Color distributions in early-type galaxies. I - BVRI observations with a scanning CCD

An RCA CCD has been used in a scanning mode on the Palomar 1.5-m telescope to obtain high precision photometric images of three early-type galaxies. Advantages of this mode of operation include much reduced pixel-to-pixel sensitivity variations and more accurate sky subtraction. Frames in B, V, R, and I of NGC 4486 (M87) have been used to show, by comparison with published high quality surface photometry and aperture photometry, that this instrument produces measurements accurate at the 0.02-mag level at surface brightnesses above that of the sky. These frames of M87 and similar images of NGC 2300 and NGC 2768 reveal several interesting features of color variations in the inner regions of these early-type galaxies. NGC 2768 is found to have a dust ring or disk perpendicular to its plane of symmetry. All the colors appear to become bluer gradually outward from the center, with M87 showing the strongest gradients. The shapes of the contours of constant color are rounder, at small radii (less than 20 arcsec), than the isophotes in NGC 2300 and NGC 2768. This is not predicted by any galaxy formation picture, but would seem to support dissipationless collapse or mergers as the dominant processes in the formation of early-type galaxies.