Faint object camera observations of M 87 : the jet and nucleus.

Abstract

Ultraviolet and optical images of the central region and jet of the nearby elliptical galaxy M87 have been obtained with ~0.1 arcsec resolution in several spectral bands with the Faint Object Camera (FOC) on the Hubble Space Telescope, including polarisation images. Deconvolution enhances the contrast of the complex structure and filamentary patterns in the jet already evident in the aberrated images. Morphologically there is close similarity between the FOC images of the extended jet and the best 2 cm radio maps obtained at similar resolution, and the magnetic field vectors from the ultraviolet and radio polarimetric data also correspond well. We observe structure in the inner jet within a few tenths arcsec of the nucleus which also has been well studied at radio wavelengths. Our ultraviolet and optical photometry of regions along the jet shows little variation in spectral index from the value 1.0 between markedly different regions and no trend to a steepening spectrum with distance along the jet. The new results strongly support the model for the jet in which there is no in situ particle acceleration localized at strong shocks but electrons accelerated at the nucleus propagate with low dissipation along the inside of the jet and the observed synchrotron emission occurs primarily in a boundary layer between the jet and its external medium. We observe no evidence for a central star cluster. In the nucleus, our data show the presence of an unresolved bright continuum source of optical to ultraviolet spectral index ~1.4 and size <= 0.6 pc core radius (at distance 16 Mpc), and an [O III]-emitting region of ~1.6 pc core radius.