2007ApJ...668..699O

The Spitzer spectrum of the giant FR II radio galaxy 3C 326 is dominated by very strong molecular hydrogen emission lines on a faint IR continuum. The H₂ emission originates in the northern component of a double-galaxy system associated with 3C 326. The integrated luminosity in H₂ pure rotational lines is 8.0x10⁴¹ erg/s, which corresponds to 17% of the 8-70 µm luminosity of the galaxy. A wide range of temperatures (125-1000 K) is measured from the H₂0-0 S(0)-S(7) transitions, leading to a warm H₂ mass of 1.1x10⁹ M_☉. Low-excitation ionic forbidden emission lines are consistent with an optical LINER classification for the active nucleus, which is not luminous enough to power the observed H₂emission. The H₂ could be shock heated by the radio jets, but there is no direct indication of this. More likely, the H₂ is shock heated in a tidal accretion flow induced by interaction with the southern companion galaxy. The latter scenario is supported by an irregular morphology, a tidal bridge, and a possible tidal tail imaged with IRAC at 3-9 µm. Unlike ultraluminous infrared galaxies, which in some cases exhibit H₂ line luminosities of comparable strength, 3C 326 shows little star formation activity (∼0.1 M_☉/yr). This may represent an important stage in galaxy evolution. Starburst activity and efficient accretion onto the central supermassive black hole may be delayed until the shock-heated H₂ can kinematically settle and cool.