Adaptive Optics Imaging and Spectroscopy of Cygnus A. I. Evidence for a Minor Merger

We present Keck II adaptive optics near infrared imaging and spectroscopicobservations of the central regions of the powerful radio galaxy Cygnus A. The0.05" resolution images clearly show an unresolved nucleus between twospectacular ionization/scattering cones. We report the discovery of arelatively bright (K'~19) secondary point source 0.4" or 400 pc in projectionsouthwest of the radio nucleus. The object is also visible in archival HubbleSpace Telescope optical images, although it is easily confused with theunderlying structure of the host. Although the near infrared colors of thissecondary point source are roughly consistent with those of an L-dwarf, itsspectrum and optical-to-infrared spectral energy distribution (SED) virtuallyrule out the possibility that it may be any foreground projected object. Weconclude that the secondary point source is likely to be an extragalacticobject associated with Cygnus A. We consider several interpretations of thenature of this object, including: a young star cluster peering through the dustat the edge of one of the ionization cones; an older, large globular cluster; acompact cloud of dust or electrons that is acting as a mirror of the hiddenactive nucleus; and the dense core of a gas stripped satellite galaxy that ismerging with the giant elliptical host. The data presented here are mostconsistent with the minor merger scenario. The spectra and SED of the objectsuggest that it may be a densely packed conglomeration of older stars heavilyextincted by dust, and its high luminosity and compact nature are consistentwith those of a satellite that has been stripped to its tidal radius. Furtherspectroscopic observations are nevertheless necessary to confirm thispossibility.Comment: To appear in November 10 issue of ApJ. 22 pages, 5 figures (2 color