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

Abstract

We present Keck/LRIS spectroscopy of seven isolated galaxy-quasar pairs at $0.4\le z\le 0.6$, each exhibiting ultra-strong MgII absorption ($W_{r,2796}\ge 3$ 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 $C_{f,\mathrm{out}}=0.5$, whereas inflows show a lower average of $C_{f,\mathrm{in}}=0.3$. Mass flow rates span ${\dot{M}}_{\mathrm{in}}=0.01-1.18$ $M_{\odot }{\mathrm{yr}}^{-1}$ for inflows and ${\dot{M}}_{\mathrm{out}}=0.23-1.03$ $M_{\odot }{\mathrm{yr}}^{-1}$ 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 photoionised gas phase traced by MgII; 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 MgII absorbers trace baryon cycling out to impact parameters of $D=15$-31 kpc. Moreover, the unexpectedly high prevalence of inflows suggests that ultra-strong MgII absorbers offer a powerful strategy for future surveys to systematically map inflow and outflow cycles across cosmic time.

16 pages, 7 figures

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