恒星形成星系、绿谷星系与宁静星系过渡阶段中的冷星系周介质（CGM）MgII吸收线研究
Cool CGM MgII Absorption Across the Star-Forming, Green Valley, and Quiescent Transition
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中文总结 AI 辅助
本研究提出恒星形成偏移度量$σ_{\text{SFO}}$，结合716个星系的MgII吸收线数据，发现冷CGM的吸收强度、覆盖因子随星系从恒星形成经绿谷到宁静阶段逐渐下降，揭示恒星形成淬灭是渐变过程而非突变。
中文摘要 AI 辅助
我们利用MgII吸收线，针对红移范围$0.07<z<2.7$内的716个星系（其中169个为新观测星系，547个为档案库星系），探究了从恒星形成阶段到宁静阶段过渡过程中冷星系周介质（circumgalactic medium，简称CGM）气体的分布与运动学特征。为了在跨越100亿年的宇宙时间尺度上对星系进行统一比较，我们提出了一种恒星形成偏移度量指标（$σ_{\text{SFO}}$），用于衡量单个星系相对于其所处时代恒星形成主序的偏离程度。$σ_{\text{SFO}}$的一个核心优势是，它能够在不同红移下将处于过渡阶段的绿谷星系识别为一个独立的星系统计群体，而传统的“恒星形成-宁静”二元分类法会掩盖这类过渡星系的特征。我们拟合了经位力半径归一化的径向分布曲线，发现MgII吸收线的强度随与宿主星系投影距离的增加而减弱。该平均分布曲线周围的弥散程度与恒星形成活动强相关：径向分布残差与$\text{log}\thinspace\text{sSFR}$、$σ_{\text{SFO}}$均呈正相关，其中恒星形成星系的吸收强度高于分布曲线对应值，残差为正；绿谷星系的残差处于中间水平；宁静星系的吸收强度则普遍低于曲线对应值。内区CGM（$R/R_{200} < 0.25$）的MgII覆盖因子也遵循相同的变化序列，从恒星形成星系到绿谷星系再到宁静星系单调下降。MgII吸收线的运动学特征与冷CGM主要处于束缚态的结论一致。恒星形成星系还表现出双峰方位角依赖特性：MgII吸收在极向和盘方向均有增强，这与双极外向流和共面吸积的物理图像相符。综合这些结果可知，冷CGM的状态与星系中恒星形成的淬灭过程同步演化；借助$σ_{\text{SFO}}$指标可以发现，绿谷阶段的冷气体含量是逐渐下降的，而非从恒星形成状态到宁静状态的 abrupt 突变。
英文摘要
We investigate the distribution and kinematics of cool circumgalactic medium (CGM) gas across the star-forming to quiescent transition using MgII absorption for 716 galaxies spanning $0.07<z<2.7$ (169 new and 547 archival galaxies). To compare galaxies uniformly across 10 billion years of cosmic time, we introduce a star-formation offset metric ($σ_{\mathrm{SFO}}$), which measures a galaxy's deviation from the star-forming main sequence of its epoch. A key advantage of $σ_{\mathrm{SFO}}$ is its ability to identify transitional green valley galaxies as a distinct population across redshift, which would otherwise be obscured by binary star-forming-passive classifications. We fit a virial-radius-normalized radial profile and find that the MgII absorption strength declines with increasing projected distance from the host galaxy. The scatter around this mean profile correlates strongly with star-formation activity: radial profile residuals correlate positively with $\log sSFR$ and $σ_{\mathrm{SFO}}$, with star-forming galaxies showing excess absorption above the profile, green-valley galaxies showing intermediate residuals, and quiescent galaxies falling preferentially below it. The MgII covering fraction in the inner CGM ($R/R_{200} < 0.25$) follows the same sequence, declining monotonically from star-forming through green-valley to quiescent systems. MgII absorption kinematics are consistent with a predominantly bound cool CGM. Star-forming galaxies further exhibit a bimodal azimuthal dependence, with MgII absorption enhanced along both the polar and disk directions, consistent with bipolar outflows and co-planar accretion. Together, these results indicate that the cool CGM tracks the quenching of star-formation in galaxies, with $σ_{\mathrm{SFO}}$ revealing a gradual decline in cool gas across the green valley rather than an abrupt star-forming-to-passive transition.
