直面风暴：利用 ULLYSES 量化大麦哲伦星系的盘宽电离外流
Braving the Storm: Quantifying Disk-wide Ionized Outflows in the Large Magellanic Cloud with ULLYSES
AI总结：
本研究利用 ULLYSES 的 110 条恒星视线和高斯过程连续谱拟合，量化 LMC 盘宽电离外流，给出外流率、质量加载因子及与恒星形成率面密度的普适速度标度关系。
摘要：
大麦哲伦星系（LMC）拥有许多 HII 区，这些区域可能引发显著的外流。我们利用来自 HST 的“作为基本标准的年轻恒星紫外遗产库”（Ultraviolet Legacy Library of Young Stars as Essential Standards，ULLYSES）项目的 110 条恒星视线，考察了 LMC 在 HI、SII、SiIV 和 CIV 中的多相气体（$T\sim10^{4-5}$ K）。我们开发了一种基于高斯过程回归概念的连续谱拟合算法，并在 $v_{\rm helio}=175-375$ km s$^{-1}$ 范围内识别出可靠的 LMC 星际吸收。我们的分析表明，SiIV 和 CIV 在整个 LMC 中呈现盘宽电离外流，其整体速度为 $|v_{\rm out, bulk}|\sim20-60$ km s$^{-1}$，这表明大多数外流物质在引力上是束缚的。外流柱密度与 LMC 的恒星形成率面密度（$Σ_{\rm SFR}$）相关，并且具有更高 $Σ_{\rm SFR}$ 的外流往往电离程度更高。考虑到 CIV 所追踪的来自 LMC 两侧的外流，我们保守估计总外流率为 $\dot{M}_{\rm out}\gtrsim 0.03~M_\odot {\rm yr}^{-1}$，质量加载因子为 $η\gtrsim 0.15$。我们将 LMC 的外流与在星暴星系中探测到的外流以及模拟预测进行比较，发现在广泛的恒星形成条件范围（$Σ_{\rm SFR}\sim10^{-4.5}-10^{2}~M_\odot {\rm yr}^{-1} {\rm kpc}^{-2}$）内，存在 $|v_{\rm out, bulk}|\propto Σ_{\rm SFR}^{0.23}$ 的普适标度关系。最后，我们发现外流与 LMC 的年轻恒星盘共同旋转，速度场似乎并未受到外力的显著影响；因此我们推测 LMC 前方存在弓形激波，在 LMC 围绕银河系运行时，该弓形激波可能保护外流免受冲压影响。
英文摘要：
The Large Magellanic Cloud (LMC) is home to many HII regions, which may lead to significant outflows. We examine the LMC's multiphase gas ($T\sim10^{4-5}$ K) in HI, SII, SiIV, and CIV using 110 stellar sight lines from the HST's Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) program. We develop a continuum fitting algorithm based on the concept of Gaussian Process regression and identify reliable LMC interstellar absorption over $v_{\rm helio}=175-375$ km s$^{-1}$. Our analyses show disk-wide ionized outflows in SiIV and CIV across the LMC with bulk velocities of $|v_{\rm out, bulk}|\sim20-60$ km s$^{-1}$, which indicates that most of the outflowing mass is gravitationally bound. The outflows' column densities correlate with the LMC's star formation rate surface densities ($Σ_{\rm SFR}$), and the outflows with higher $Σ_{\rm SFR}$ tend to be more ionized. Considering outflows from both sides of the LMC as traced by CIV, we conservatively estimate a total outflow rate of $\dot{M}_{\rm out}\gtrsim 0.03~M_\odot {\rm yr}^{-1}$ and a mass loading factor of $η\gtrsim 0.15$. We compare the LMC's outflows with those detected in starburst galaxies and simulation predictions, and find a universal scaling relation of $|v_{\rm out, bulk}|\propto Σ_{\rm SFR}^{0.23}$ over a wide range of star-forming conditions ($Σ_{\rm SFR}\sim10^{-4.5}-10^{2}~M_\odot {\rm yr}^{-1} {\rm kpc}^{-2}$). Lastly, we find that the outflows are co-rotating with the LMC's young stellar disk and the velocity field does not seem to be significantly impacted by external forces; we thus speculate on the existence of a bow shock leading the LMC, which may have shielded the outflows from ram pressure as the LMC orbits the Milky Way.
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