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

   https://doi.org/10.3847/1538-4357/ac923b  

   Song, Y. ; Linden, S. T. ; Evans, A. S. ; Barcos-Muñoz, L. ; Murphy, E. J. ; Momjian, E. ; Díaz-Santos, T. ; Larson, K. L. ; Privon, G. C. ; Huang, X. ; et al ( November 2022 , The Astrophysical Journal)

   We present the analysis of ∼100 pc scale compact radio continuum sources detected in 63 local (ultra)luminous infrared galaxies (U/LIRGs;L_IR≥ 10¹¹L_⊙), 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 ∼ 0.14–13M_⊙yr⁻¹, Σ_SFR∼ 13–1600M_⊙yr⁻¹kpc⁻²) and the thermal free–free emission from Hiiregions (median SFR_th∼ 0.4M_⊙yr⁻¹,${\mathrm{\Sigma }}_{{\mathrm{SFR}}_{\mathrm{th}}}\sim 44M_{\odot }$yr⁻¹kpc⁻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.

    

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2. A 2–3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946

   https://doi.org/10.1051/0004-6361/202142624  

   Eibensteiner, C. ; Barnes, A. T. ; Bigiel, F. ; Schinnerer, E. ; Liu, D. ; Meier, D. S. ; Usero, A. ; Leroy, A. K. ; Rosolowsky, E. ; Puschnig, J. ; et al ( March 2022 , Astronomy & Astrophysics)

   The complex physical, kinematic, and chemical properties of galaxy centres make them interesting environments to examine with molecular line emission. We present new 2 − 4″ (∼75 − 150 pc at 7.7 Mpc) observations at 2 and 3 mm covering the central 50″ (∼1.9 kpc) of the nearby double-barred spiral galaxy NGC 6946 obtained with the IRAM Plateau de Bure Interferometer. We detect spectral lines from ten molecules: CO, HCN, HCO + , HNC, CS, HC 3 N, N 2 H + , C 2 H, CH 3 OH, and H 2 CO. We complemented these with published 1 mm CO observations and 33 GHz continuum observations to explore the star formation rate surface density Σ SFR on 150 pc scales. In this paper, we analyse regions associated with the inner bar of NGC 6946 – the nuclear region (NUC), the northern (NBE), and southern inner bar end (SBE) and we focus on short-spacing corrected bulk (CO) and dense gas tracers (HCN, HCO + , and HNC). We find that HCO + correlates best with Σ SFR , but the dense gas fraction ( f dense ) and star formation efficiency of the dense gas (SFE dense ) fits show different behaviours than expected from large-scale disc observations. The SBE has a higher Σ SFR , f dense , and shocked gas fraction than the NBE. We examine line ratio diagnostics and find a higher CO(2−1)/CO(1−0) ratio towards NBE than for the NUC. Moreover, comparison with existing extragalactic datasets suggests that using the HCN/HNC ratio to probe kinetic temperatures is not suitable on kiloparsec and sub-kiloparsec scales in extragalactic regions. Lastly, our study shows that the HCO + /HCN ratio might not be a unique indicator to diagnose AGN activity in galaxies. 

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3. The PARADIGM project I: a multiscale radio morphological analysis of local U/LIRGS

   https://doi.org/10.1093/mnras/stae744  

   Lucatelli, Geferson ; Beswick, Robert J. ; Moldón, Javier ; Pérez-Torres, Miguel A. ; Conway, John E. ; Alberdi, Antxon ; Romero-Cañizales, Cristina ; Varenius, Eskil ; Klöckner, Hans-Rainer ; Barcos-Muñoz, Loreto ; et al ( March 2024 , Monthly Notices of the Royal Astronomical Society)

   Disentangling the radio flux contribution from star formation (SF) and active-galactic-nuclei (AGNs) activity is a long-standing problem in extragalactic astronomy, since at frequencies of ≲ 10 GHz, both processes emit synchrotron radiation. We present in this work the general objectives of the PARADIGM (PAnchromatic high-Resolution Analysis of DIstant Galaxy Mergers) project, a multi-instrument concept to explore SF and mass assembly of galaxies. We introduce two novel general approaches for a detailed multiscale study of the radio emission in local (ultra) luminous infrared galaxies (U/LIRGs). In this work, we use archival interferometric data from the Very Large Array (VLA) centred at ∼ 6 GHz (C band) and present new observations from the e-Multi-Element Radio-Linked Interferometer Network (e-MERLIN) for UGC 5101, VV 705, VV 250, and UGC 8696. Using our image decomposition methods, we robustly disentangle the radio emission into distinct components by combining information from the two interferometric arrays. We use e-MERLIN as a probe of the core-compact radio emission (AGN or starburst) at ∼ 20 pc scales, and as a probe of nuclear diffuse emission, at scales ∼100–200 pc. With VLA, we characterize the source morphology and the flux density on scales from ∼200 pc up to and above 1 kpc. As a result, we find deconvolved and convolved sizes for nuclear regions from ∼10 to ∼200 pc. At larger scales, we find sizes of 1.5–2 kpc for diffuse structures (with effective sizes of ∼ 300–400 pc). We demonstrate that the radio emission from nuclear extended structures (∼ 100 pc) can dominate over core-compact components, providing a significant fraction of the total multiscale SF output. We establish a multiscale radio tracer for SF by combining information from different instruments. Consequently, this work sets a starting point to potentially correct for overestimations of AGN fractions and underestimates of SF activity.

    

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4. CHANG-ES. XXIV. First Detection of a Radio Nuclear Ring and Potential LLAGN in NGC 5792

   https://doi.org/10.3847/1538-4357/ac4ae7  

   Yang 杨, Yang 阳. ; Irwin, Judith ; Li, Jiangtao ; Wiegert, Theresa ; Wang, Q. Daniel ; Sun, Wei ; Damas-Segovia, A. ; Li, Zhiyuan ; Shen, Zhiqiang ; Walterbos, René A. M. ; et al ( March 2022 , The Astrophysical Journal)

   We report the discoveries of a nuclear ring of diameter 10″ (∼1.5 kpc) and a potential low-luminosity active galactic nucleus (LLAGN) in the radio continuum emission map of the edge-on barred spiral galaxy NGC 5792. These discoveries are based on the Continuum Halos in Nearby Galaxies—an Expanded Very Large Array (VLA) Survey, as well as subsequent VLA observations of subarcsecond resolution. Using a mixture of Hαand 24μm calibrations, we disentangle the thermal and nonthermal radio emission of the nuclear region and derive a star formation rate (SFR) of ∼0.4M_☉yr⁻¹. We find that the nuclear ring is dominated by nonthermal synchrotron emission. The synchrotron-based SFR is about three times the mixture-based SFR. This result indicates that the nuclear ring underwent more intense star-forming activity in the past, and now its star formation is in the low state. The subarcsecond VLA images resolve six individual knots on the nuclear ring. The equipartition magnetic field strengthB_eqof the knots varies from 77 to 88μG. The radio ring surrounds a point-like faint radio core ofS_{6 GHz}= (16 ± 4)μJy with polarized lobes at the center of NGC 5792, which suggests an LLAGN with an Eddington ratio of ∼10⁻⁵. This radio nuclear ring is reminiscent of the Central Molecular Zone of the Galaxy. Both of them consist of a nuclear ring and LLAGN.

    

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5. Infrared Spectral Energy Distributions and Dust Masses of Sub-solar Metallicity Galaxies at z ∼ 2.3

   https://doi.org/10.3847/1538-4357/ac54a9  

   Shivaei, Irene ; Popping, Gergö ; Rieke, George ; Reddy, Naveen ; Pope, Alexandra ; Kennicutt, Robert ; Mobasher, Bahram ; Coil, Alison ; Fudamoto, Yoshinobu ; Kriek, Mariska ; et al ( March 2022 , The Astrophysical Journal)

   Abstract We present results from Atacama Large Millimeter/submillimeter Array (ALMA) 1.2 mm continuum observations of a sample of 27 star-forming galaxies at z = 2.1–2.5 from the MOSFIRE Deep Evolution Field survey with metallicity and star formation rate measurements from optical emission lines. Using stacks of Spitzer, Herschel, and ALMA photometry (rest frame ∼8–400 μ m), we examine the infrared (IR) spectral energy distributions (SED) of z ∼ 2.3 subsolar-metallicity (∼0.5 Z ⊙ ) luminous infrared galaxies (LIRGs). We find that the data agree well with an average template of higher-luminosity local low-metallicity dwarf galaxies (reduced χ 2 = 1.8). When compared with the commonly used templates for solar-metallicity local galaxies or high-redshift LIRGs and ultraluminous IR galaxies, even in the most favorable case (with reduced χ 2 = 2.8), the templates are rejected at >98% confidence. The broader and hotter IR SED of both the local dwarfs and high-redshift subsolar-metallicity galaxies may result from different grain properties or a harder/more intense ionizing radiation field that increases the dust temperature. The obscured star formation rate (SFR) indicated by the far-IR emission of the subsolar-metallicity galaxies is only ∼60% of the total SFR, considerably lower than that of the local LIRGs with ∼96%–97% obscured fractions. Due to the evolving IR SED shape, the local LIRG templates fit to mid-IR data overestimate the Rayleigh–Jeans tail measurements by a factor of 2–20. These templates underestimate IR luminosities if fit to the observed ALMA fluxes by >0.4 dex. At a given stellar mass or metallicity, dust masses at z ∼ 2.3 are an order of magnitude higher than z ∼ 0. Given the predicted molecular gas fractions, the observed z ∼ 2.3 dust-to-stellar mass ratios suggest lower dust-to-molecular gas masses than in local galaxies with similar metallicities. 

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