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

Abstract

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 SFRth ∼ 0.4 M⊙ yr⁻¹, Σ_[(SFR)_(th)] ~ 44 M_⊙ 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.