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The determination of the extinction in both passbands requires comparing
the unaffected light with the extinguished one. Assuming that the isophotes of
the observed galaxies are intrinsically ellipses, it is possible
to build a model image adjusting ellipses to the isophotes of the galaxy. This
fitting is done with the isophote routines in the STSDAS package within
IRAF. This model image represents the light not extinguished by the
dust, since it is not affected by local intensity irregularities caused by the dust
absorption and scattering. With this model image it is possible to construct
extinction maps in both filters for each galaxy
where F is the count level for each image pixel. The flux calibration and sky background
are not relevant for the extinction maps, since the model incorporates them.
In the galaxies with isophotes strongly affected by the dust (e.g. NGC 3489,
NGC 5044), the first model was used to identify the dust distribution to
be masked in further fittings, at least in the regions where the
isophote routines do not easily converge.
The mean AV and AR values derived from the dust maps
are listed in Table 2 as well as the mean (V-R) values.
Note that the standard deviations are not due to errors, but due to the fact
that the extinction is not uniform over the region used in the measurements. The
particle density of the dust cloud can be estimated by making
simple assumptions about the physical properties of the dust grains together with the
mean AV value inside the dust region, as follows.
The theoretical model to compute AV assumes that it is
proportional to the optical cross section weighted by the grain size distribution n(a), integrated over the grain sizes
(a-<a<a+) along the dust cloud length l in the line of sight. Assuming
n(a) to be the same for the whole cloud and using the definition of the
i.e., the ratio between the extinction and the geometrical cross sections,
can be written (Spitzer 1978) as:
The efficiency factor is related to the grain sizes and composition and varies
with wavelength. must be parameterized
according to the sizes and optical properties of the grains. In the optical
region, assuming that the extinction is caused mainly by silicates
(Mathis et al. 1977), can be assumed to vary as
(Goudfrooij et al. 1994c):
Since the extinction curves of elliptical galaxies in the optical region are
similar to that of our Galaxy (Goudfrooij et al. 1994),
we can also use the same grain size distribution. Mathis et al.
(1977) demonstrated that the function which best reproduces
the extinction curve of our Galaxy over a wide range of wavelengths is
By means of Eq. (2) and Eq. (4)
can then be evaluated. Eq. (5) is used to
compute the normalization factor n0 introducing into
Eq. (2). We measured the extinction AV as the mean value
of the absorption features in the extinction maps and estimated the associated
dust cloud size l from the colour map (V-R). Using these parameters and those
previously derived in this section, we evaluate the mass density and the total mass
of the dust cloud.
The derived dust mass ranges between 103 and 105 .
These values are comparable to the ionized gas mass derived in Paper I.
The good correlation between and
(Fig. 2) suggests that both ISM components are intrinsically related.
Measured values of the extinction AV and AR, the colour index
(V-R), the mean size l of the dust cloud, the dust mass and the cloud
morphology for the galaxies of the sample. Morphology: SD - Small discs, F -
Filamentary, RE - Regular extended; Scale relative to gas distribution (Paper I): - larger, - equal, - smaller. (Values inside parentheses
are at our limit of detection)
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