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 $\lambda \lambda$1239, 1240 doublet from 482 archival HST/COS extragalactic sightlines to probe this cool gas phase. We identify 43 low-velocity absorbers ($|v_{\mathrm{LSR}}|<40\mathrm{km}{\mathrm{s}}^{-1}$), yielding a covering fraction ($C_f$) of $32\pm 5\%$ for $\log N_{\mathrm{MgII}}>15$. 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 $h_{\mathrm{MgII}}=0.12\pm 0.02\mathrm{kpc}$ and mid-plane density $n_{0,\mathrm{MgII}}=(3.9\pm 0.4)\times 10^{-6}\mathrm{c}{\mathrm{m}}^{-3}$. A pronounced north-south asymmetry exists, with the northern hemisphere displaying a significantly higher mid-plane density ($n_{0,n}\approx 4.7\times 10^{-6}\mathrm{c}{\mathrm{m}}^{-3}$) than the south ($3.2\times 10^{-6}\mathrm{c}{\mathrm{m}}^{-3}$). 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.

18 pages, 11 figures and 3 tables, Accepted by SCPMA

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