The Radio AGN

Historically, radio emission was a key way to identify active galactic nuclei (AGN). However, we have since seen that strong radio emission is not necessary to classify an object as an AGN, and astronomers have divided AGN into two classes – "radio loud" and "radio quiet". The distinction is based on a comparison of the radio to the optical luminosity using the following relation:

where is frequency observed in the radio band. Then, radio-loud galaxies can be defined, for example, as those with for [1]. Naturally, things are not quite that clear-cut, and radio-loudness can be measured based on a different radio observing frequency or using a different cut-off for R. However, the differences really only affect those sources in the middle of the distribution, but well-accounts for the bulk of radio observations of AGN. Radio galaxies can be further classified. Radio-loud AGN are subclassifed by whether they exhibit large, resolved radio lobes on scales of several hundred kiloparsec or whether the radio emission is unresolved on Very Large Array (VLA) scales (e.g. smaller than about 20 kpc, depending on cosmological parameters). Even within these divisions, there are further subclassifications: the lobe-dominated galaxies can be classified by the location of the brighest radio spots versus the overall source size. Radio-quiet AGN fall into the familiar Seyfert classifications and QSOs.

References

¹ Krolik, J. H. (1999) Active Galactic Nuclei. Princeton University press, Princeton.

Other Resources

Radio Structure in Radio Galaxies
Wikipedia entry on AGN
Double Lobed Radio Galaxies
A Bestiary of Active Galaxies
Posted in Tutorials by Barb at February 9, 2007 9:01 PM