Johannes Zabl

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.

Using CANUCS imaging we found an apparent major merger of two $z$ ∼ 5 ultra-low-mass galaxies (M⋆∼107M⊙ each) that are doubly imaged and magnified ∼12–15× by the lensing cluster MACS 0417. Both galaxies are experiencing young (∼100 Myr), synchronised bursts of star formation with log (sSFR/Gyr−1) ∼1.3–1.4, yet SFRs of just ∼0.2M⊙ yr−1. They have sub-solar (Z ∼ 0.2Z⊙) gas-phase metallicities and are connected by an even more metal-poor star-forming bridge. The galaxy that forms from the merger will have a mass of at least M⋆∼2 × 107 M⊙, at least half of it formed during the interaction-induced starburst. More than half of the ionizing photons produced by the system (before and during the merger) will have been produced during the burst. This system provides the first detailed look at a merger involving two high-$z$ ultra-low-mass galaxies of the type believed to be responsible for reionizing the Universe. It suggests that such galaxies can grow via a combination of mass obtained through major mergers, merger-triggered starbursts, and long-term in-situ star formation. If such high-$z$ mergers are common, then merger-triggered starbursts could be significant contributors to the ionizing photon budget of the Universe.