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

3. Results

3.1. Broad-band images

The J, H, and K-band images and a image are reproduced in Fig. 1.

Fig. 1. Near-infrared continuum images of the central of NGC 3079 in the K-band (top left), the H-band (top right) and the J-band (bottom left). Contour levels are 0.1, 0.2, 0.4, 0.6, 1, 1.5, 2, 4, 6, 10 and 20 W m- 2 µm-1 sr-1 (K-band), 0.5, 0.75, 1, 1.5, 2, 2.5, 5, 10 and 20 W m-2 µm- 1 sr-1 (H-band), 0.75, 1.5, 2.25, 3, 4, 5, 7.5, 10 and 20 W m- 2 µm-1 sr-1 (J-band). The frame at bottom right shows the -band flux ratio, with contours at ratios of 0.3 to 1.5 in steps of 0.2; the reddest regions are darkest. Positions are relative to the K-band peak.

The differences between the broad-band images primarily reflect the wavelength-dependent effects of extinction. The strong east-west asymmetry, most clearly seen in the J-band image, is caused by dust extinction in the western half of the galaxy. With increasing wavelength, the light distribution becomes more symmetrical and more pronouncedly peaked, although symmetry is not perfect even at K-band, suggesting obscuration of the nucleus even at this wavelength. The J and images indicate the presence of at least two dust lanes: one cuts off the bulge light, the second betrays its presence by bays of reduced emission in the J-band image, corresponding to an extended reddened zone in the image about 10" west of the nucleus, parallel to the midplane. The relatively well-defined western edge in the J-band image may represent a third dust lane, seen as a thin layer of enhanced reddening between the other two dust lanes in the image. The north-south asymmetry apparent in the images suggests higher extinction north of the nucleus than south of it. East of the nucleus, the bulge light appears to suffer less extinction, and a bulge with a peanut-shaped light distribution is apparent (Shaw et al. 1993). The asymmetries along the major and along the minor axes are further illustrated by the near-infrared colour profiles shown in Fig. 2.

Fig. 2. Near-infrared colour profiles in apertures along the major axis (top panel) and along the minor axis (bottom panel) of NGC 3079. Positions are with respect to the peak of the K-band peak.

Lawrence et al. (1985) noted a displacement of the 10m and radio peak (presumably marking the true nucleus) from the visual peak by 5" in a northwestern direction. The near-infrared images at wavelengths intermediate between 10µm and the visual show the same effect but, as expected, to a lesser degree. Going from J to K, the intensity peak shifts to the west by .

All three images (especially the J-band image) show a secondary maximum 8" south of the main peak. The lower resolution 0.8 mm continuum map by HIGW shows the central source to be elongated in this direction, and the radio continuum maps by Duric et al. (1983) and Baan & Irwin (1995) show enhanced emission at this position, with a 1.4-5.0 GHz radio flux density of about 0.5 mJy. The apparently flat radio continuum spectrum, implying mostly thermal emission, is consistent with the intensity of the bright H+[] knot visible in Fig. 1 of Veilleux et al. (1994) at this position. The object thus appears to be a massive star forming region; its location, especially in the H+[] image, suggests that it is part of a spiral arm, and at a projected distance from the nucleus of about 700 pc. Its radio intensity implies the presence of a few hundred OB stars. The J-band image contains a few more, weaker peaks farther out in the disk of NGC 3079 (e.g., at -2.5, ), which may likewise represent concentrations of luminous stars.

3.2. H₂ images

In Fig. 3 we show the line surface brightness maps of the H₂ emission from the centre of NGC 3079. As the velocity separation is only 10 less than the velocity resolution, the "channel" images are largely independent. The H₂ distribution has a relatively sharp western edge and a more wispy eastern boundary. The peak of the total H₂ emission is located north of the 2.13 µm continuum peak marked by a cross in Fig. 3. This offset is real and accurate, since both the continuum and line images are extracted from the same data set. In Fig. 4 we further illustrate this offset by plotting the outline of the total H₂ emission on a contour map of the 2.13 µm continuum distribution.

Fig. 3. H2 S(1) emission line surface brightness images of the centre of NGC 3079. "Channel" maps represent emission integrated over velocity intervals, at central velocities (relative to the systemic velocity of NGC 3079) indicated in the figures. The sum of the three channel maps is at bottom right. Contour values are 2, 4, 7, 10, 13, 16 and 19 in units of 10-8 W m-2 sr- 1 for the channel maps, and 4, 8, 12, 18, 24, 30, 36, 42 and 48 again in units of 10-8 W m-2 sr- 1 for the map of integrated emission. Positions are relative to the peak position of the integrated H2 emission. The cross marks the position of peak broad-band 2.1 µm emission.

Fig. 4. The outline of the compact total H2 S(1) line emission plotted on top of a contour map of the more extended 2.1 µm continuum emission. H2 contour values are 5, 20 and 40 in units of W m-2 sr-1; continuum contours are 10, 20, 30, 50, 75, 100, 150, 200, 300, 400, 600, 800 and 1000 in arbitrary units.

The peak of the H₂ emission is found at accurately the same position in all of the three channels maps of Fig. 3. Because the emission line is widest at this position, we identify it with the dynamical centre of the H₂ emission. This position thus likely marks the obscured nucleus of NGC 3079. The position offset of the K-band continuum nucleus towards the south indicates that extinction still plays a role in this wavelength region and that the H₂ emission may suffer less extinction than the continuum.

The H₂ distribution in Fig. 3 can be schematically described by a bright central component superposed on more extended emission. The bright central component measures 32" (260175 pc) in the integrated H₂ map. In the central channel, this component is elongated with diameters of at position angle , nominally deviating from the extended disk position angle. At the outlying velocities, the peak surface brightness of this component has dropped by a third and the shape is more circular with a diameter of . Thus at the outlying velocities, the central bright H₂ component is less extended in the plane of the galaxy and more extended perpendicular to the plane than at the systemic velocity. This result will be discussed further in Sect. 4.2.2.

The more extended emission is fainter. In integrated H₂ it measures along , closer to the position angle of the disk (Table 1) In the perpendicular direction, its extent is hard to determine because of the dominating presence of the central component. The channel maps show the extended component to be rotating, with the south receding and the north approaching.

The integrated intensity of the H₂ S(1) line emission in Fig. 3 is W m^-2, which is about two thirds of the line strength found by HIGW from spectrophotometry.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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