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Do Starburst galaxies contain buried AGN?

We have observed 210 Seyfert galaxies and 81 starburst galaxies with the 275-km Parkes - Tidbinbilla Interferometer (PTI) at 2.3 GHz (Norris et al., 1988). This has resulted in a high-resolution survey with uniform sensitivity ( > 2 mJy) and resolution (0.1 arcsec, corresponding to 20 - 200 pc over the redshift range of the sample). Thus the PTI is sensitive to structures with brightness temperatures greater than K, and so can detect radio emission from compact nuclear objects associated with the AGN, but is blind to extended star-formation activity.

In Seyferts we typically detect about 41 % of the objects observed (e.g. Roy et al. 1994) but in starburst galaxies (Norris & Roy 1996) we detect only 5%. This result shows a significant difference between the two samples, and shows clearly that, although a few starburst galaxies may contain a buried AGN, the majority do not. There is a tendency for these buried AGN to occur in high luminosity objects, such as those classified as ultra-luminous infrared galaxies. Lonsdale et al. (1993) carried out a similar survey on 15 ultra-luminous starburst galaxies and detected eight of them. The difference between this result and that of Norris & Roy may be attributed partly to the fact that the Lonsdale sample were all high-luminosity objects, although the Norris and Roy result demonstrates that even at these high luminosities, some ultra-luminous galaxies are still powered by starburst activity.