Colour distributions in E-S0 galaxies^*

IV. Colour data and dust in E's from Nieto's B, R frames

Observatoire de la Côte d'Azur, Dept. Augustin Fresnel, BP. 229, F-06304 Nice Cedex 4, France

Send offprint request to: R. Michard

Received: 23 October 1998
Accepted: 9 March 1999

Abstract

The colours distributions (with R in Cousins's system) have been measured in 44 E classified galaxies in the Local Supercluster, from pairs of frames collected by Nieto and co-workers in 1989-91. These are nearly all from the CFHT, and of sub-arsec resolution. Great attention has been given to the effects of unequal PSF's in the B and R frames upon colour distributions near centre; such effects are illustrated from model calculations and from pseudo-colours obtained from pairs of frames taken in the same band but with different seeing conditions. Appropriate corrections were systematically applied in order to derive central colours and inner gradients, although still affected by the limited resolution of the frames. The radial colour distributions have been measured in more detail than usual, considering separately the near major axis and near minor axis regions of the isophotal contours. Azimuthal colour distributions, in rings limited by selected isophotes, were also obtained. Dust "patterns", i.e. patches, lanes, arcs, ..., have been detected and mapped from the colour distributions. An ad hoc dust pattern importance index (or DPII) in a scale of 0 to 3, has been introduced to qualify their size and contrast. We have tried to find evidence of a diffuse dust concentration towards the disk, if one is apparent. Positive results (noted by the dd symbol) have been obtained for disky E's, whenever the inclination of their disk to the line of sight is large enough, and eventually also in the small isolated disks sometimes present in both boxy and disky galaxies. The red central peak occurring in many E-galaxies might be the signature of a central concentration of dust, also in cases where this peak is isolated rather than embedded in some extensive colour pattern. The properties of the near centre colour profiles have been related to a classification of nuclear photometric profiles into "flat topped" and "sharply peaked" (equivalent to "core-like" and "power-law" in the terminology of Faber et al. 1997). The published here data include the following: . Short descriptions and codes for the characters of the distribution of each object, and comparison to the results of recent surveys. . A table of the mean at the centre and at two selected isophotes, a "core colour gradient" and the usual logarithmic gradient. . Maps of near core isochromes and B isophotes for comparison. Images of the colour distribution are made available in electronic form.