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<p>The physical properties of Milky Way Mg<span>ii</span>-bearing gas remain poorly constrained due to the saturation of the near-ultraviolet doublet. We utilize the weaker Mg<span>ii</span> <i>λλ</i>1239, 1240 doublet from 482 archival Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) extragalactic sightlines to probe this cool gas phase. We identify 43 low-velocity absorbers (∣<i>v</i><sub>LSR</sub>∣ &lt; 40 km s<sup>−1</sup>), yielding a covering fraction (<i>C</i><sub><i>f</i></sub>) of 32% ± 5% for log <i>N</i><sub>MgII</sub> &gt; 15. We find that <i>C</i><sub><i>f</i></sub> 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 Mg<span>ii</span> scale height <i>h</i><sub>MgII</sub> = 0.12 ± 0.02 kpc and mid-plane density <i>n</i><sub>0,MgII</sub> = (3.9 ± 0.4) × 10<sup>−6</sup> cm<sup>−3</sup>. A pronounced north-south asymmetry exists, with the northern hemisphere displaying a significantly higher mid-plane density (<i>n</i><sub>0,<i>n</i></sub> ≈ 4.7 × 10<sup>−6</sup> cm<sup>−3</sup>) than the south (3.2 × 10<sup>−6</sup> cm<sup>−3</sup>). 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 Mg<span>ii</span> is tightly confined to the disk, governed by a unified depletion law and restricted vertical extent.</p>

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The vertical structure and asymmetry of Mgii-enriched Gas in the Milky Way disk

  • 期刊论文

  • 发表日期：

    2026年5月25日

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  • Xiaochuan Jiang,
  • Taotao Fang,
  • Shulan Yan

摘要

The physical properties of Milky Way Mgii-bearing gas remain poorly constrained due to the saturation of the near-ultraviolet doublet. We utilize the weaker Mgii λλ1239, 1240 doublet from 482 archival Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) extragalactic sightlines to probe this cool gas phase. We identify 43 low-velocity absorbers (∣vLSR∣ < 40 km s−1), yielding a covering fraction (Cf) of 32% ± 5% for log NMgII > 15. We find that Cf 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 hMgII = 0.12 ± 0.02 kpc and mid-plane density n0,MgII = (3.9 ± 0.4) × 10−6 cm−3. A pronounced north-south asymmetry exists, with the northern hemisphere displaying a significantly higher mid-plane density (n0,n ≈ 4.7 × 10−6 cm−3) than the south (3.2 × 10−6 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.