HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF THE DOUBLE NUCLEUS OF NGC 4486B

Hubble Space Telescope WFPC2 images show that the low-luminosity ellip tical galaxy companion to M87, NGC 4486B, has a double nucleus that re sembles the one discovered in M31. The NGC 4486B nucleus comprises two peaks separated by similar to 0.''15 or 12 pc. Neither peak is coinci dent with the galaxy photocenter, which falls between them. The nuclea r morphology is independent of color; thus, the double structure is no t likely to arise from dust absorption. It is also unlikely that the p eaks are a binary stellar system (such as an ongoing merger of the nuc leus of a less luminous system with the nucleus of NGC 4486B), since t he decay timescale is short (<10(8) yr) and the present environment of NGC 4486B should inhibit mergers. We suggest that the nuclear morphol ogy of NGC 4486B may be explained by the eccentric-disk model of Trema ine, which was originally advanced to account for the central structur e of M31. This model requires that NGC 4486B contains a central massiv e dark object, which is suggested by the spectroscopic observations of Kormendy et al. The eccentric disk might be related to the symmetric disk seen at larger radii.