Characterizing Compact 15-33 GHz Radio Continuum Sources in Local U/LIRGs

Abstract

We present the analysis of $\sim 100$pc-scale compact radio continuum sources detected in 63 local (Ultra) Luminous Infrared Galaxies (U/LIRGs; $L_{\mathrm{IR}}\ge 10^{11}{L}_{\odot }$), using FWHM $\lesssim 0^{\prime \prime }.1-0^{\prime \prime }.2$ resolution 15 and 33 GHz observations with the Karl G. Jansky Very Large Array. We identify a total of 133 compact radio sources with effective radii of 8 - 170pc, which are classified into four main categories -- "AGN" (AGN), "AGN/SBnuc" (AGN-starburst composite nucleus), "SBnuc" (starburst nucleus) and "SF" (star-forming clumps) -- based on ancillary datasets and the literature. We find that "AGN" and "AGN/SBnuc" more frequently occur in late-stage mergers and have up to 3 dex higher 33 GHz luminosities and surface densities compared with "SBnuc" and "SF", which may be attributed to extreme nuclear starburst and/or AGN activity in the former. Star formation rates (SFRs) and surface densities ($\hat{I}{\pounds }_{\mathrm{SFR}}$) are measured for "SF" and "SBnuc" using both the total 33 GHz continuum emission (SFR $\sim 0.14-13$ M${}_{\odot }$ yr${}^{-1}$, $\hat{I}{\pounds }_{\mathrm{SFR}}\sim 13-1600$ M${}_{\odot }$ yr${}^{-1}$ kpc${}^{-2}$) and the thermal free-free emission from HII regions (median SFR${}_{\mathrm{th}}\sim 0.4$ M${}_{\odot }$ yr${}^{-1}$, $\hat{I}{\pounds }_{\mathrm{SF}{\mathrm{R}}_{\mathrm{th}}}\sim 44$ M${}_{\odot }$ yr${}^{-1}$ kpc${}^{-2}$). These values are 1 - 2 dex higher than those measured for similar-sized clumps in nearby normal (non-U/LIRGs). The latter also have much flatter median 15 - 33 GHz spectral index ($\sim -0.08$) compared with "SBnuc" and "SF" ($\sim -0.46$), which may reflect higher non-thermal contribution from supernovae and/or ISM densities in local U/LIRGs that directly result from and/or lead to their extreme star-forming activities on 100\,pc scales.

Accepted for publication in ApJ