Compact radio emission in 60-mum peaker galaxies

Abstract

Accepted 1998 July 9. Received 1998 July 2; in original form 1998 March 10 ABSTRA C T We present radio interferometric observations of a well-defined sample of IRAS galaxies with warm far-infrared colours ‐ 60-mm peakers (60PKs). We find the surprising result that the core radio power of Seyfert 60PKs is between those of ‘normal’ Seyfert 2 galaxies and radio ellipticals, and follows the same relationship with respect to total and extended radio emission as low- and high-power radio galaxies. This is consistent with the suggestion that 60PKs represent nascent radio elliptical galaxies of low to intermediate power. The compact radio cores (assumed to be associated with the AGN core) in 60PKs contain typically ,37 per cent of the total radio flux of the galaxy (generally attributed to synchrotron radiation from cosmic ray electrons accelerated by distributed star formation in the galaxy). If this result is true more generally, then it implies that the compact radio core in an active galaxy ‘knows’ about the rate of star formation in the galaxy. Alternatively, the presence of a ‘radio excess’ for Seyfert 60PKs suggests that the extended radio flux may be attributed to radio jets rather than star formation, which explains its relationship with the radio core emission. Taking into consideration the sensitivity limit for the Parkes‐Tidbinbilla Interferometer, we find that none of the 60PKs classified as starburst by optical spectroscopy is detected, while over 90 per cent of the Seyfert 60PKs contain compact radio cores. This leads us to conclude that the starburst 60PKs are not just heavily dust-obscured AGN. Although we have smallnumber statistics, these results are consistent with previous surveys that indicate that the optical spectroscopic classification of emission-line galaxies discriminates between galaxies that contain high brightness temperature radio cores and those that do not. The data presented in this paper are insufficient to determine if there is an evolutionary relationship between the starburst and Seyfert classes of 60PKs.