Ultrastrong Mg  <scp>ii</scp> absorbers trace both inflowing and outflowing gas: insights from dual down-the-barrel and quasar sightlines

ABSTRACT We present Keck/Low-Resolution Imaging Spectrometer spectroscopy of seven isolated galaxy–quasar pairs at $0.4 \le z \le 0.6$, each exhibiting ultrastrong Mg ii absorption ($W_{\mathrm{ r},2796} \ge 3$ 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 $C_{\mathrm{ f}, \rm out}=0.5$, whereas inflows show a lower average of $C_{\mathrm{ f}, \rm in}=0.3$. Mass flow rates span $\dot{M}_{\rm in} = 0.01-1.18\, {\rm M}_{\odot }\, \mathrm{yr}^{-1}$ for inflows and $\dot{M}_{\rm out} = 0.23-1.03\, {\rm M}_{\odot }\,\mathrm{yr}^{-1}$ 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 $\sim 10^4$ K photoionized 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 ultrastrong Mg ii absorbers trace baryon cycling out to impact parameters of $D = 15$–31 kpc. Moreover, the unexpectedly high prevalence of inflows suggests that ultrastrong Mg ii absorbers offer a powerful strategy for future surveys to systematically map inflow and outflow cycles across cosmic time.