MusE GAs FLOw and Wind (MEGAFLOW) X. The cool gas and covering fraction of Mg ii in galaxy groups

Abstract

ABSTRACT We present a study of the cool gas (≈104 K) traced by Mg ii absorptions around groups of galaxies in the MEGAFLOW (MusE GAs FLOw and Wind) survey. Using a combination of two algorithms, we blindly identify 32 groups of more than 5 galaxies at 0.3 < z < 1.5 with $10.7 \lt \log _{10}(M/\rm {\rm M}_{\odot }) \lt 13.7$. Among them 26 can be used to study potential counterpart Mg ii absorptions. We report that 21 out of the total 120 Mg  ii absorption systems present in MEGAFLOW are associated with groups. We observe that the Mg ii rest-frame equivalent width ($W_{\rm r}^{2796}$) drops at an impact parameter of ≈150 projected kpc from the closest galaxy and ≈ one virial radius from the identified group centre indicating that Mg ii haloes scale with the mass of the groups. The impact parameter where the covering fraction exceeds 50 per cent is $\log _{10}(b/\rm kpc) = 2.17 \pm 0.47$ (2σ) and (b/Rvir) = 1.67 ± 0.98, which is ≈3 times larger than for field galaxies ($\log _{10}(b/\rm kpc)=1.67\pm 0.15$). Finally, we estimate the cool gas column density profile in groups (from the $W_{\rm r}^{2796}$) and show that its shape follows closely the typical dark matter column density profile for haloes at similar redshift and masses.