The Vertical Structure and Asymmetry of Mg ii-enriched Gas in the Milky Way Disk

Abstract

The physical properties of Milky Way Mgii-bearing gas remain poorly constrained due to the saturation of the near-UV doublet. We utilize the weaker Mgii $λλ$1239, 1240 doublet from 482 archival HST/COS extragalactic sightlines to probe this cool gas phase. We identify 43 low-velocity absorbers ($|v_{\rm LSR}|<40\ {\rm km\ s^{-1}}$), yielding a covering fraction ($C_f$) of $32\pm5\%$ for $\log N_{\rm MgII} > 15$. We find that $C_f$ follows an exponential decay relative to equivalent width thresholds, marking a transition from a diffuse medium to localized, dense structures (e.g., cold neutral medium cores). The steep decline of the distribution at high column densities likely reflects the saturation of the turbulent log-normal spectrum and dust depletion. By integrating stellar data, we derive a Mgii scale height $h_{\rm MgII} = 0.12\pm0.02\ \rm\ kpc$ and mid-plane density $n_{0,\rm MgII} = (3.9\pm0.4)\times 10^{-6}\ \rm cm^{-3}$. A pronounced north-south asymmetry exists, with the northern hemisphere displaying a significantly higher mid-plane density ($n_{0,n} \approx 4.7 \times 10^{-6}\ \rm cm^{-3}$) than the south ($3.2 \times 10^{-6}\ \rm cm^{-3}$). This discrepancy suggests that the northern interstellar medium is more spatially concentrated and clumpy, whereas the southern gas is more ubiquitously distributed with a lower average density. These results indicate that Mgii is tightly confined to the disk, governed by a unified depletion law and restricted vertical extent.