A CO detection in the off-plane region of the edge-on galaxy NGC 4565 with the Nobeyama 45 m telescope

Understanding the cycling of interstellar medium (ISM) between the Galactic plane and off-plane regions is crucial for tracing the evolution of disk galaxies. We present ¹²CO(J = 1–0) multipointing observations of the edge-on, Milky Way–like galaxy NGC 4565 obtained with the Nobeyama 45 m telescope (angular resolution 14 ~ 0.8 kpc). Along a prominent dust filament, we detect significant CO emission at three off-plane positions located above the Galactic plane. After evaluating possible beam-pattern contamination, the detections remain robust. The derived per-beam molecular masses are Mₘₒₗ (2.1 \!-\!4.3)× 10⁷\, M_. While the off-plane spectra show broader effective line widths (σ eff=83–115 km s⁻¹) than the disk spectra, they contain CO components consistent with the local disk rotation. The mean observed off-plane CO intensity fraction is about 0.34. Comparison with geometrically thin-disk models suggests that this large fraction is best explained by gas above the disk. The large σ eff values are partly attributable to a high-velocity component with molecular gas mass MₘₒₗHV~ 10⁷\, M_ that is offset by ~ 100~ km~s⁻¹ from the local disk velocity. The kinetic energy of this component is estimated to be Eₖᵢₙ~ 10⁵⁴ erg. Such a large energy requirement is difficult to explain by disk-driven feedback in NGC 4565, which has a Milky Way–like star formation rate. External inflow is therefore one possibility.