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

We present the analysis of ~100 pc scale compact radio continuum sources detected in 63 local (ultra)luminous infrared galaxies (U/LIRGs; L _IR ≥ 10¹¹ L _⊙), using FWHM ≲ 0.″1-0.″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 8-170 pc, which are classified into four main categories-"AGN" (active galactic nuclei), "AGN/SBnuc" (AGN-starburst composite nucleus), "SBnuc" (starburst nucleus), and "SF" (star-forming clumps)-based on ancillary data sets 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 (Σ_SFR) are measured for "SF" and "SBnuc" using both the total 33 GHz continuum emission (SFR ~ 0.14-13 M _⊙ yr^-1, Σ_SFR ~ 13-1600 M _⊙ yr^-1 kpc^-2) and the thermal free-free emission from H II regions (median SFR_th ~ 0.4 M _⊙ yr^-1, ${{\rm{\Sigma }}}_{{\mathrm{SFR}}_{\mathrm{th}}}\sim 44\,{M}_{\odot }$ 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 a much flatter median 15-33 GHz spectral index (~-0.08) compared with "SBnuc" and "SF" (~-0.46), which may reflect higher nonthermal contribution from supernovae and/or interstellar medium densities in local U/LIRGs that directly result from and/or lead to their extreme star-forming activities on 100 pc scales.