Constructing a color map from images of a galaxy obtained in two different filters is the most direct way of looking for dust using broadband imaging. The B-R color map is obtained using a procedure analogous to obtaining the B-R color at each pixel and can be described as follows: (1) degrade the image which has the PSF with the smaller FWHM to the PSF with the larger FWHM. It is the R images which most often have the smaller FWHM; (2) divide the B image by the R image and convert the quotient image to the magnitude scale.
Because of the greater absorption at shorter wavelengths, the B image of a galaxy is more likely to be distorted by the presence of a dust feature than the R image. The effect of obtaining a color map is to remove the smooth background provided by the longer wavelength R image from the B image, and highlighting the dust affected regions. A color map can thus reveal dust features (as also blue features) anywhere in the galaxy.
We have obtained B-R images for all the galaxies
in the radio and control samples. We find that six ( 
) radio
galaxies have prominent dust lanes (E(B-R);SPMgt;0.1,
dimension 
). Five ( 
) more radio galaxies have
prominent dust patches (E(B-R);SPMgt;0.1, size between 3 and 4 
).
For the control sample we find that only 
galaxies have dust
lanes, but 
galaxies have dust patches. Thus, the number of
galaxies with dust features in both the samples is the same
( 
), but the dust in radio galaxies is more organized. The
possible scenarios that can explain this are:
(1) dust in radio galaxies has recently settled into a
plane while the dust in normal ellipticals has passed that phase and
has now settled in the central region following the dissipation
of angular momentum.
This implies that normal ellipticals have taken their time
to settle down, that all elliptical
galaxies pass through an active phase and that the dust is accreted.
This is supported by the observation that at larger semi-major
axis lengths, all dust lanes are seen to be warped indicating that
the outer regions are still in the process of settling while the
dust in the central region is better settled, and
(2) the dust in lanes and disks is accreted while the dust found at the
centers of galaxies is locally generated from mass loss.
Detailed kinematic observations of dust lanes could resolve the issue.
The dust properties inferred from color maps are summarized in
Table 
and
the B and B-R images of the radio
galaxies with dust lanes are presented in Figures and .
[Statistics of objects with prominent dust features as indicated by B-R color maps] Statistics of objects with prominent dust features (E(B-R);SPMgt;0.1) in the radio and control samples as indicated by B-R color maps and of nuclear dust as indicated by extrapolated profiles.
| radio | control | |
| sample | sample | |
| Dust lanes | 20% | 7% |
| Dust patches | 17% | 30% |
| Dust in the center | ||
| (from extrapolated | ||
| profiles) | 66% | 43% |
Another way of looking for dust using the 
images,
recommended by Sparks et al. (1985), is to smooth
the R image and then divide the B image with it before proceeding
to convert it to the magnitude scale. This helps to smooth out any
distortions that may be present in the R image and the distortions
in the B image are better highlighted. The resulting smoothed
color map is qualitatively similar to the ordinary color map but
the features are sharper.
Yet another way to obtain the distortions introduced in a B image by the presence of dust is to obtain a residual image by subtracting out a smooth model of the image from the original image:
The smooth model obtained by interpolation of the surface brightness, ellipticity and position angle profiles provided by ellipse following the fitting of ellipses to the galaxy isophotes.
The disadvantage in the two methods outlined here is
that the data no longer remain calibrated and tasks related
to photometry cannot be carried out. In case of the radio
and control samples, we have obtained residuals as outlined above,
but have used them only for the purpose of locating features. When
a feature was found, we always went back to the B-R color map
or the original images to obtain quantitative estimates.
We have described in Section how we fit
the surface brightness profile of a galaxy by leaving out the
central region, so that the high signal-to-noise region where
de Vaucouleurs' law may not hold true does not affect the fit. We also
described how we then extrapolate the fits inward to quantitatively
measure the departure of the observed profile from the fitted
profile. The departure is a combination of effects due to star
formation, dust and genuine departures from de Vaucouleurs' law. We have
shown in the lower part of Table that as per the
extrapolated profiles
radio galaxies are likely to have nuclear dust, whereas
the incidence is 
for the control sample.
The extrapolation uses a profile averaged along
elliptical isophotes. It can hence detect dust that is spread over
a region with optical depth insufficient to be seen by a color map.
>From the numbers given above,
radio galaxies are seen to have dust features
near the center far more often
than the control sample galaxies.
We present these numbers in Table along with
those expressing the incidence of
larger scale dust features deduced from the B-R color maps.
Dust lane galaxies (images and color maps)
More dust lane galaxies (images and color maps)
