The importance of studying the unified distribution of dust grains is clear. In Centaurus A, where the overall extinction is asymmetric and rather chaotic, lies embedded a beautifully symmetric barred mini-spiral. This specific case illustrates particulary well the importance of obtaining information on dust grains of all sizes to clearly understand the large scale distribution of dust: if the spiral were to be seen face-on, the column densities at equivalent galactocentric radii would then essentially be the same. However, if one tilts a barred spiral embedded in light sources (stars and an active galactic nucleus), a break in symmetry is introduced by projection effects as different positions on the spiral now lie at varying optical depths in the line of sight. That an intrinsically symmetric dust distribution can produce an extremely asymetric dust lane has been clearly demonstrated by Elmegreen & Block (1999).
In Centaurus A, we have shown that the V-15 bright structures are very well correlated with the H-K model color maps of Quillen et al. (1993). We note that this correlation is much better in the SE side than in the NW side, indicating an asymmetry in the distribution of the dust in the disk plane that is not taken into account by the models. This asymmetry is very well understood if the dust is distributed in a bar+arms structure as evidenced by Mirabel et al. (1999).
Detailed comparison of the emission and extinction structures also reveal that the warp observed in the emission map closely follows the southern ridge of extinction, implying that the plane of the mini-barred galaxy disk is nearly perpendicular to the plane of the sky on approximately 2-3 kpc.
Finally a dust mass can be computed assuming that the dust resides
mostly in the bar+arms structure. The result,
though compatible with the dust mass of a small spiral or that found
in elliptical galaxies, is markedly smaller than that derived by
Mirabel et al. (1999). We interpret this result as implying that a
substantial amount of dust is diffuse and resides in the disk of the
mini-barred spiral galaxy.
© European Southern Observatory (ESO) 2000
Online publication: December 8, 1999