MusE GAs Flow and Wind (MEGAFLOW) VI. A study of CIV and MgII absorbing gas surrounding [OII] emitting galaxies

Using the MEGAFLOW survey, which consists of a combination of MUSE and UVES observations of 22 quasar fields selected to contain strong MgII absorbers, we measure covering fractions of CIV and MgII 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 CIV and MgII covering fractions evolve with redshift. In the MUSE data, we found 215 $z=1-1.5$ [OII] 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) CIV (MgII) absorption systems with rest-frame equivalent width (REW) $W_r>$0.05\AA\ associated with at least one [OII] emitter. The covering fractions of $z\approx1.2$ CIV (MgII) absorbers with mean $W_r\approx$0.7\AA\ (1.0\AA), exceeds 50\% within 23$^{+62}_{-16}$ (46$^{+18}_{-13}$) kpc. Together with published studies, our results suggest that the covering fraction of CIV (MgII) becomes larger (smaller) with time, respectively. For absorption systems that have CIV but not MgII, we find in 73\% of the cases no [OII] counterpart. This may indicate that the CIV comes from the intergalactic medium (IGM), i.e. beyond 250 kpc, or that it is associated with lower-mass or quiescent galaxies.