3.1.1. Radio

One measure commonly used to distinguish starbursts from AGN is the FIR-to-radio correlation parameter q₀; starburst galaxies show a tight correlation around a value of q₀ = 2.34 ( ~ 0.1), while radio-loud AGN often have values as small as q₀ = 0 to -1. The majority of ULIGs have values of q₀ within the range q₀ = 2.0-2.6, however so do the majority of RQQSOs, which presents a problem when using a global parameter such as q₀ to distinguish starbursts from radio-quiet AGN.

A more direct method of distinguishing AGN from starbursts at radio wavelengths is to look for the presence or absence of a true high-brightness temperature AGN-like radio core. VLBA observations of RQQSOs find typical radio-AGN core fractions in the range of 20-50 % at 1.4 GHz (P. Barthel and H. Smith, private communication). Two of the five ULIGs in Table 2 (Mrk 231 and UGC 05101) have AGN-core strengths within this range; perhaps not surprisingly, these two objects also have the smallest q₀ values.

A somewhat unexpected finding is the detection with the VLA of large scale (i.e. 20-50 kpc) radio ``plumes'' in three of the five ULIGs (UGC 05101, Mrk 231, Mrk 273), similar to the bipolar plumes or extended jets seen in radio galaxies (M. Yun, this workshop) suggesting the presence of a powerful radio-AGN core in these objects.