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1. Radio Morphology of Red Geysers

   https://doi.org/10.3847/1538-4357/ac24a0  

   Roy, Namrata; Moravec, Emily; Bundy, Kevin; Hardcastle, Martin J.; Gürkan, Gülay; Diego Baldi, Ranieri; Leslie, Sarah K.; Masters, Karen; Gelfand, Joseph; Riffel, Rogerio; et al (December 2021, The Astrophysical Journal)

   Abstract We present 150 MHz, 1.4 GHz, and 3 GHz radio imaging (LoTSS, FIRST, and VLASS) and spatially resolved ionized gas characteristics (SDSS IV-MaNGA) for 140 local ( z < 0.1) early-type red geyser galaxies. These galaxies have a low star formation activity (with a star formation rate, SFR, ∼ 0.01 M ⊙ yr −1 ), but show unique extended patterns in spatially resolved emission-line maps that have been interpreted as large-scale ionized winds driven by active galactic nuclei (AGN). In this work, we confirm that red geysers host low-luminosity radio sources ( L 1.4GHz ∼ 10 22 WHz −1 ). Out of 42 radio-detected red geysers, 32 are spatially resolved in LoTSS and FIRST, with radio sizes varying between ∼5–25 kpc. Three sources have radio sizes exceeding 40 kpc. A majority display a compact radio morphology and are consistent with either low-power compact radio sources (FR0 galaxies) or radio-quiet quasars. They may be powered by small-scale AGN-driven jets that remain unresolved at the current 5″ resolution of radio data. The extended radio sources, not belonging to the “compact” morphological class, exhibit steeper spectra with a median spectral index of −0.67, indicating the dominance of lobed components. The red geysers hosting extended radio sources also have the lowest specific SFRs, suggesting they either have a greater impact on the surrounding interstellar medium or are found in more massive halos on average. The degree of alignment of the ionized wind cone and the extended radio features are either 0° or 90°, indicating possible interaction between the interstellar medium and the central radio AGN. 

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2. Dwarf Galaxies with Radio-excess Active Galactic Nuclei in the Very Large Array Sky Survey

   https://doi.org/10.3847/1538-4357/ad9584  

   Eberhard, John-Michael; Reines, Amy E; Gim, Hansung B; Darling, Jeremy; Greene, Jenny E (January 2025, The Astrophysical Journal)

   Abstract We present a systematic search for radio active galactic nuclei (AGNs) in dwarf galaxies using recent observations taken by the Very Large Array Sky Survey (VLASS). To select these objects, we first establish a criterion to identify radio-excess AGNs using the infrared-radio correlation parameter,q, that describes the tight relation between radio and IR emission in star-forming galaxies. We find a 2σthreshold ofq< 1.94 to select radio-excess AGNs, which is derived from a sample of ∼7000 galaxies across the full mass range in the NASA-Sloan Atlas that have radio and IR detections from VLASS and the Wide-Field Infrared Survey Explorer, respectively. We create catalogs of radio-excess AGNs and star-forming galaxies and make these available to the community. Applying our criterion to dwarf galaxies with stellar massesM_⋆≲ 3 × 10⁹M_⊙and redshiftsz≤ 0.15, and carefully removing interlopers, we find 10 radio-excess AGNs with radio-optical positional offsets between ∼0″ and 2.′3 (0–2.7 kpc). Based on statistical arguments and emission line diagnostics, we expect the majority of these radio-excess AGNs to be associated with the dwarf host galaxies rather than background AGNs. Five of the objects have evidence for hosting AGNs at other wavelengths, and five objects are identified as AGNs in dwarf galaxies for the first time. We also identify eight variable radio sources in dwarf galaxies by comparing the VLASS epoch 1 and epoch 2 observations to Faint Images of the Radio Sky at Twenty-centimeters detections presented in A. E. Reines et al. (2020). 

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

   https://doi.org/10.3847/1538-4357/ac05c2  

   Song, Y.; Linden, S. T.; Evans, A. S.; Barcos-Muñoz, L.; Privon, G. C.; Yoon, I.; Murphy, E. J.; Larson, K. L.; Díaz-Santos, T.; Armus, L.; et al (July 2021, The Astrophysical Journal)

   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 ⊙ yr −1 and 402 M ⊙ 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. 

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4. The radio galaxy population in the simba simulations

   https://doi.org/10.1093/mnras/stab654  

   Thomas, Nicole; Davé, Romeel; Jarvis, Matt J; Anglés-Alcázar, Daniel (March 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We examine the 1.4 GHz radio luminosities of galaxies arising from star formation and active galactic nuclei (AGNs) within the state-of-the-art cosmological hydrodynamic simulation Simba. Simba grows black holes via gravitational torque limited accretion from cold gas and Bondi accretion from hot gas, and employs AGN feedback including jets at low Eddington ratios. We define a population of radio loud AGNs (RLAGNs) based on the presence of ongoing jet feedback. Within RLAGN, we define high and low excitation radio galaxies (HERGs and LERGs) based on their dominant mode of black hole accretion: torque limited accretion representing feeding from a cold disc, or Bondi representing advection-dominated accretion from a hot medium. Simba predicts good agreement with the observed radio luminosity function (RLF) and its evolution, overall as well as separately for HERGs and LERGs. Quiescent galaxies with AGN-dominated radio flux dominate the RLF at $$\gtrsim 10^{22-23}$$ W Hz−1, while star formation dominates at lower radio powers. Overall, RLAGNs have higher black hole accretion rates and lower star formation rates than non-RLAGN at a given stellar mass or velocity dispersion, but have similar black hole masses. Simba predicts an LERG number density of 8.53 Mpc−3, ∼10× higher than for HERGs, broadly as observed. While LERGs dominate among most massive galaxies with the largest black holes and HERGs dominate at high specific star formation rates, they otherwise largely populate similar-sized dark matter haloes and have similar host galaxy properties. Simba thus predicts that deeper radio surveys will reveal an increasing overlap between the host galaxy demographics of HERGs and LERGs. 

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5. Detecting HII Regions in the Outer Scutum-Centaurus Arm

   Johnson, A.; Armentrout, W.; Anderson, L.; Bania, T.; Balser, D.; Wenger, T. (January 2021, American Astronomical Society meeting #237)

   There is relatively little known about Galactic star formation in the outer edges of the Milky Way, particularly in the Outer Scutum-Centaurus spiral arm (OSC). Lying about 15 kpc from the center of the Galaxy, the OSC was discovered in 2011 and is the most distant molecular spiral arm of the Milky Way. The OSC warps up to 4 degrees above the Galactic plane and as a result, has been excluded from the scope of many surveys of the Galactic plane, typically confined to a single degree above or below the plane. The goal of our study is to identify radio continuum from HII regions in the OSC in order to better understand the population of high-mass star formation regions in the outer Galaxy. We observed 12 HII Regions in the OSC using the Very Large Array at 10 GHz. Of our 12 targets, 7 are re-observations of undetected sources from Armentrout et al. (2017). The remaining 5 targets are sources without previously observed 10 GHz radio continuum data. We identify 10 GHz radio continuum associated with 7 of our OSC HII region targets for the first time. Assuming one dominant ionizing source per HII region, we assign spectral types from O9 to O5.5 for these sources, depending on their distance and continuum intensity. The remaining 5 nondetections represent lower-mass (B-type) star-forming regions below the sensitivity limit of our survey. These regions represent very high-mass star formation on the outer edge of the Galaxy, where densities and metallicities might be more similar to that of a much younger Milky Way or lower mass galaxies like the Magellanic Clouds. 

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