Imaging the disk-halo interface of NGC 891: a 2.7 kpc-thick molecular gas disk

TL;DR

This study maps and analyzes the vertical distribution of molecular gas in NGC 891, revealing a significant thick disk component up to 1.4 kpc high, driven by star formation feedback, and compares it with other gas phases.

Contribution

First detailed characterization of the vertical extent and kinematics of molecular gas in NGC 891 using CO(2-1) observations, highlighting a substantial thick disk component.

Findings

Molecular gas extends up to 1.4 kpc above the disk.

Thick disk contains up to 27% of total molecular gas.

Star formation feedback likely lifts molecular gas into the halo.

Abstract

Halos surrounding spiral galaxies act as the bridges connecting the galactic disk and the intergalactic medium (IGM). They host a significant fraction of the baryonic mass in the Universe, and feedback from star formation (SF) or active galactic nuclei (AGN) likely plays an important role in regulating this vertical baryonic component. Despite its importance, the contribution of extraplanar molecular gas remains poorly understood. We aim to characterize the vertical extent and the kinematics of molecular gas traced by CO(2-1) emission in the nearby (D = 9.5 Mpc) spiral galaxy NGC 891, one of the best studied edge-on galaxies. We also compare our results with HI, H$\alpha$-traced DIG and dust maps from the literature. Our analysis is based on new CO(2-1) observations of NGC 891 obtained with the IRAM 30m telescope. We mapped two 6 kpc $\times$ 6 kpc regions on the northeastern side and the area surrounding the galactic center. We apply a careful method to estimate and remove the residual contribution of the error beam to the CO cube. The vertical extent of the molecular gas is best described by a two-component Gaussian fit, consisting of a bright thin disk component (deconvolved FWHM $\simeq$ 360 pc) and a fainter thick disk component (deconvolved FWHM $\simeq$ 1.1 kpc). Statistically significant CO(2-1) emission is detected up to 1.3-1.4 kpc above the disk midplane. We estimate that the thick molecular disk component contains up to 27% of the total molecular gas mass of the galaxy. Our results demonstrate that SF-driven feedback in a non-starburst galaxy can lift significant amounts of molecular gas to large vertical distances. We interpret the presence of extraplanar molecular gas in NGC 891 in the framework of a galactic fountain scenario, in which material is expelled from star-forming regions and transported toward the outer halo.