利用 ALCHEMI 研究星暴星系 NGC 253 西南外流流带的物理性质
Physical Properties of the Southwest Outflow Streamer in the Starburst Galaxy NGC 253 with ALCHEMI
AI总结：
本文基于 ALCHEMI 高分辨率观测，利用多种分子谱线比值分析 NGC 253 西南外流流带的光深、致密气体比例与激波强度，表明流带底部巨分子云中的恒星形成可触发激波并驱动分子外流。
摘要：
星系分子外流的物理性质十分重要，因为它们可以约束外流形成机制。我们研究了星暴星系 NGC 253 中心分子区中西南（SW）外流流带的性质，包括气体运动学、光深、致密气体比例和激波强度。基于 ALCHEMI 项目的数据，我们以约 27 pc 的空间分辨率对分子辐射进行成像。我们利用 CO(1-0) 谱线追踪分子气体的运动学；利用 CO/13CO(1-0) 比值约束 CO 辐射的光深，利用 HCN/CO(1-0) 比值约束致密气体比例，并利用 SiO(2-1)/13CO(1-0) 比值约束激波强度。在 SW 流带区域，CO/13CO(1-0) 积分强度比约为 21，接近 C/13C 同位素丰度比。与星系盘相比更高的积分强度比，可归因于 SW 流带内部 CO(1-0) 辐射所处的光学薄环境。在外流流带底部的三个巨分子云（GMC）中，HCN/CO(1-0) 与 SiO(2-1)/13CO(1-0) 积分强度比均接近约 0.2，这意味着这些 GMC 比星系盘具有更高的致密气体比例和增强的快速激波强度。这两个积分强度比的等值线沿外流流带方向延伸，将增强的致密气体比例和激波强度与分子外流联系起来。此外，位于 SW 流带底部、具有增强致密气体比例和激波强度的分子气体与外流具有相同速度。这些现象表明，GMC 内部的恒星形成能够触发激波，并进一步驱动分子外流。
英文摘要：
The physical properties of galactic molecular outflows are important as they could constrain outflow formation mechanisms. We study the properties of the southwest (SW) outflow streamer including gas kinematics, optical depth, dense gas fraction, and shock strength in the central molecular zone of the starburst galaxy NGC 253. We image the molecular emission at a spatial resolution of $\sim$27 pc based on data from the ALCHEMI program. We trace the kinematics of molecular gas with CO(1-0) line. We constrain the optical depth of CO emission with CO/$^{13}$CO(1-0) ratio, the dense gas fraction with HCN/CO(1-0) ratio, as well as the shock strength with SiO(2-1)/$^{13}$CO(1-0) ratio. The CO/$^{13}$CO(1-0) integrated intensity ratio is $\sim$21 in the SW streamer region, which approximates the C/$^{13}$C isotopic abundance ratio. The higher integrated intensity ratio compared to the disk can be attributed to the optically thinner environment for CO(1-0) emission inside the SW streamer. The HCN/CO(1-0) and SiO(2-1)/$^{13}$CO(1-0) integrated intensity ratios both approach $\sim$0.2 in three giant molecular clouds (GMCs) at the base of the outflow streamers, which implies the higher dense gas fraction and enhanced strength of fast shocks in those GMCs than in the disk. The contours of those two integrated intensity ratios are extended towards the directions of outflow streamers, which connects the enhanced dense gas fraction and shock strength with molecular outflow. Moreover, the molecular gas with enhanced dense gas fraction and shock strength located at the base of the SW streamer shares the same velocity with the outflow. These phenomena suggest that the star formation inside the GMCs can trigger the shocks and further drive the molecular outflow.
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