A Comparison between Nuclear Ring Star Formation in LIRGs and in Normal Galaxies with the Very Large Array

Abstract

Nuclear rings are excellent laboratories for studying intense star formation. We present results from a study of nuclear star-forming rings in five nearby normal galaxies from the Star Formation in Radio Survey (SFRS) and four local LIRGs from the Great Observatories All-sky LIRG Survey at sub-kiloparsec resolutions using Very Large Array high-frequency radio continuum observations. We find that nuclear ring star formation (NRSF) contributes 49%-60% of the total star formation of the LIRGs, compared to 7%-40% for the normal galaxies. We characterize a total of 57 individual star-forming regions in these rings, and find that with measured sizes of 10-200 pc, NRSF regions in the LIRGs have star formation rate (SFR) and Σ SFR up to 1.7 M e yr −1 and 402 M e yr −1 kpc −2 , respectively, which are about 10 times higher than in NRSF regions in the normal galaxies with similar sizes, and comparable to lensed high-z star-forming regions. At ∼100-300 pc scales, we estimate low contributions (<50%) of thermal free-free emission to total radio continuum emission at 33 GHz in the NRSF regions in the LIRGs, but large variations possibly exist at smaller physical scales. Finally, using archival sub-kiloparsec resolution CO (J = 1-0) data of nuclear rings in the normal galaxies and NGC 7469 (LIRG), we find a large scatter in gas depletion times at similar molecular gas surface densities, which tentatively points to a multimodal star formation relation on sub-kiloparsec scales.

High-resolution Hubble Space Telescope Imaging Survey of Local Star-forming Galaxies. I. Spatially Resolved Obscured Star Formation with Hα and Paschen-β Recombination Lines

“…Other studies directly suggest that one cannot use rest-optical lines to estimate physical properties of entire starburst systems, and one should instead aim to obtain rest-frame near-IR observations (Puglisi et al 2017;Calabrò et al 2018). Other recent works have used radio emission (specifically free-free emission) to measure star formation, in addition to IR and recombination lines (e.g., Murphy et al 2018;Linden et al 2019Linden et al , 2021Song et al 2021). Freefree emission does not have the caveat of being affected by extinction as is the case for recombination lines.…”

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

“…To ensure that we only identify physically meaningful structures, we ran Astrodendro on both the 15 and 33 GHz images of each system with min_value = 5σ rms and min_delta = 1σ rms where σ rms is the rms noise measured in an emission-free region of the image before primary-beam correction. We follow Song et al (2021) and set min_npix to be a quarter of the area of the synthesized beam, to avoid identifying noise spikes yet allowing detection of small unresolved regions. Despite that extended diffuse emission is largely filtered out in these observations, complex structures encompassing trunk, branch, and leaf are identified in several systems.…”

“…of heavily obscured luminous AGNs in local U/LIRGs (Iwasawa et al 2011;Treister et al 2012;Ricci et al 2017Ricci et al , 2021Koss et al 2018;Torres-Albà et al 2018) has provided strong supporting evidence for this evolutionary scenario (see also review by U 2022), details regarding the interplay between star formation and AGN activity, as well as how they together (or separately) act upon the transformation of these extreme systems, still remain ambiguous. However, the extraordinary star-forming properties of local U/LIRGs relative to nearby normal galaxies (i.e., galaxies with L IR < 10 11 L e ; e.g., Lonsdale et al 1984;Howell et al 2010;Stierwalt et al 2014;Piqueras López et al 2016;Díaz-Santos et al 2017;Linden et al 2019Linden et al , 2021Larson et al 2020;Song et al 2021), and the prevalence of outflows observed in starburst-dominated local U/LIRGs (e.g., Rupke et al 2005;Cazzoli et al 2016;Barcos-Muñoz et al 2018;Fluetsch et al 2019Fluetsch et al , 2020Vivian et al 2019) highlight the pivotal role of star-formation-driven feedback in regulating their evolution. To better quantify the physical processes governing the evolution of local U/LIRGs, a robust characterization of the most energetic regions in these systems is necessary.…”

“…Despite finding extranuclear star formation enhancement relative to SFRS galaxies on kiloparsec scales, they concluded that nuclear star formation must drive GOALS systems above the star formation main sequence (e.g., Elbaz et al 2011;Speagle et al 2014) occupied by the SFRS galaxies. Subsequently in Song et al (2021), four nuclear rings detected in our survey were examined at ∼100 pc (0 1) scales and compared to five nuclear rings detected in the SFRS galaxies. The nuclear ring star formation was found to contribute more significantly to the total star formation of the host galaxies for LIRGs compared with normal galaxies.…”

“…The nuclear ring star formation was found to contribute more significantly to the total star formation of the host galaxies for LIRGs compared with normal galaxies. In this paper, we extend the methodology adopted in Song et al (2021) to study the ∼100 pc scale compact radio continuum sources detected in 63 U/LIRGs in the survey, with the aim of constraining the nature and physical properties of these energetic regions at frequencies unimpeded by dust extinction.…”