Continuum and Line Emission in Cygnus A
Abstract
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 Hβ, [O III] λ5007, Hα, [N II] λ6583, and [S II] λλ6716,6731, 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 fnu ~ ν^-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 Hβ 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. A comparison with simple models for sources embedded in dust indicates that ~50% of the emergent blue flux from the late-type stars at the center may be due to scattered radiation. This large scattered contribution (which probably applies as well to the FSC and the emission-line component), together with a modest asymmetry in the radiation field, could plausibly account for the observed polarization properties of the inner region of Cyg A. Systematic differences are present between the distribution of [O III] on the one hand and [N II] (and other low-ionization species) on the other. After a large common component to the distribution is removed, the remaining [O III] excess is aligned with the radio axis, while the [N II] excess is distributed in what appears to be a planar structure perpendicular to the radio axis. The association of dust with this latter component indicates that it is a distinct morphological structure and not simply the result of shadowing effects. The emission-line intensities in the high-ionization regions are consistent with photoionization by a power-law continuum; those in the low-ionization region are similar to those seen in LINERs and appear to be consistent with ratios expected from x-ray photoionization. We find a systemic redshift for Cyg A of 0.05562+/-0.00015, from a measurement of the strongly diluted Mg I b feature in high-S/N spectra of the central region. The velocity field of the ionized gas is quite complex, showing at least two components along the line of sight in many regions. In general, discrete clouds show velocity curves consistent with rotation rather than with outflow, but high-velocity gas, probably associated with the radio jets, is seen on both sides of the galaxy nucleus. The ionized gas centered on the nucleus is counter-rotating relative to the bright cloud to the northwest and most of the fainter material in the same area. We conclude that, while there are strong reasons for believing that Cyg A harbors a hidden quasar (the presence of a point x-ray source, evidence from the emission lines of a flat photoionizing continuum, and the probable recent detection of scattered broad Mg II), there is no evidence that a significant amount of quasar optical or IR continuum has been scattered into our line of sight. Although the optical continuum peaks are roughly aligned with the radio structure, it is unlikely that there is any physical correspondence with the alignment between continuum and radio structure seen in luminous radio galaxies at high redshifts. Finally, the counter-rotating gas in the nuclear region, the secondary IR peak 1" north of the nucleus, the evidence for star formation in the nuclear mini-spiral, and the extensive ragged swath of low-ionization gas and dust stretching to the northeast and south all suggest that Cyg A has suffered a recent merger.
- Publication:
-
The Astronomical Journal
- Pub Date:
- August 1994
- DOI:
- 10.1086/117080
- Bibcode:
- 1994AJ....108..414S
- Keywords:
-
- Astronomical Spectroscopy;
- Cygnus Constellation;
- Emission Spectra;
- Imaging Techniques;
- Line Spectra;
- Radio Galaxies;
- Charts;
- Morphology;
- Radio Sources (Astronomy);
- Spectral Line Width;
- Tables (Data);
- Astrophysics;
- QUASARS: EMISSION LINES;
- GALAXIES: INDIVIDUAL: CYGNUS A;
- RADIO CONTINUUM: GALAXIES