We detect bright [ C ii ] \lambda 158 \mu m line emission from the radio galaxy 3C 326N at z = 0.09 , which shows no sign of ongoing or recent star formation ( \mbox { SFR } < 0.07 M _ { \odot } yr ^ { -1 } ) despite having strong H _ { 2 } line emission and a substantial amount of molecular gas ( 2 \times 10 ^ { 9 } M _ { \odot } , inferred from the modeling of the far-infrared ( FIR ) dust emission and the CO ( 1-0 ) line emission ) .
The [ C ii ] line is twice as strong as the 0-0S ( 1 ) 17 \mu m H _ { 2 } line , and both lines are much in excess of what is expected from UV heating .
We combine infrared Spitzer and Herschel photometry and line spectroscopy with gas and dust modeling to infer the physical conditions in the [ C ii ] -emitting gas .
The [ C ii ] line , like rotational H _ { 2 } emission , traces a significant fraction ( 30 to 50 % ) of the total molecular gas mass .
This gas is warm ( 70 < T < 100 K ) and at moderate densities 700 < n _ { H } < 3000 cm ^ { -3 } , constrained by both the observed [ C ii ] -to- [ O i ] and [ C ii ] -to-FIR ratios .
The [ C ii ] line is broad , asymmetric , with a redshifted core component ( FWHM = 390 km s ^ { -1 } ) and a very broad blueshifted wing ( FWHM = 810 km s ^ { -1 } ) .
The line profile of [ C ii ] is similar to the profiles of the near-infrared H _ { 2 } lines and the Na D optical absorption lines , and is likely to be shaped by a combination of rotation , outflowing gas , and turbulence .
If the line wing is interpreted as an outflow , the mass loss rate would be higher than 20 M _ { \odot } yr ^ { -1 } , and the depletion timescale close to the orbital timescale ( \approx 3 \times 10 ^ { 7 } yr ) .
If true , we are observing this object at a very specific and brief time in its evolution , assuming that the disk is not replenished .
Although there is evidence of an outflow in this source , we caution that the outflow rates may be overestimated because the stochastic injection of turbulent energy on galactic scales can create short-lived , large velocity increments that contribute to the skewness of the line profile and mimic outflowing gas .
The gas physical conditions raise the issue of the heating mechanism of the warm gas , and we show that the dissipation of turbulent energy is the main heating process .
Cosmic rays can also contribute to the heating , but can not be the dominant heating source because it requires an average gas density that is higher than the observational constraints .
After subtracting the contribution of the disk rotation , we estimate the turbulent velocity dispersion of the molecular gas to be 120 < \sigma _ { turb } < 330 km s ^ { -1 } , which corresponds to a turbulent heating rate that is higher than the gas cooling rate computed from the line emission .
The dissipation timescale of the turbulent energy ( 2 \times 10 ^ { 7 } -10 ^ { 8 } yrs ) is comparable to or larger than the jet lifetime or the dynamical timescale of the outflow , which means that turbulence can be sustained during the quiescent phases when the radio jet is shut off .
The strong turbulent support maintains a very high gas scale height ( 0.3 to 4 kpc ) in the disk .
The cascade of turbulent energy can inhibit the formation of gravitationally bound structures on all scales , which offers a natural explanation for the lack of ongoing star formation in 3C 326N , despite its having sufficient molecular gas to form stars at a rate of a few solar mass per year .
To conclude , the bright [ C ii ] line indicates that strong AGN jet-driven turbulence may play a key role in enhancing the amount of molecular gas ( positive feedback ) but still can prevent star formation on galactic scales ( negative feedback ) .