1 Introduction

In 1989-91, the late J.-L. Nieto and co-workers obtained at the CFHT frames of many elliptical galaxies, plus a few S0 objects, in order to investigate the core profiles of these objects. Preliminary results were published by Nieto et al. (1991a, 1991b). They found that the cores of E-galaxies could be empirically sorted out into two classes, one well or nearly resolved at the CFHT resolution, the other remaining quite unresolved: the disky ellipticals (or diE), like the S0's, were in the sharp nucleus class. The problem of galaxian central profiles has now been tackled successfully by several groups using HST frames, with special emphasis upon E-S0 galaxies, providing more quantitative confirmations of the conjectures of Nieto's et al. However core colour profiles and patterns of such objects have not yet been much studied at high resolution. It was therefore deemed useful to analyse the best B and R frames in Nieto's CFHT collection, restricting the sample to relatively nearby galaxies, i.e. objects in the Local Supercluster with V₀ < 3000 km s^-1. Suitable pairs for our goal were available for 38 E-type galaxies, that is objects so classified in one of the usual catalogues. The data was supplemented by Pic du Midi B, R frames for 6 E objects, obtained by E. Davoust and kindly put at our disposal.

In previous papers of this series some results were presented, based largely or exclusively upon the present "Nieto's sample". Such results are shortly summarized below. These papers also described the background and purposes of our work. The corresponding comments and references will not be repeated here. Review papers of interest have been published by Goudfrooij (1995) and (1996).

Our previous results are here recalled:

1.

In Paper I of this series (Michard 1998a) a dust pattern importance index (or DPII) was obtained for a nearly complete sample of 67 ellipticals, using the present Nieto's sample supplemented by literature data. The frequency and importance of dust patterns for various brands of E-classified galaxies were studied from statistics of this index. It was found that dust is much more frequently detected, and with more important patterns, among diE's than boE's, the unE's beeing intermediate.

2.

In Paper II (Michard 1998b) evidence for dust concentration in the disk was sought, for galaxies containing such a component: the criteria were an asymmetry in light and colour along the minor axis, as described for S0's by Michard & Simien (1993) (MS93). and/or a flattening of the isochromes as compared to the isophotes. In case of positive evidence, the symbol dd for "dust in disk'' was introduced. This occurs in diE's, whenever the inclination of their disk to the line of sight is large enough, and eventally also in the small isolated disks sometimes present in both boE's and diE's galaxies.

3.

In Paper III (Michard 1998c) the colour amplitude of the central red peak detected in most E-galaxies was described by the parameter $\Delta C_{0,3}$, i.e. the difference of colour between the centre and the isophote of mean radius 3 arcsec. For a sample of 39 galaxies, the central profiles were classified into two classes, shown to be equivalent to the "core-like'' and "power-law'' types of Faber et al. (1997). It was found that the red peak amplitude is very small for "core-like'' objects, unless they show clear evidence of a dust pattern. This agrees with recent HST results by Carollo et al. (1997), from a special subsample of objects with kinematically decoupled cores, and possibly rule out a model for the formation of flat cores proposed by Silva & Wise (1996).

This paper will present our data in more detail than in the previously published papers. Section 2 will describe the

observations. Section 3 will introduce the techniques to be used in order to derive reliable 1D and 2D colour distributions. A special emphasis is given to errors induced by the different PSF's of the two frames involved in colour measurements, and to the techniques applied to reduce such errors. Section 4 describes various typical features of the colour distributions in E-galaxies, and introduces useful ad hoc parameters. Section 5 compiles the data of interest for statistical discussions and presents comparisons with other works.

Further papers of this series will deal with:

. An examination of colour distributions in strongly inclined S0 galaxies, a project started in cooperation with Dr. P. Poulain in Toulouse. The available material for this kind of objects is rather scanty, except for the study of NGC 3115 by Silva et al. (1989), and the observations of bulges by Balcells & Peletier (1994).

. Statistics of the colour material gathered at Observatoire de Haute-Provence in collaboration with the late J. Marchal.

1.1 Notations and abbreviations

It may be useful to collect here the notations and abbreviations currently used below.

. SuBr surface brightness.

. majA, minA, major and minor axis respectively

. a, c major and minor axis of the Reference Ellipse for the representation of an isophote according to Carter (1978).

. $r\,=\,(ac)^{1/2}$ mean radius of the isophote.

. e_i, f_i coefficients of cosine and sine terms in Carter's harmonic representation of deviations from the Reference Ellipse.

. PA position angle of major axis.

. q axis ratio of the Reference Ellipse.

. $\epsilon\,=1-q$ ellipticity.

. diE, boE, unE subclassification of ellipticals as disky, boxy or undeterminate; p added for peculiar envelopes.