Characterizing Compact 15â€"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.″1â€"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 8â€"170 pc, which are classified into four main categoriesâ€"“AGN” (active galactic nuclei), “AGN/SBnuc” (AGN-starburst composite nucleus), “SBnuc” (starburst nucleus), and “SF” (star-forming clumps)â€"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 (ÎŁ_SFR) are measured for “SF” and “SBnuc” using both the total 33 GHz continuum emission (SFR ∼ 0.14â€"13 M _⊙ yr^âˆ'1, ÎŁ_SFR ∼ 13â€"1600 M _⊙ yr^âˆ'1 kpc^âˆ'2) and the thermal freeâ€"free emission from H ii regions (median SFR_th ∼ 0.4 M _⊙ yr^âˆ'1, ÎŁ SFR th ∼ 44 M ⊙ yr^âˆ'1 kpc^âˆ'2). These values are 1â€"2 dex higher than those measured for similar-sized clumps in nearby normal (non-U/LIRGs). The latter also have a much flatter median 15â€"33 GHz spectral index (∼âˆ'0.08) compared with “SBnuc” and “SF” (∼âˆ'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. [ABSTRACT FROM AUTHOR]