4 Remarkable features in the $\vec B-R$ distributions in E-galaxies

According to the present study, the colour distributions in E-galaxies may feature the following properties:

1.

Colour patterns are made up of localized red patches, and of straight or curved lanes, sometimes forming a complex system. Such local features are unambiguously assigned to dust.

2.

The dust in disk phenomenon corresponds to a set of properties in the colour distributions of galaxies containing a disk, and denoting a concentration of dust in this component. The most obvious of such properties is a redder lane along the disk or close to it.

3.

The central red peak stands out in many colour distributions, even after careful matching of the PSF's, as described in Sect. 3. Arguments will be given suggesting that a central dust concentration might be the cause of this feature in many cases, but no conclusive evidence is available.

4.

Blue features are scarce enough to be considered as anomalous in E-galaxies.

It should be emphasized that the above listed features are not exclusive of one another, and can very well occur together in a single object.

4.1 Local red "colour patterns"

Local dust features are detected in 16 galaxies out of 42 in our sample, leaving aside the two anomalous blue objects NGC 3156 and 4742. These formations differ widely in size and contrast, and it was deemed useful, for classification purposes, to introduce a dust pattern importance index (DPII), in a scale of 0 to 3. The various steps of the DPII can be defined as follows:

. 0: No evidence of local dust in B-R data,

. 1-: Some evidence for local dust is detected,

. 1: Clear dust pattern around core,

. 2: Marked dust pattern around core,

. 3: Outstanding dust pattern.

For the indices 2 and 3, the dust produces visible distortions in the B isophotes.

In Paper I of this series, this index was derived for 67 E galaxies of the Local Supercluster, both from the present sample and from literature data (including many multiple observations), and a statistical discussion was given, relating the DPII to the morphological subclasses, diE, unE and boE, and to other suggestive groupings of objects.

The colour patterns in E's are centrally located, and the galaxian centre is usually the reddest point in the distribution. The pattern of NGC 2974 is an exception, with the peak colour displaced by some 1.5 arcsec.

The strong dust patterns in E galaxies show a geat variety of mostly irregular geometries. Regular rings, as seen in S0 and Sa galaxies at various inclinations are scarcely encountered. The complex colour pattern of NGC 2974 shows however several such rings.

It is perhaps useful to bring attention to a number of dust patterns of special interest as displayed in the maps of Figs. A1 to A32. Outstanding dust patterns (DPII=3) occur in NGC 1052, 2768, 2974, 4125 and 4374. The case DPII=2 is represented only by NGC 5831. Is it a subjective trend of our selection or the result of a gap in the real distribution? The classification of a much larger sample would be necessary to decide.

For DPII=1, we note, among others, NGC 0821, with a short dust lane nearly along the minor axis; and NGC 4660 with a dust lane nearly along the major axis, so that it is also classified among the objects with "dust in disk".

The DPII value 1- has been given in cases where the isochromes appear to deviate significantly from the isophotes, without a well contrasted colour pattern. Examples are NGC 3377 and 3379. Another case of interest is NGC 4261, where dust is not clearly seen in the B-R distribution, but appears as a small absorption arclet in the B image. This minute feature as been registered with the HST (see Jaffe et al. 1994). There are doubtful cases, where the 1- index could become 0 with a better S/N ratio, or conversely. For NGC 4494 we find DPII=0 (no dust), while Goudfrooij et al. (1994b) and van Dokkum & Franx (1995) see dust near the nucleus of this object.

As noted in Paper I, there is generally no evidence for dust patterns in boE galaxies. We refer to the maps of NGC 4365, 4406, 4472 for examples of giant dustless boE's, and to NGC 4387 and 4478 for minor boE's with rotational support (see Nieto & Bender 1989). The latter have no colour pattern but a sharp red nucleus (see below).

4.2 "Dust in disk'' phenomenon

In Paper II of this series, the evidence has been considered for dust concentration near the disk of E galaxies containing a disk component. This involved the disk of diE's at sufficient inclinations, but also the inner "decoupled'' disks seen in both boE's and diE's. This evidence may include the "minor axis asymmetry'' in light and colour, described and modelled for S0 galaxies in MS93. A distinct reddening of the major axis in the region where the disk contributes significantly to the SuBr might be also taken as evidence for "dust in disk'', but is somewhat ambiguous, because a similar appearance may result from very different colour gradients between the spheroid and disk of a two-component galaxy.

Looking at the set of maps below, the two above properties appear as a flattening of the isochromes as compared to the isophotes, and eventually their displacement in the minor axis direction, relative to the centre of symmetry of the isophotes. These appearances can be recognized in the graphs pertaining to NGC 0584, 3585, 3610, 4473, 4621, 5322, 5845. The latter two objects have inner decoupled disks, and their HST deconvolved images appear in Lauer et al. (1995).

In the summary data table of Sect. 5, galaxies showing evidence of the "dust in disk'' phenomenon are noted with the dd symbol.

4.3 Red nuclei

Red nuclei in E-galaxies have been considered by Sparks et al. (1985): they note "an unresolved nucleus 0.3 magnitudes redder than the surrounding galaxy'' for NGC 6958. This description obviously refers to an artefact of differential seeing, according to the experiments described in Sect. 3, and to previous studies referenced there. In the present work, the PSF's of the two frames involved in a colour measurement were equalized, and the red nuclei can be objectively studied. The resolution is limited by the worse of the two frames, the B one in general, and the accuracy is estimated to 0.02-0.03 magnitude at the centre and nearby regions.

  

Figure 9: Sample of objects with flat radial B-R distributions. The label of each graph is the name of the plotted file, which identifies the galaxy. Abscissae: log of isophotal mean radius r in arcsec. Ordinates: Colour in magnitude. We note that such objects are large galaxies with flat topped cores

  

Figure 10: Sample of objects with radial B-R distributions involving a central isolated red peak. The label of each graph is the name of the plotted file, which identifies the galaxy. Abscissae: log of isophotal mean radius r in arcsec. Ordinates: Colour in magnitude. We note that such objects also have a sharply peaked central light distribution

The central regions show the following types of colour distributions:

1.

nearly flat. This is the case for a few large galaxies, i.e. NGC 0720, 4365, 4406, 4472, 4649, 5846 and perhaps also for the compact dwarf NGC 4486B. Such cases are illustrated in Fig. 9. In this situation, the logarithmic colour gradient tends to decrease inwards of a limiting radius of a few arcsec (much less for the dwarf). We note from Table 6 that such objects are in the "flat topped core" (ftc) class of central SuBr profiles;

2.

isolated red peaks. In this case, illustrated in Fig. 10, the logarithmic colour gradient increases inwards of a break radius $r_{\rm b}$, with typical values $1.5<r_{\rm b}<3$ arcsec, for the range of distances and object sizes of our sample. The amplitude of the red nucleus is parametrized by the difference $\Delta C_{0,3}$ of the B-R colours at the centre and at the radius r=3 arcsec. Objects with isolated red peaks of large $\Delta C_{0,3}$, notably the examples of Fig. 10, usually belong to the "sharp peak" (shp) class of central SuBr sprofiles;

3.

embedded red peaks. In this case, the red nucleus is part of a larger colour pattern, and the radius $r_{\rm b}$ is no more defined. The colour difference $\Delta C_{0,3}$ can still be measured but cannot be looked at as a property of the red nucleus itself. The 1D colour graphs are not of much use in such cases. One example has been given in Figs. 1, 2 for NGC 4125.

It is tempting to speculate that central red peaks may be caused by a concentration of dust near the centre of the object (although population variations might also be partly responsible?). This is probably the case for red peaks embedded in a larger dust pattern. Since minute central dust patterns have been discovered from HST frames one might consider these as a likely cause of the red peaks seen at a lesser resolution.

Remark: There are two atypical cases deviating from the above classification of red nuclei. NGC 0636 exhibits a small blue nucleus at the centre of a normal red nucleus; for NGC 2974, the peak B-R is displaced some 1.5 arcsec from the galaxian centre.

4.4 Anomalous objects

In our sample, there are 2 galaxies with a central region much bluer than its surroundings, most probably due to recent star formation.

NGC 3156 is an E5 in the RSA, but has type T=-2 in the RC3. From the same catalogue, its colours are very blue, 0.75 in ${(B-V)_{\rm e}}$ and 0.29 in ${(U-B)_{\rm e}}$. The radial B-R distribution is shown in Fig. 11. This galaxy also shows a small dust arclet.

NGC 4742 is an E4 in the RSA, and has type T=-5 in the RC3. The catalogue colours are 0.80 for ${ (B-V)_{\rm T}}$ and 0.30 for ${(U-B)_{\rm T}}$. Colours in the effective radius are not available. The radial B-R distribution is shown in Fig. 12. A sharp red peak occurs at the centre of the object.

  

Figure 11: Radial distribution of B-R for NGC 3156. Abscissae: log of isophotal mean radius r in arcsec. Ordinates: Uncorrected colour in magnitude. Circles: majA; crosses: minA. This anomalous galaxy contains a centrally located very blue region and also some dust

  

Figure 12: Radial distribution of B-R for NGC 4742. Abscissae: log of isophotal mean radius r in arcsec. Ordinates: Uncorrected colour in magnitude. Circles: majA; crosses: minA. This anomalous galaxy contains a centrally located very blue region with a red peak at the very centre. The available field was quite small so that the colour farther than some 8 arcsec is very uncertain