Jet-related Excitation of the [CII] Emission in the Active Galaxy NGC 4258 with SOFIA

We detect widespread [CII]157.7um emission from the inner 5 kpc of the active galaxy NGC 4258 with the SOFIA integral field spectrometer FIFI-LS. The emission is found associated with warm H2, distributed along and beyond the end of southern jet, in a zone known to contain shock-excited optical filaments. It is also associated with soft X-ray hot-spots, which are the counterparts of the `anomalous radio arms' of NGC~4258, and a 1 kpc-long filament on the minor axis of the galaxy which contains young star clusters. Palomar-CWI H-alpha integral field spectroscopy shows that the filament exhibits non-circular motions within NGC 4258. Many of the [CII] profiles are very broad, with the highest line width, 455 km/s, observed at the position of the southern jet bow-shock. Abnormally high ratios of L([CII])/L(FIR) and L([CII])/L(PAH7.7um) are found along and beyond the southern jet and in the X-ray hotspots. These are the same regions that exhibit unusually large intrinsic [CII] line widths. This suggests that the [CII] traces warm molecular gas in shocks and turbulence associated with the jet. We estimate that as much as 40% (3.8 x 10^39 erg/s) of the total [CII] luminosity from the inner 5 kpc of NGC 4258 arises in shocks and turbulence (< 1% bolometric luminosity from the active nucleus), the rest being consistent with [CII] excitation associated with star formation. We propose that the highly-inclined jet is colliding with, and being deflected around, dense irregularities in a thick disk, leading to significant energy dissipation over a wide area of the galaxy.