Astron. Astrophys. 336, 433-444 (1998)

1. Introduction

NGC 3079 is a bright and highly inclined late-type spiral galaxy (SB(s)c: De Vaucouleurs et al. 1976; Sc(s)II-III: Sandage & Tammann 1987) accompanied by the lesser galaxies NGC 3073 and MCG . The group distance is estimated to be 16 Mpc for (Irwin & Seaquist 1991). For consistency with Hawarden et al. (1995; hereafter HIGW), we will assume a distance of 18 Mpc (Aaronson & Mould 1983) in the remainder of this paper. Some basic properties of NGC 3079 are given in Table 1.

Table 1. Properties of NGC 3079
Notes:
RSA (Sandage & Tammann 1987)
observations, Irwin & Seaquist (1991)
Baan & Irwin (1995)
Sofue & Irwin (1992); Braine et al. (1997)
Aaronson & Mould (1983), for Mpc^-1
Fabbiano et al. (1992)
Hawarden et al. (1995)

On both sides of NGC 3079, strong lobes of radio continuum emission extend several kiloparsecs from the plane along the minor axis (De Bruyn 1977; Seaquist et al. 1978; Duric et al. 1983; Duric & Seaquist 1988), in the inner parts associated with filamentary H and [] emission interpreted as the signature of a powerful outflow from the nucleus with velocities of up to in a cone of large opening angle (Heckman et al. 1990; Filippenko & Sargent 1992; Veilleux et al. 1994). In contrast to the radio lobes, the optical filaments are only seen at the eastern side of the disk, indicating that the western side suffers higher extinction.

In the disk of NGC 3079, radio continuum emission extended over (kpc) surrounds a very compact (size about 1 pc) radio core (Seaquist et al. 1978; Irwin & Seaquist 1988; Duric & Seaquist 1988; Baan & Irwin 1995) which agrees in position with an X-ray point source (Fabbiano et al. 1992) and strong H₂O masers (Henkel et al. 1984 and references therein). Based on VLBI observations, Irwin & Seaquist (1988) argued that the outflow originates from a central compact object rather than from a more extended starburst region.

In NGC 3079, CO emission is concentrated in the centre (Irwin & Sofue 1992; Sofue & Irwin 1992). Out to radii of (470 pc), the molecular gas exhibits solid body rotation with a maximum rotational line-of-sight velocity of . The steep velocity gradient and the finite angular resolution of about 4" create the false impression of an unresolved CO emission peak at the nucleus. However, position-velocity maps (in particular Fig. 5b in Sofue & Irwin 1992) show the existence of a small nuclear hole in the CO distribution, similar to the situation found in the equally highly inclined galaxy NGC 253 (Israel et al. 1995).

Duric & Seaquist (1988) explained the then-observed phenomena with a model in which the observed radio structures result from a strong nuclear wind focussed into a bipolar outflow by a dense circumnuclear disk. This model was supported by HIGW on the grounds that the observed properties of the centre of NGC 3079 cannot be explained by a (circum)nuclear starburst, but rather point to the existence of an active nucleus vigorously interacting with its gaseous surroundings. HIGW conclude that the H₂ vibrational line emission is not excited by X-rays or UV photons. Instead, they argue that kinetic energy of fast shocks generated by wind impact on the molecular gas disk is converted into H₂ line mission, with the low efficiency expected for such a mechanism, and that the extended mid-infrared emission from NGC 3079 arises from shock-heated dust.

HIGW did not obtain images of the H₂ distribution, and only barely resolved its emission. Because NGC 3079 is seen almost edge-on, its centre suffers considerable extinction (Forbes et al. 1992; Veilleux et al. 1994), leading to some uncertainty in the near-infrared luminosities discussed by HIGW. In order to verify the conclusions reached by them, a further investigation of the nuclear H₂ emission and the properties of the central region of NGC 3079, by high resolution imaging of the J, H and K-band continuum emission as well as the S(1) H₂ line emission, was deemed desirable.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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