Molecular shocks and the massive outflow in NGC 3079

The edge-on SBc galaxy NGC 3079 is the site of a very energetic nuclear outflow, which has been modelled as a disc-focused wind. Our 2-µm spectra of the central regions show strong emission in the v = 1–0 S(1) line of molecular hydrogen and weaker emission in other H2 transitions. The Brγ recombination line of HI is just detected; all the lines are superimposed on a strong stellar continuum with absorption features of CO and neutral metals characteristic of late-type giant stars. The S(1) line is ~630 km s−1 wide (at 20 per cent of the peak intensity); this is consistent with the rotation of the molecular complex near the nucleus. A K-band image shows a resolved (~2-arcsec FWHM) central feature but no signs of complex circumnuclear structure; the central feature is probably a small stellar ‘bulge’ heavily obscured to the west. A map of the inner 45 arcsec of the galaxy at 800-µm wavelength shows a central source with flux density ~350 mJy, superimposed on emission from the galaxian disc. The strength of the H2 emission relative to the far-infrared luminosity of the nucleus of NGC 3079 is surpassed only by that of the ultraluminous post-merger galaxy NGC 6240. The nuclear bulge and probably also the emission-line source suffer an extinction AV~7.6 mag and are probably located in a relatively dust-free cavity in a dense circumnuclear molecular disc. It is argued that, somewhat surprisingly, star formation is not the dominant process in the nuclear vicinity of NGC 3079; the wind originates in the active nucleus itself. We show that neither X-ray nor UV irradiation appears to be an important factor in the excitation of the H2; instead, we conclude that the kinetic energy of fast shocks (velocities ≥120 km s−1) generated by the impact of the outflow from the nucleus on the nearby molecular material is being converted to H2line emission at about 3 per cent of the efficiency expected in low-velocity shocks, in evident confirmation of the disc-focused wind model. Dust heating in the shocks may explain the extended mid-IR source surrounding the nucleus.