Molecular shocks and the massive outflow in NGC 3079

Details Molecular shocks and the massive outflow in NGC 3079 Molecular shocks and the massive outflow in NGC 3079

Oct 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995mnras.276.1197h&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 276, Issue 4, pp. 1197-1214.

Astronomy and Astrophysics

Astronomy

21

Shock Waves, Ism: Kinematics And Dynamics, Galaxies: Active, Galaxies: Individual: Ngc 3079, Infrared: Galaxies, Radio Continuum: Galaxies

Scientific paper

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 upsilon=1-0 S(1) line of molecular hydrogen and weaker emission in other H_2 transitions. The Brgamma 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 H_2 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 A_V~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 H_2; 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 H_2 line 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.

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