Physical properties of the southwest outflow streamer in the starburst galaxy NGC 253 with ALCHEMI

Aims: The physical properties of galactic molecular outflows are important as they could constrain outflow formation mechanisms. In this work, we study the properties of the southwest (SW) outflow streamer including gas kinematics, optical depth, dense gas fraction, and shock strength through molecular emission in the central molecular zone of the starburst galaxy NGC 253.
Methods: We imaged the molecular emission in NGC 253 at a spatial resolution of 1.6″(∼27 pc at D ∼ 3.5 Mpc) based on data from the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) large program. We traced the velocity and velocity dispersion of molecular gas with the CO(1-0) line and studied the molecular spectra in the region of the SW streamer, the brightest CO streamer in NGC 253. We constrained the optical depth of the CO emission with the CO/¹³CO(1-0) ratio, the dense gas fraction with the HCN/CO(1-0), H¹³CN/¹³CO(1-0) and N₂H⁺/¹³CO(1-0) ratios, as well as the shock strength with the SiO(2-1)/¹³CO(1-0) and CH₃OH(2_k-1_k)/¹³CO(1-0) ratios.
Results: The CO/¹³CO(1-0) integrated intensity ratio is ∼21 in the SW streamer region, which approximates the C/¹³C isotopic abundance ratio. The higher integrated intensity ratio compared to the disk can be attributed to the optically thinner environment of CO(1-0) emission inside the SW streamer. The HCN/CO(1-0) and SiO(2-1)/¹³CO(1-0) integrated intensity ratios both approach ∼0.2 in three giant molecular clouds (GMCs) at the base of the outflow streamers, which implies a higher dense gas fraction and strength of fast shocks in those GMCs than in the disk, while the HCN/CO(1-0) integrated intensity ratio is moderate in the SW streamer region. The contours of those two integrated intensity ratios are extended in the directions of outflow streamers, which connect the enhanced dense gas fraction and shock strength with molecular outflow. Moreover, the molecular gas with an enhanced dense gas fraction and shock strength located at the base of the SW streamer shares the same velocity as the outflow.
Conclusions: The enhanced dense gas fraction and shock strength at the base of the outflow streamers suggest that star formation inside the GMCs can trigger shocks and further drive the molecular outflow. The increased CO/¹³CO(1-0) integrated intensity ratio coupled with the moderate HCN/CO(1-0) integrated intensity ratio in the SW streamer region are consistent with the picture that the gas velocity gradient inside the streamer may decrease the optical depth of CO(1-0) emission, as well as the dense gas fraction in the extended streamer region.