First Results from the COLA Project: The Radio–Far‐Infrared Correlation and Compact Radio Cores in Southern COLA Galaxies

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Abstract
We present the first results from the COLA (compact objects in low-power AGNs) project, which aims to determine the relationship between one facet of AGN activity, the compact radio core, and star formation in the circumnuclear region of the host galaxy. This will be accomplished by the comparison of the multiwavelength properties of a sample of AGNs with compact radio cores to those of a sample of AGNs without compact cores and a matched sample of galaxies without AGNs. In this paper we discuss the selection criteria for our galaxy samples and present the initial radio observations of the 107 southern (δ < 0°) galaxies in our sample. Low-resolution ATCA observations at 4.8, 2.5, and 1.4 GHz and high-resolution, single-baseline snapshots at 2.3 GHz with the Australian Long Baseline Array (LBA) are presented. We find that for the majority of the galaxies in our sample, the radio luminosity is correlated with the far-infrared (FIR) luminosity. However, a small number of galaxies exhibit a radio excess causing them to depart from the FIR-radio correlation. Compact radio cores are detected at fluxes greater than 1.5 mJy in nine of the 105 galaxies observed with the LBA. The majority (8/9) of these galaxies exhibit a radio excess, and 50% (7/14) of the galaxies that lie above the radio-FIR correlation by more than 1 σ have compact radio cores. The emission from the compact cores is too weak to account for this radio excess, implying that there are radio structures associated with the compact cores that extend farther than the 005 resolution (corresponding to a linear scale 11-22 pc) of the LBA. There is no evidence that the radio luminosity of the compact cores is correlated with the FIR galaxy luminosity, indicating that the core contributes little to the overall FIR emission of the galaxy. The galaxies with compact cores tend to be classified optically as AGNs, with two-thirds (6/9) exhibiting Seyfert-like optical emission line ratios, and the remaining galaxies classified either as composite objects (2/9) or starburst (1/9). The galaxies classified optically as AGNs also exhibit the largest radio excesses, and we therefore conclude that a large radio excess on the radio-FIR correlation is a strong indication of an AGN with a compact radio core.
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