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

Y. Song, S. T. Linden, A. S. Evans, L. Barcos-Munoz, E. J. Murphy, E. Momjian, T. Diaz-Santos, K. L. Larson, G. C. Privon, X. Huang, L. Armus, J. M. Mazzarella, V. U, H. Inami, V. Charmandaris, C. Ricci, K. L. Emig, J. McKinney, I. Yoon, D. Kunneriath, T. S. -Y. Lai, E. E. Rodas-Quito, A. Saravia, T. Gao, W. Meynardie, D. B. Sanders

Submitted on 8 September 2022


We present the analysis of 100pc-scale compact radio continuum sources detected in 63 local (Ultra) Luminous Infrared Galaxies (U/LIRGs; LIR1011L), using FWHM 0.10.2 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 (ΣSFR) are measured for "SF" and "SBnuc" using both the total 33 GHz continuum emission (SFR 0.1413 M yr1, ΣSFR131600 M yr1 kpc2) and the thermal free-free emission from HII regions (median SFRth0.4 M yr1, ΣSFRth44 M yr1 kpc2). 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 (0.08) compared with "SBnuc" and "SF" (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.


Comment: Accepted for publication in ApJ

Subject: Astrophysics - Astrophysics of Galaxies