2. The V-15 µm morphology of Centaurus A
Some of the most dramatic and chaotic dust lane morphologies are to be found in optical photographs of our closest radio galaxy, Centaurus A. This is exemplified on Fig. 1 which represents an overlay of ISOCAM LW3 15 µm contours superposed on the V-band prime focus image of Centaurus A. The mid-infrared structure has been interpreted as tracing dust in a barred mini-spiral galaxy (Mirabel et al. 1999), reformed from the interstellar medium of the accreted galaxy.
In Fig. 2 we present a ratio image, secured by dividing the ISOCAM image by the optical one. The optical image was rebinned and convolved with the ISOCAM LW3 point spread function as fully described by Block et al. (1997). Several key features in these figures are worthy of note:
Firstly, an inner disk-like structure of dust of radius 1.5 arcmin (or 1.4 kpc at 3.25 Mpc) is clearly unveiled in our V-15 µm image. The disk is bright, being detected in emission at 15 µm and in extinction at V. It is quite symmetric, clearly showing the bar arms on both sides of the nucleus connecting to the spiral arms (see Mirabel et al. 1999). At the edges, the disk starts to warp. The western side of the bar shows less clearly than the eastern side, possibly indicating that this is the far-side of the structure (extinction will be less on the far-side thus breaking the symmetry of the emission structure).
There is a striking correspondence with the bright structures seen in our Fig. 2 and the dark lanes which Quillen et al. (1993) find in the H-K near-infrared regime, especially on the SE side, confirming that this should be the near side of the mini-spiral. The somewhat poorer agreement on the NW sides probably originates in the fact that modelling in Quillen et al. (1993) did not take into account the existence of a bar structure in the dust disk, which introduces a strong asymmetry in the azimuthal distribution of the dust: on the NW side, most of the dust is on the far-side of the disk, thus contributing little extinction (see Fig. 2). Nevertheless, the rather good agreement between emission and extinction structures suggests that the distribution of large, cold dust grains in the disk (responsible for the extinction at V, H and K) should closely follow the morphology of hot grains detected in emission by ISOCAM. This is now independently confirmed by the SCUBA observations of cold dust detected in emission at 850 µm and reported by Mirabel et al. (1999, see their Fig. 2).
There is a also a close similarity between the morphology of the disk seen in our V-15 µm image and the warped disk inferred from molecular gas CO(2-1) observations (eg. Fig. 10 in Quillen et al. 1992). These morphological considerations offer strong support - apart from kinematical data presented by Mirabel et al. (1999)- to believe that this gas+dust structure represents the disk of a mini-spiral galaxy reformed during the merger of a companion galaxy with the giant elliptical.
In our V-15 µm image, very small dust grains have presumably been subject to temperature spiking. There is a significant amount of UV emission from newly formed stars in the ionized gas disk (Marston & Dickens 1988, Nicholson et al. 1992) and the similarity with the molecular gas distribution likely indicates that the dust seen in emission at 15 µm resides at the interface of UV irradiated clouds.
Secondly: it is remarkable to see just how closely the southernmost contours of the disk follow the ridge of optical emission both to the SE and to the NW over the full 3 kpc projected diameter of the disk. The southernmost sector of the disk of the small spiral galaxy reformed in the merger can actually be optically delineated: that there indeed has been a piling up of dusty material on the SE ridge is confirmed by dark lanes seen in the near-infrared images of Quillen et al. (1993, see especially their Fig. 11).
The presence of dark dust lanes at K is indicative of appreciable optical depths, since imaging at the K-band (2.16 µm) penetrates dust ten times more efficiently than does visible light. The various components of extinction have been recently reviewed by Bryant & Hunstead (1999) from NIR imaging and spectroscopy of the central 30" of the galaxy. They show that extinction to the K-band point source is smaller than 10 mag in V, compatible with, for instance, the extinction that could be derived ( 3 magnitudes in V) to the line of sight of SN1986G (Phillips et al. 1987). They also clearly demonstrate that the K-band source is unlikely to be the AGN itself, but rather dust clouds located less than 20 pc away from it. This explains why much higher extinctions have been reported (e.g. 70 V mag from X-ray studies, see the discussion in Packham et al. 1996): the latter likely samples the line of sight all the way to the AGN, while the former does not include that occuring in a very compact circumnuclear ring.
In the NW there is diffuse optical emission covering a sector of the 15 µm emission, which could, in part, be attributed to forward scattering by dust grains, toward the observer, from the central engine of Centaurus A. Other dust lanes which give Centaurus A its rather chaotic appearance have no 15 µm emission counterpart and are colour coded blue in Fig. 2 (this is specifically the case of the north-eastern dust lane which forms the northern boundary of the optical dust lane). Note that these regions also lack counterparts in the SCUBA maps of Mirabel et al. (1999). This is quite puzzling given their optical appearance, and is worth elucidating. Indeed, looking at the near-infrared maps of Quillen et al. (1993), one can see that the north-eastern lane is still detectable in the K-band image and that it has a (J-K) color similar to that of the more central dust lane that we also see in emission. Therefore the total optical depths of the two lanes are likely of the same order of magnitude and, were the cold dust temperatures to be of the same order, one would expect to detect the northern lane in the submillimeter. The answer most probably lies in the actual three-dimensional location of the dust giving rise to that lane, i.e. it should be further away from the nucleus of Centaurus A. As mentionned earlier, the current view of the dust structure in Centaurus A presented by Mirabel et al. (1999) and supported by the present paper does not contradict the geometrical model developped by Quillen et al. (1993). We can therefore use that model to find the actual location of the northern dust lane. According to Quillen et al. (1993) strong extinction will occur at folds in the warped disk or tilted rings structure. At these folds, the line of sight becomes tangential to the structure, thus maximizing the optical depth of the dust. Using the parameters presented by Quillen et al. (1993) we compute the angle between the line of sight and the axis of the concentric rings as a function of distance to the nucleus. Extremas in this function will signal the presence of the folds we are searching for. We find two such extremas, the inner one corresponding to the inner dust lane that we also see in emission, and the second one 3 times further away, that gives rise to the north-eastern dust lane. This significant increase in distance is very likely to be the reason why the north-eastern dust lane has no emission counterpart: at that distance, heating by the stellar population of the giant elliptical is probably too low, and, since star formation activity, as traced by the ISOCAM emission, has ceased, internal heating sources are absent.
We can actually quantitatively check that cold dust can give rise to strong extinction feature while escaping submm detection: if we assume a Galactic gas-to-dust mass ratio of 160, and mag cm2 from Bohlin et al. (1978), we can compute the relation between the extinction and the emission. To simplifiy this computation, we use dust grains of a single size, and a single density, emitting as modified black-bodies. This gives:
From the SCUBA 450 µm image of Mirabel et al. (1999), we derive an upper limit for the flux in the dark lane region of 2 mJy.. Using a typical size of 0.1 µm for the grains, and a temperature of 15 K for the cold dust, known ranges of grain properties (Draine & Lee 1984, Mennella et al. 1998), translate in of typically 5-20, amply enough to produce the very dark lane observed in north-east side of the galaxy. This range of extinction values fits well with the fact that the NE dust lane has colors similar to the central dust lane (Quillen et al. 1993) and that the extinction in the central region (excluding that occuring in the immediate vicinity of the AGN) is 10 mag in V (Bryant & Hunstead 1999).
© European Southern Observatory (ESO) 2000
Online publication: December 8, 1999