HST observations of 3C 449: discovery of an extended nuclear disk and of possible optical jets.

Abstract

We present the results of HST observations of the radio-galaxy 3C 449. Observations were taken with the Faint Object Camera using the F372M and F342W filters both of which include the redshifted [OII]λλ3726, 3729 lines. Around the nucleus of 3C 449, we detected a ring with a projected diameter of 0.4" which corresponds to about 200 pc. Our analysis of the luminosity profile shows that the observed ring is actually the result of absorption in the inner region of the galaxy. The size of this region (100 pc radius, for H_0_=50km/s/Mpc) is consistent with it being cool material associated with an extended accretion disk. Assuming cylindrical symmetry we derived an orientation for the axis of the disk as 30 degrees from the line of sight and 10 degrees from the projected axis of the radio jets. We also discovered two bright blue knots which are aligned with the axis of the disk. They emit mostly, if not wholly, continuum radiation. The lack of line emission in the knots essentially rules out the possibility that the emission is produced by photoionization form the nuclear source and supports the interpretation that the radio-jets themselves are producing the knots. The interpretation that these knots are the optical synchrotron counter-parts to the radio-jets is consistent with the radio-observations if the ratio-to-optical spectral index is about 0.5, as expected in case of particle acceleration by shock waves; this also naturally explains the symmetrical position of the knots with respect to the nucleus. Higher resolution radio observations are needed to clarify this point. Alternative possibilities exist for both the inner ring and the optical knots: reflection of the light from the nucleus, presence of star forming regions, shock waves or merging could also explain the observed morphology but with implausible constraints on the overall properties of 3C 449.