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

Context. 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 (AGNs) likely plays an important role in regulating this vertical baryonic component. Despite its importance, the contribution of extraplanar molecular gas remains poorly understood. Aims. 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 the extraplanar distribution of other tracers of baryonic matter, including atomic gas (H I ), H α -traced diffuse ionized gas (DIG), and dust maps from the literature. Methods. Our analysis is based on new CO(2–1) observations of NGC 891 obtained with the IRAM 30 m telescope. We mapped two 6 kpc × 6 kpc regions in the northeastern side and the area surrounding the galactic center. We applied a careful method to estimate and remove the residual contribution of the error beam to the CO cube. Results. The vertical extent of the molecular gas is best described by a two-component Gaussian fit, consisting of a bright thin disk component with a deconvolved full width at half maximum (FWHM) of ≤360 pc and a fainter thick disk component with a deconvolved FWHM of ≃1.1 kpc. Statistically significant (> 3 σ ) 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. The vertical extent of H α emission is similar to that of the molecular gas, whereas H I extends much further. Conclusions. 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. This is supported by optical images where dust columns rise from the inner disk, showing spatial correlation between CO (2–1), H α , and dust.