse how the C IV and Mg II covering fractions evolve with redshift. In the MUSE data, we found 215 z = 1-1.5 [O II] emitters with fluxes >10^–17 erg s^–1 cm^–2 and within 250 kpc of quasar sight-lines. Over this redshift path z = 1-1.5, we have 19 (32) C IV (Mg II) absorption systems with rest-frame equivalent width (REW) W_r> 0.05 Å associated with at least one [O II] emitter. The covering fractions of z~1.2 C IV (Mg II) absorbers with mean W_r~0.7 Å (1.0 Å), exceeds 50 per cent within 23⁺⁶²_–16 (46⁺¹⁸_–13) kpc. Together with published studies, our results suggest that the covering fraction of C IV (Mg II) becomes larger (smaller) with time, respectively. For absorption systems that have C IV but not Mg II, we find in 73 per cent of the cases no [O II] counterpart. This may indicate that C IV comes from the intergalactic medium (IGM), i.e. beyond 250 kpc, or that it is associated with lower mass or quiescent galaxies.