PKS 1814-637: A powerful radio-loud AGN in a disk galaxy

We present a detailed study of PKS 1814-637, a rare case of powerful radio source (P5 GHz = 4.1 × 1025 W Hz-1) hosted by a disk galaxy. Optical images have been used to model the host galaxy morphology confirming it to be dominated by a strong (and warped) disk component that is observed close to edge-on to the line of sight. This is the first radio galaxy found to reside in a disk dominated galaxy with radio luminosity equivalent to powerful FRII objects. At radio wavelengths, PKS 1814-637 is about 480 pc in diameter and it is classified as a compact steep spectrum (CSS) source; such sources are usually considered to be radio sources observed in the early stages of their evolution. However, the optical and mid-IR spectroscopic properties of PKS1814-637 show more in common with Seyfert galaxies than they do with radio galaxies, with the detection of H2, and PAH emission features, along with HI and silicate absorption features, providing evidence for a rich ISM which is likely to be related to the disk morphology of the host galaxy. We argue that the interaction between the radio plasma and the rich ISM in this and similar objects may have boosted their radio emission, allowing them to more easily enter flux limited samples of radio sources. In this case, PKS 1814-637 represents a type of "imposter": an intrinsically low power object that is selected in a radio flux limited sample because of the unusually efficient conversion of jet power into radio emission. This would make PKS 1814-637 an extreme example of the effects of jet-cloud interactions in galaxies containing a rich ISM, and perhaps a missing link between radio galaxies and radio-loud Seyfert galaxies. However, it is unlikely that jet-cloud interactions alone can account for the unusually powerful radio emission compared to Seyfert galaxies, and it is probable that the jet in PKS 1814-637 is also intrinsically more powerful than in typical Seyfert galaxies, perhaps due to a higher bulge and black hole mass. The estimated BH mass is indeed higher than the majority of Seyfert galaxies in the local Universe. We speculate that sources similar to PKS1814-637 are likely to be more common at high redshifts, because of the enhanced probability at earlier epochs of triggering radio sources in moderately massive bulges that are also gas-rich.