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An Impostor among Radio Galaxies

Submitter: Raffaella Morganti
Description: The origin of the dichotomy between radio-quiet and radio-loud objects is still a matter of debate: is it a question of nature or of nurture? Both intrinsic differences in the central engine and extrinsic differences in the surrounding medium have been considered. It is intriguing that optical AGN (Seyfert-like type) are mainly found in late-type, disk galaxies, while this is not the case for radio-loud AGN. These are mainly hosted by early-type galaxies.

However, in a paper now accepted for publication in Astronomy and Astrophysics we present the discovery of a rare and extreme example of powerful radio source (with radio luminosity equivalent to powerful FRII objects) hosted by a disk dominated galaxy. PKS1814-637 is the most powerful radio source (P_5 GHz = 4.1 x 10^25 W/Hz) known so far to be hosted by a disk galaxy.

At radio wavelengths, PKS1814-637 is about 480 pc in diameter and it is classified as a compact and young steep spectrum source (see contour plot on the right side of the figure). 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 H_2, and PAH emission features (detected using Spitzer), along with HI (from the Australian LBA) and silicate absorption features, providing evidence for a rich ISM which is likely to be related to the disk morphology of the host galaxy.

In the paper, we argue that the interaction between the radio plasma and the rich ISM may have boosted its radio emission. If this is the case, PKS~814-637 would represent a type of "impostor": 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.

We also 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 galaxies that are also gas-rich.

"PKS1814-637: a powerful radio-loud AGN in a disk galaxy" R. Morganti, J. Holt, C. Tadhunter, C. Ramos Almeida, D. Dicken, K. Inskip, T. Oosterloo, T. Tzioumis A&A accepted http://xxx.lanl.gov/abs/1109.0630)
Copyright: ASTRON
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