Shocked molecular hydrogen in the 3C 326 radio galaxy system

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 H-2 emission originates in the northern component of a double-galaxy system associated with 3C 326. The integrated luminosity in H-2 pure rotational lines is 8.0 x 10(41) erg s(-1), which corresponds to 17% of the 8-70 mu m luminosity of the galaxy. A wide range of temperatures (125-1000 K) is measured from the H-2 0-0 S(0)-S(7) transitions, leading to a warm H-2 mass of 1.1 x 10(9) M circle dot. 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 H2 emission. The H2 could be shock heated by the radio jets, but there is no direct indication of this. More likely, the H-2 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 mu m. Unlike ultraluminous infrared galaxies, which in some cases exhibit H-2 line luminosities of comparable strength, 3C 326 shows little star formation activity (similar to 0.1 M circle dot yr(-1)). 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 H-2 can kinematically settle and cool.