EXTENDED OPTICAL EMISSION-LINE GAS IN POWERFUL RADIO GALAXIES

Abstract

We present the results of a search for extended optical emission-line gas in 43 powerful radio galaxies. Spatially extended optical emission-line gas is common in the host galaxies of powerful radio sources. Both the extent and the luminosity of the emission-line gas in powerful radio galaxies is roughly an order of magnitude greater than in normal elliptical galaxies of similar optical magnitudes. The total emission-line luminosity is roughly half of the radio luminosity and the radio luminosity correlates with the narrow-line luminosity over four decades. The near nuclear emission-line gas is often distributed in a smooth roughly elliptical feature centered on, and symmetric about, the nucleus. The distribution of axial ratios found in these small emission-line nebulae (ELN) is inconsistent with their being disks seen from different orientations. The minor axes of the small regions of emission-line gas show a weak tendency to align with the position angle of the extended radio source and the minor axis of the stellar isophotes. The very extended emission-line gas (d > 10 kpc) is filamentary and is found preferentially within the regions (quadrants) occupied by the radio source. The small (d 100 kpc) radio sources with very extended ELN show no signs that they have been disturbed by their surrounding media. Lower limits to the density of the emission-line gas at distances of ~ 10 kpc from the galaxy nucleus are ~ 0.1 cm3 and upper limits to the total mass in emission-line gas are ~ ΙΟ9 3ΚΘ. The nonstellar, optical nuclear continuum is strongly correlated with the narrow emission-line luminosity and is sufficient to photoionize the extended ELN. The tendency of the very extended emission-line gas to be found in the radio quadrants may be due to the preferential escape of the nuclear ionizing continuum along the radio-source axis. Alternately, the nuclear ionizing radiation may be emitted isotropically, but the relatively high-density gas observed in emission lines may be found only along the edges of the radio source.