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L.D. Bradley II, M.E. Kaiser (The Johns Hopkins University), W.A. Baan (NFRA, Westerbork Observatory), D.M. Crenshaw (Catholic University of America)
We present {\em VLA} radio observations of the near-nuclear region of the Seyfert\,2/Liner galaxy M51 obtained at 3.6~cm with a resolution of 0.24\arcsec. These data complement our high spatial resolution {\em HST} observations obtained with the Space Telescope Imaging Spectrograph (STIS). Both the STIS and archival {\em HST}/WFPC2 narrow band [OIII] images (Ford 1995) exhibit multiple emission-line clouds. Previous radio observations at 6~cm with the {\em VLA} suggest the presence of a faint southern radio jet coincident with the southern optical bicone seen in the WFPC2 narrow band [OIII] image and directed toward an extended diffuse radio structure (Crane and van der Hulst 1992, ApJ). The southern jet and diffuse structure seen in our 3.6~cm observations are in agreement with the lower resolution (0.4\arcsec) 6~cm data. The southern jet extends 2.3\arcsec (107\,pc; D=9.6\,Mpc Sandage and Tammann 1974, ApJ) from the nucleus along a position angle of 140\circ and connects to the diffuse structure located ~2-6\arcsec\ (93-279\,pc) from the nucleus. Close to the northern edge of this diffuse (lobe) structure, at a distance 2.5\arcsec\ from the nucleus, lies a radio knot which is identified with the extra-nuclear cloud (XNC) detected with narrow band H\alpha\,+\,[NII] imaging (Ford et al. 1985, ApJ) and WFPC2 narrow band [OIII] imaging (Grillmair et al. 1997, AJ). We also detect weak radio emission extending to the north of the nucleus roughly opposite the southern jet. This northern 10\muJy radio contour encompasses the [OIII] emission structure ~1.2\arcsec\ north of the nucleus.
Qualitatively, our 3.6\,cm radio map and archival WFPC2 [OIII]\,\lambda5007 image of the near-nuclear region of M51 characterize the correspondence between the optical and radio emission. In addition, we present photoionization modelling detailing the relative importance of photoionization and shocks for the individual emission-line clouds in this region.
Support for these observations was provided by contract NAS5-30403.