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

December 2018 • 2018ApJ...869...61A

Authors • Appleton, P. N. • Diaz-Santos, T. • Fadda, D. • Ogle, P. • Togi, A. • Lanz, L. • Alatalo, K. • Fischer, C. • Rich, J. • Guillard, P.

Abstract • We detect widespread [C II] 157.7 μm emission from the inner 5 kpc of the active galaxy NGC 4258 with the SOFIA integral field spectrometer FIFI-LS. The emission is found to be associated with warm H₂, distributed along and beyond the end of the southern jet, in a zone known to contain shock-excited optical filaments. It is also associated with soft X-ray hotspots, 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 that contains young star clusters. Palomar CWI Hα integral field spectroscopy shows that the filament exhibits non-circular motions within NGC 4258. Many of the [C II] profiles are very broad, with the greatest line width, 455 km s^-1, observed at the position of the southern jet bow-shock. Abnormally high ratios of L([C II])/L(FIR) and L([C II])/L(PAH 7.7 μm) are found along and beyond the southern jet and in the X-ray hotspots. These are the same regions that exhibit unusually large intrinsic [C II] line widths. This suggests that the [C II] traces warm molecular gas in shocks and turbulence associated with the jet. We estimate that as much as 40% (3.8 × 10³⁹ erg s^-1) of the total [C II] 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 [C II] 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.

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Phil Appleton

Senior Scientist