Extended Optical Line Emitting Gas in Powerful Radio Galaxies: What is the Radio Emission-Line Connection?
Abstract
We use the results of the emission-line and radio imaging survey of radio galaxies presented in Baum et al. and Baum and Heckman to look for statistical evidence of energetic and spatial relationships between the extended emission-line gas and the radio source in powerful radio galaxies. We find that the radio luminosity correlates with the optical narrow emission-line luminosity over roughly four orders of magnitude in line luminosity and five in radio luminosity. This correlation most likely points to a common energy source for both the optical line and the radio emission, and suggests that the radio and line luminosities of radio galaxies are determined, to first order, by the properties of their central engines. The total emission-line luminosity (emitted in the infrared, the optical and the ultraviolet) of the powerful radio galaxies in the representative sample is roughly half of the luminosity of the associated radio source. While there exist a host of active galaxies with much higher ratios of line to radio luminosities than is found in radio galaxies (e.g., Seyfert galaxies and radio-quiet quasars), we find no steep-spectrum radio galaxies with high radio luminosities and low line luminosities. Combined with the strong correlation of the ionizing continuum with the emission-line luminosity, this implies that in powerful radio galaxies: (1) the central engine typically produces at least as much ionizing continuum luminosity as it does radio luminosity, and (2) the host galaxies always have cold gas near their nuclei. We find that there is a better correlation between the radio and narrow line luminosity than between the radio and the total (broad plus narrow) emission-line luminosities in radio galaxies. Two possible explanations are (1) there is a much stronger physical link between the radio jets and the narrow-line region than between the radio jets and the broad-line region, and (2) the broad emission lines are aspect angle dependent, while the narrow emission lines are aspect angle independent. We find statistical evidence of a spatial relationship between the very extended emission-line gas and the radio source. In all cases but one, the radio source extent is similar to or greater than the extent of the emission-line nebula. The very extended emission-line gas shows a statistical tendency to align with the radio source axis in both radio galaxies and radio-loud, steep-spectrum quasars, and the luminosity of the very extended emission-line gas comes preferentially, although not exclusively, from the radio quadrants. If the very extended emission-line gas is photoionized by the active nucleus, there are two possible explanations for the preference of the very extended emission-line gas for the radio quadrants. Some fraction of the ionizing radiation from the nucleus may be emitted, or escape, preferentially along the radio source ejection axis. Alternately, the nuclear ionizing radiation may be emitted isotropically, but the relatively high density gas which we observe in emission lines may be found preferentially along the edges of the radio source. It is also possible that the radio source contributes to the ionization of this gas. Possibilities include shock ionization, ionization by an ultraviolet extension of the radio synchrotron emission, and heating/ionization by cosmic rays associated with the radio source. The former two possibilities are likely to be most important in the few cases where the emission-line gas is found adjacent to regions of bright radio emission. We show that cosmic-ray ionization is energetically feasible; unfortunately, due to the large number of unknowns, we cannot estimate its actual contribution to the ionization of the gas. We find that small (d_radio_ < 150 kpc) radio sources with very extended emission-line gas (1) commonly have distorted radio morphologies and low apparent fractional radio polarizations, (2) often are associated with the dominant galaxies in rich clusters of galaxies where the cooling time of the intracluster gas is shorter than the Hubble time, and (3) commonly have emission-line gas which is cospatial, in projection, with the radio emitting plasma. Thus, in this class of "small" radio galaxies with very extended emission-line gas, the morphology, physical extent, and polarization properties of the radio sources appear to have been affected by the gas-rich environment through which the radio sources propagate. By contrast, we find that large (d_radio_ > 150 kpc) radio sources with very extended emission-line gas (1 ) have undistorted, FR2 radio morphologies and normal levels of fractional polarization, (2) are either the dominant member of small groups of galaxies, or isolated galaxies, and (3) have emission line nebula in which the brightest (off nuclear) regions of line emission are sometimes found along a position angle which is skewed to the radio source axis and are only sometimes found adjacent to bright radio features or directly along the path between the core and the hotspots. In a few sources the emission-line gas appears to be located preferentially along the low surface brightness regions of the radio source, perhaps associated with the regions of the radio source thought to be formed by the backflow of radio plasma from the hotspots toward the galaxy nucleus. Thus in these "large" radio sources with very extended emission-line gas, there is typically no evidence that the morphologies of the radio sources have been affected by the cold gas in the surrounding medium.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1989
- DOI:
- 10.1086/167044
- Bibcode:
- 1989ApJ...336..702B
- Keywords:
-
- Active Galactic Nuclei;
- Emission Spectra;
- Radio Emission;
- Radio Galaxies;
- Gas Density;
- Ionized Gases;
- Luminosity;
- Nebulae;
- Polarized Radiation;
- Astrophysics;
- RADIATION MECHANISMS;
- RADIO SOURCES: GALAXIES;
- RADIO SOURCES: EXTENDED