Ellipticals with Kinematically Distinct Cores: V – I Color Images with WPFC2^*

We have analyzed Hubble Space Telescope WFPC2 F555W and F814W (i.e., V and I) images for 15 elliptical galaxies with kinematically distinct cores. For each of them, we have derived surface brightness and isophotal parameter profiles in the two bands, color maps, and radial profiles in V - I. Most galaxies show patchy dust absorption close to their nuclei. However, there are generally no indications of homogeneous, diffuse dust components close to the nuclei. The nuclear colors in the unobscured regions are most likely representative of the central stellar populations.

We have detected photometric evidence for faint stellar disks, on scales of a few tens to a few arcseconds, in seven galaxies, namely NGC 1427, 1439, 1700, 4365, 4406, 4494, and 5322. In NGC 1700, the isophotes are slightly boxy at the scale of the counterrotating component and disky at larger radii. We find no difference in V - I color greater than 0.02 mag between these disks and the surrounding galactic regions. Hence, the stellar populations in the kinematically distinct cores are not strongly deviant from the population of the main body. Specifically, there is no evidence for a dominating population of blue, very metal weak stars as predicted by some of the formation scenarios. This argues against models in which small galaxies fall in and survive in the nuclei, unless supermassive black holes are present. These would likely disrupt the accreted small systems.

For one galaxy, NGC 4365, the innermost region is bluer than the surrounding regions. This area extends to ~15 pc and contains a luminosity of ~2.5 × 10⁶ L_☉. If interpreted as a stellar population effect, an age difference of ~3-4 Gyr, or an [Fe/H] variation of about 0.2 dex, is derived.

The nuclear intensity profiles show a large variety: some galaxies have steep cusp profiles, while others have shallow cusps and a "break radius." The nuclear cusps of galaxies with kinematically distinct cores follow the same trends as the nuclei of normal galaxies.

We have not been able to identify a unique, qualifying feature in the WFPC2 images that distinguishes the galaxies with kinematically distinct cores from the kinematically normal cores. It is possible that statistical differences exist: possibly, the kinematically distinct cores have a higher fraction of nuclear disks. The similarity of both types of cores puts strong constraints on the formation scenarios. Simulations of galaxy mergers, with the inclusion of star formation and nuclear black holes, are needed to resolve the question of how these structures may have formed. Spectra with high spatial resolution are needed to study the nuclear structure of the distinct component in detail.