Astron. Astrophys. 333, 13-16 (1998)

4. Discussion

The trivial explanation for the localised H I absorption is an isolated cloud which fortuitously aligns with component 6. We consider it more likely, however, that the absorbing gas lies in the galaxy disk surrounding the nucleus. For example, this result compares favorably with the localised H I absorption observed towards the radio jet of NGC 4151 (Mundell et al. 1995). The interesting question is whether, as was proposed for NGC 4151, the absorbing gas might be located in small-scale (  pc) disc surrounding the AGN. In the case of Mkn 6, however, we find that absorption from gas distributed on kpc-scales is more consistent with the observations. The first evidence is that the linewidth is very narrow,  km s^-1, which is less than half the H I absorption linewidth of NGC 4151. In contrast, H I absorption linewidths towards Seyfert and starburst galaxies often exceed 100 km s^-1, particularly in those cases where the H I absorption is known to trace gas deep in the nucleus (Pedlar et al. 1996; Mundell et al. 1995; Gallimore et al. 1994; Dickey 1986). This evidence is not sufficient, however, since we cannot rule out the possibility that the absorption arises from a compact, circularly rotating disc viewed nearly face-on. Nevertheless, the narrowness of the line is consistent with that expected from a larger scale ring or disc.

We next examine the displacement of the absorption from the AGN. Unfortunately, the correspondence between components in the optical and radio images is not accurately known. Moreover, the continuum spectra and sizes of the radio features are indistinct, and so there is currently no clear radio candidate for the AGN proper (Kukula et al. 1996). Clements (1983) places the optical nucleus somewhere between component 5 and (the H I absorbed) component 6, but the uncertainties are roughly one quarter the length of the radio jet. Nevertheless, the Clements position is significantly displaced southward from component 6 (Kukula et al. 1996). Capetti et al. (1995) propose an alignment between the radio and optical images based on Hubble Space Telescope images. They found a linear extension of [O III ] emission that agrees well both in orientation and detailed shape with the southern part of the radio jet (i.e., components 1-5). Aligning the radio and optical jet structures places the AGN (  pc in projection) south of component 6, somewhere nearer component 3 (from Kukula et al.: , ;  mJy). Adopting this alignment, and further considering the narrowness of the absorption line, we are drawn to the conclusion that the H I absorption in Mkn 6 arises from neutral gas displaced from the nucleus by  pc. For reference, the strongest absorption lines observed towards the Seyfert nucleus of NGC 1068 similarly trace a  pc radius, central disc (Gallimore et al. 1994).

From a more detailed study of the optical and radio continuum structures of the nucleus (Holloway et al. in preparation), we have discovered a conspicuous candidate for the H I absorber. Illustrated in Fig. 3, there is an obvious band of increased extinction which crosses north of the optical nucleus. For convenience, we refer to this dark region simply as a dust lane. According to the alignment of Capetti et al. (1995), the dust lane encompasses the position of the H I absorbed radio feature. The high aspect ratio of the dust lane suggests a disk or spiral arms viewed edge-on.

Fig. 3. Illustration of the H I absorbing medium of Mkn 6. The left panel is an overlay of the 21 cm radio continuum and an archival HST image taken in the F606W (wide V-band) filter. We have subtracted an elliptical isophote model of the smooth, bulge light from the HST image in order to enhance the contrast of the underlying structure. The halftone rendering of the HST image is displayed in the positive sense: the dark band across the nucleus is an apparent band of high extinction, presumably arising in a dust lane. We chose the Capetti et al. (1995) alignment between the MERLIN and HST images. Only component 6 among the brighter jet features lies, in projection, within the dust lane. The cartoon in the right panel depicts a plausible ring geometry for the neutral, absorbing gas. The proposed location of the AGN, near radio component 3, is indicated by the dot. This cartoon is purely illustrative and is not intended to be a detailed model for the MERLIN H I absorption and HST data.

The simplest picture is that the dust lane traces a kpc-scale disc or ring surrounding the nucleus, or perhaps a spiral arm segment lying in front of the nucleus. The radio jet must be oriented with component 6 lying behind the disc to the north and components 1-5 in front of the disc to the south. There are two important implications of this result. Firstly, the location of the H I absorbed radio feature within the newly discovered dust lane lends self-consistent support for the Capetti et al. alignment, which, as a corollary, strengthens their argument for an interaction between the radio jet and the NLR gas. The second implication is that the northern jet and NLR structures fall behind the galaxian disc, contrary to our earlier model for the northern ionisation cone (Kukula et al. 1996). More specifically, there is a strong correspondence between [O III ] emission and radio emission only at the southern end of the jet. The lack of [O III ] emission towards the northern end of the jet (i.e., component 6) is naturally explained by extinction in our model for the H I absorption. We will explore a revised model for the ionisation cone structure in a follow-up paper (Holloway et al. 1997).

© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
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