Ultra-strong Mg ii absorbers trace both inflowing and outflowing gas: insights from dual down-the-barrel and quasar sightlines

We present Keck/LRIS spectroscopy of seven isolated galaxy–quasar pairs at 0.4 ≤ z ≤ 0.6, each exhibiting ultra-strong Mg ii absorption (Wr, 2796 ≥ 3 Å), probing both down-the-barrel and transverse gas flows. Down-the-barrel galaxy spectra reveal outflows in three galaxies (v = 19 to 311 km s−1) and inflows in five (v = 61 to 361 km s−1), including one system showing inflows and outflows simultaneously. All galaxies with detected inflows are below the star-forming main sequence, suggesting that they might be actively replenishing their gas reservoirs. Outflows have a mean covering fraction of Cf, out=0.5, whereas inflows show a lower average of Cf, in=0.3. Mass flow rates span Ṁᵢₙ = 0.01-1.18 M yr⁻¹ for inflows and Ṁₒᵤₜ = 0.23-1.03 Myr⁻¹ for outflows, yielding mass loading factors below unity and implying these galaxies cannot sustain their current level of star-formation rates. These results are based on the T ∼104 K photoionised gas phase traced by Mg ii; additional accreting/outflowing material in other gas phases may also be present, but remains undetected in this study. Quasar sightlines consistently show redshifted inflow components and blueshifted outflow components, demonstrating that ultra-strong Mg ii absorbers trace baryon cycling out to impact parameters of D = 15 – 31 kpc. Moreover, the unexpectedly high prevalence of inflows suggests that ultra-strong Mg ii absorbers offer a powerful strategy for future surveys to systematically map inflow and outflow cycles across cosmic time.