3 Observations, colour-index (V - R) , A_V and A_R extinction maps

 

3.1 Observations

 The images of our galaxies were acquired using broad-band V and R filters matching the Cousins standard bands during the same runs described in Paper I. The observational set-ups and filters specifications are given in Tables 1, 2 and 4 of Paper I. The exposure times for each galaxy were chosen to be 30 and 300 s, with 30 s optimized to obtain the images in the very central part of each galaxy and 300 s optimized for the external regions, since in these longer exposures the center was saturated. The data was processed with IRAF, using the standard techniques for bias and dark-current subtraction, division by normalized twilight flat-fields, and interpolation over bad columns and pixels. No pattern was found in the bias frames.

3.2 Colour maps (V-R)

 To build the (V-R) colour maps, we first realigned the V and R images, by measuring the centers of field stars with similar point spread function (PSF) in each image. The centering was accurate up to a tenth of a pixel for most galaxies, except for NGC 2865 and NGC 4472 whose frames contain too few field stars to perform the alignment. In this case the central part of the galaxy and other obvious morphological visible features were used to define the alignment. For images with different seeing the PSFs were matched before the subsequent analysis. The background counts were estimated by measuring boxes in regions of the images not affected by the galaxy luminosity. This is a reliable procedure since most of our images have no gradient in the backgrounds, except for NGC 3311 and NGC 4472, where a power law of degree 1 was fitted to the outer galaxy profiles in order to estimate and subtract the background. The images were calibrated using multi-colour and multi-aperture photometry of Poulain (1986, 1988) and Poulain & Nieto (1994) as described in Paper I. Once the background was subtracted we created the (V-R) colour-index map, used to determine the morphology and size of the dust distribution when calculating the dust masses. The contribution of the ionized gas emission to the colour maps can be neglected, since the R filter width is roughly 1500 Å and the equivalent width of the emission lines, summed together, is not more than 15 Å, thus, this contamination is less than 1% or 0.01 mag. Figure 5 shows the V and (V-R) isophotes as well as the $\mathrm{H}\alpha+[\mathrm{NII}]$ emission maps from Paper I for each galaxy, except for NGC 2865, NGC 6483, NGC 6909 and IC 4797, which do not show $\mathrm{H}\alpha+[\mathrm{NII}]$ emission. North is up and East to the left for all images. The V and V-R contour levels are listed while for $\mathrm{H}\alpha+[\mathrm{NII}]$ we show the lowest contour level, which corresponds to the flux threshold level, in units of $10^{-16}\,\mathrm{erg}\,\mathrm{s}^{-1}\mathrm{arcsec}^{-2}$. The central blank regions in the colour maps of some galaxies are due to an overexposure of the nucleus.

3.3 Dust morphology and description of individual cases

 The redder (V-R) values in the colour index maps (Fig. 5) correspond to larger A_V and A_R extinction values. From these maps we observe that the dust is in the form of filaments (e.g. NGC 533, NGC 4472 and NGC 5044), small disks (e.g. NGC 3379) and regular extended clouds (e.g. NGC 636, NGC 3311). The dust morphology follows very closely that of the ionized gas. We have measured the typical size l of the dust distribution (Table 2) which corresponds to the size of the most external isophote as shown in the (V-R) maps, and found that the dust has roughly the same size and distribution as that of the ionized gas.

Since the dust morphology varies considerably amongst the galaxies of our sample, it is worth to describe the most interesting cases separately. We use the same classification of Paper I, according to morphology and size:

• small disk (SD): faint and short filaments, mean diameter $\leq\, 4$Kpc;
• regular extended (RE): similar to SD but larger in size (4 to 8 Kpc);
• filaments (F): conspicuous filamentary structure, often extending as far as 10 Kpc from the galaxy center.

NGC 533. This galaxy has a filamentary dust distribution. Its morphology and distribution corresponds to that of the ionized gas.

NGC 636. The dust distribution is asymmetric with a cone-like shape oriented in the SE direction, while the ionized gas is in the form of a small disk, whose major axis orientation is perpendicular to the axis of the cone.

NGC 1600. The dust extends farther than $15\hbox{$^{\prime\prime}$}$ with an asymmetric filamentary structure in the W direction, while the H$\alpha$ emission is located in a small $5\hbox{$^{\prime\prime}$}\times 5\hbox{$^{\prime\prime}$}$ central disk.

NGC 2865. There is a large fragmentary dust cloud extending up to $15\hbox{$^{\prime\prime}$}$ from the nucleus. The asymmetric pattern of the cloud is due to the presence of a very bright field star in the SW direction. No ionized gas is observed in this galaxy.

NGC 3268. A small disk of dust and ionized gas is observed in the inner $5\hbox{$^{\prime\prime}$}$ of this galaxy. Note that the dust distribution is asymmetric with respect to the major axis of the galaxy in the SE direction, the opposite is observed for the ionized gas. The dust disk absorption can be seen in the inner V isophote.

NGC 3311. The dust cloud has an elongated structure oriented along the NS direction while the ionized gas cloud is roughly spherical, with a weak absorption feature which coincides with the dust pattern.

NGC 3379. The dust is concentrated in a large asymmetric cloud in the S direction. The H$\alpha$ image shows a very clear absorption feature South of the nucleus of the galaxy.

NGC 3489. This is an S0/Sa type galaxy. It has a very complex absorption pattern which has the form of patches (in the E side) and extended filamentary structure (in the W side). The dust absorption is observed also in the V image of the galaxy, producing box shaped isophotes in the central $20\hbox{$^{\prime\prime}$}$ of the galaxy (see the V isophotes map in Fig. 5). The ionized gas distribution in the form of filaments is rather regular in the inner $20\hbox{$^{\prime\prime}$}$, in which some patches of absorption can be seen.

NGC 3607. This is another case where the dust and ionized gas have the form of an asymmetric small disk. The dust absorption is stronger in the NE direction, while the ionized gas emission is stronger in the SW direction.

NGC 4472. This galaxy dust cloud is in the form of a small disk, similar to that of the ionized gas, although smaller in size.

NGC 4473. It shows a very small and edge on dust disk which is clearly observed in the ionized gas isophotal maps.

NGC 4552. This is an S0 galaxy with a very small amount of absorption at our limit of detection, in the form of filaments, more elongated to the NE direction, while the gas extends farther to the SW.

NGC 5044. The dust distribution has an irregular morphology, concentrated in the inner $10\hbox{$^{\prime\prime}$}$. Two central dark clouds can be seen. This galaxy has a very bright ionized gas emission in the form of extended filaments up to $40\hbox{$^{\prime\prime}$}$ from the center, being larger in the southern part of the galaxy. This galaxy seems to have gas and dust mixed, although the morphology of the ionized gas is not produced by dust absorption, since the dust cloud is smaller than the ionized gas emission region.

NGC 5812. The dust is distributed regularly in an inner region of $13\hbox{$^{\prime\prime}$}$ and is more extended than the ionized gas cloud. However, both have the same overall morphology.

NGC 5813. This is another case of an asymmetric distribution of the dust absorption in the form of filaments. The denser dust clouds follow the ionized gas distribution.

NGC 5903. The dust lies in a region smaller than that spanned by the ionized gas cloud, showing a similar morphology.

NGC 6483. This galaxy shows a irregular and filamentary dust distribution, more elongated in the SW direction. No gas is observed in this galaxy.

NGC 6758. An asymmetric dust disk can be seen in this galaxy, extending to the SW direction, while the gas is distributed in a more homogeneous cloud.

NGC 6909. The dust is in a small disk oriented with its edge on, elongated in the E direction. No gas is observed for this galaxy.

IC 4797. This galaxy has a filamentary dust distribution which seems to be obscured in the NE direction, although no gas is observed in this case.

IC 4889. This S0 galaxy shows a very large absorption cloud extending to the NE side of the galaxy. It also has a very bright and extended ionized gas emission. The unextinguished area in the southern part of the galaxy coincides with the brightest emission region of the galaxy.

IC 5105. This galaxy shows a very strong morphological correlation between the dust and the ionized gas clouds. Two large absorption regions extend to the NW and SE sides of the galaxy, separated by a narrow emission region. The inverse pattern is generally observed in the central parts of the other galaxies in our sample.