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

Abstract

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 $^{12}$CO($J=1-0$) multi-pointing observations of the edge-on, Milky Way-like galaxy NGC 4565 obtained with the Nobeyama 45-m telescope, with an angular resolution of 14 arcsec, corresponding to about 0.8 kpc. Along a prominent dust filament, we detect significant CO emission at three off-plane positions above the galactic plane. After evaluating possible beam-pattern contamination, the detections remain robust. The derived per-beam molecular masses are $M_{\rm mol} \simeq (2.1-4.3) \times 10^7 M_\odot$. While the off-plane spectra show broader effective line widths, $σ_{\rm eff}=83-115$ 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 $σ_{\rm eff}$ values are partly attributable to a high-velocity component with molecular gas mass $M_{\rm mol}^{\rm HV} \sim 10^7 M_\odot$ that is offset by about 100 km s$^{-1}$ from the local disk velocity. The kinetic energy of this component is estimated to be $E_{\rm kin} \sim 10^{54}$ 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.