Structure, colours, and populations of NGC 3115

Mysteries of a well-known object

Observatoire de Paris, LERMA, 77 av. Denfert-Rochereau, 75015 Paris, France e-mail: raymond.michard@obspm.fr

Received: 7 September 2006
Accepted: 21 November 2006

Abstract

Visible and near IR colour distributions in NGC 3115 have been studied from a set of frames of the Las Campanas Observatory and the 2MASS Atlas, plus other material from CFHT and OHP. The extended outer wings of the PSF were measured and their effect on such features as axis-ratio and colours carefully checked. In our data, the red-halo CCD phenomenon is the unique cause of a bluer major-axis in . Morphological indices suggest that the disk of NGC 3115 was once a complete little spiral, before being “swallowed” by a much bigger object. All colour distributions show the major-axis slightly redder than the minor-axis regions, at least not far from centre, in the radial range where the disk stands out. This is interpreted in the framework of a bulge+disk model as a metallicity effect, the disk having locally a larger [Fe/H] by some 0.16 ± 0.04.