>From the analysis presented so far in this chapter, it is seen that dust is present at the centers of radio galaxies more frequently than in the control sample galaxies. Also, the dust is more likely to be organized in radio galaxies. Previous studies (e.g. Kotanyi and Ekers, 1979; Möllenhoff et al., 1992; van Dokkum and Franx, 1995) have indicated that dust lanes are likely to be perpendicular to the radio axis.
However, from the small number of dust lane galaxies in our sample,
we do not find such a correlation (see Table 
).
Though it is an attractive scenario to be able to link the AGN
with the large scale dust lanes, dynamical considerations do not
necessiate it: Rees (1978; 1984) has
shown that the direction of outflow can be insensitive to the
direction of angular momentum vector of the infalling matter owing
to the Lense-Thirring precession
of the inertia frame around the black hole.
[Correlation of dust mass with radio and optical power] Correlation coefficients for dust mass in galaxies with dust lanes and dust patches with radio power and absolute B magnitude. Five points each were available for the dust lane and dust patch correlations. The combined 10 points were used for the lanes+patches correlation. The confidence value for each correlation is also shown. The correlation of radio power with dust mass in dust lane galaxies is the strongest.
| dust lanes | dust patches | lanes + patches | ||||||
|
| correlation | two-sided | correlation | two-sided | correlation | two-sided | ||
| coefficient | confidence | coefficient | confidence | coefficient | confidence | |||
| radio-dustmass | 1.00 | 0.99 | 0.20 | 0.38 | 0.64 | 0.99 | ||
| optical-dustmass | 0.60 | 0.86 | 0.60 | 0.86 | 0.51 | 0.96 | ||
| radio-optical | 0.60 | 0.86 | 0.20 | 0.38 | 0.24 | 0.67 | ||
We have investigated the possibility of the dust content being
related to radio properties. We show in Figure
the dependence of the dust mass on the radio power at 408 MHz\
as well as on the absolute B magnitude. We have included here
dust lane galaxies (shown by filled diamonds) as well as
galaxies with dust patches at the center (shown by open
diamonds). We find that the dust mass is correlated with
the radio power and that more powerful radio galaxies contain
a larger amount of dust. The correlation coefficients are
listed in Table .
Owing to the ambiguity in orientation
parameters, the estimated dust mass can have an error of upto 50%.
We have verified that the correlation is not an artifact
introduced by the presence of the distance term in radio power
as well as dust mass.
The correlation of dust mass with the absolute B
magnitude is weak. This latter
result is in confirmation of similar
earlier results (e.g. Goudfrooij, 1994).
The picture that seems to emerge is that the incidence of dust as well as its amount is correlated with radio emission in a galaxy. However, either the dust lanes that we see have not dynamically stabilized, or their orientations bear no correlation with the radio axis. Before accepting this conclusion, however, it should be remembered that we have only six dust lane galaxies in our radio sample and that the radio maps are not very good, which makes the radio axis directions rather ambiguous.
