Continuum and line emission in Cygnus A

Details Continuum and line emission in Cygnus A Continuum and line emission in Cygnus A

Aug 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aj....108..414s&link_type=abstract

The Astronomical Journal, vol. 108, no. 2, p. 414-436

Astronomy and Astrophysics

Astronomy

35

Astronomical Spectroscopy, Cygnus Constellation, Emission Spectra, Imaging Techniques, Line Spectra, Radio Galaxies, Charts, Morphology, Radio Sources (Astronomy), Spectral Line Width, Tables (Data)

Scientific paper

We present the results from (1) imaging observations of Cygnus A in five essentially line-free continuum bands with central wavelengths ranging from 0.34 to 2.1 microns. (2) imaging observations in five narrowband filters centered on the emission lines H beta(O III) lambda5007, H alpha(N II) lambda6583, and (S II) lambda lambda6716, 6731, and (3) deep spectroscopy covering the entire central region of Cyg A. We confirm that the featureless spectrum component is to be identified with the prominent double morphology at the center of Cyg A, but uncertainties in the distribution of the dust in this region tend to limit the accuracy with which we can determine its morphology and spectral-energy distribution (SED). From regions that appear to be least affected by obscuration, we find fv is approximately v-0.1 for this component. This SED could be consistent with free-free emission, a population of young stars, or a quasar continuum scattered by electrons, but probably not with a quasar continuum scattered by dust, which would be bluer. Our spectroscopy places an upper limit on the equivalent width of broad H beta that is well below that of typical quasars, showing that this flat-spectrum component (FSC) is almost certainly not dominated by scattered quasar radiation. Appeals to scattering by hot electrons to smear the scattered broad lines into invisibility appear to fail because the large density scale height of the electrons and the inefficiency of electron scattering should result in smoother and more extensive structure than we observe. Although the relative SED is consistent with free-free emission, the required amount of hot gas violates other observational constraints. At high angular resolution, the apparent morphology of the FSC is spiral-like. Although this impression may be partly due to obscuration, the distribution of the dust itself only serves to reinforce the spiral-like nature of the material with which it is associated. We conclude that the FSC is most likely radiation from a starburst at the center of Cyg A. Fits to the radial surface-brightness profile of the late-type component after our best efforts at removing the FSC allow us to estimate the wavelength dependence of extinction in the central region.

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