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Abstract Type Ia-CSM supernovae (SNe) are a rare and peculiar subclass of thermonuclear SNe characterized by emission lines of hydrogen or helium, indicative of high-density circumstellar medium (CSM). Their implied mass-loss rates of ∼10⁻⁴–10⁻¹M_⊙yr⁻¹(assuming ∼100 km s⁻¹winds) from optical observations are generally in excess of values observed in realistic SN Ia progenitors. In this paper, we present an independent study of CSM densities around a sample of 29 archival Ia-CSM SNe using radio observations with the Karl G. Jansky Very Large Array at 6 GHz. Motivated by the late (∼2 yr) radio detection of the Ia-CSM SN 2020eyj, we observed old (>1 yr) SNe, when we are more likely to see the emergent synchrotron emission that may have been suppressed earlier by free–free absorption by the CSM. We do not detect radio emission down to 3σlimits of ∼35μJy in our sample. The only radio-detected candidate in our sample, SN 2022esa, was likely misclassified as a Ia-CSM with early spectra, and appears more consistent with a peculiar Ic based on later epochs. Assuming wind-like CSM with temperatures between 2 × 10⁴K and 10⁵K, and a magnetic field-to-shock energy fractionϵ_B= 0.01 − 0.1, the radio upper limits rule out mass-loss rates between ∼10⁻⁴and 10⁻²M_⊙yr⁻¹(100 km s⁻¹)⁻¹. This is somewhat in tension with the estimates from optical observations, and may indicate that more complex CSM geometries and/or lower values ofϵ_Bmay be present. 

Abstract The effect of supernovae (SNe) on star formation in the interstellar medium (ISM) depends sensitively on where SNe explode with respect to ISM clouds. Observationally, SN ISM environments characterized by spatially resolved gas maps can empirically guide the placement of SNe in subgrid models, but unfortunately such measurements remain scarce, as SNe are rare and often distant. Here we demonstrate a new approach—mapping the ISM around evolved massive stars that are soon to explode. These provide a substantially larger sample of “explosion sites” (than just historical SNe) in nearby galaxies that have high-resolution atomic and molecular ISM maps from the Jansky Very Large Array and Atacama Large Millimeter/submillimeter Array. We demonstrate this technique in the well-resolved Local Group spiral M33 by analyzing the 50 pc scale projected ISM densities around red supergiants (RSGs; 8–30M_⊙stars) Wolf–Rayet stars (W-Rs; >30M_⊙stars), and supernova remnants. We find amass-dependentcorrelation between stars and gas clouds, with at least 45% of W-Rs and up to 77% of RSGs having no detectable H₂at their pixel locations. In the sample with H₂detections, we find that more-massive younger progenitors are coincident with denser gas. We show that the density distributions for stars >15M_⊙are statistically distinct from random alignment of stars and gas in M33. Our work provides the firstobservationally derivedestimate of the fraction of the SN-producing stellar population correlated with ISM density peaks. We demonstrate how this can be compared with galaxy simulations, and advocate similar comparisons to the community for constraining subgrid models. 

Abstract We present 0.6–3.2 pc resolution mid-infrared (MIR) JWST images at 7.7μm (F770W) and 21μm (F2100W) covering the main star-forming regions of two of the closest star-forming low-metallicity dwarf galaxies, NGC 6822 and Wolf–Lundmark–Melotte (WLM). The images of NGC 6822 reveal filaments, edge-brightened bubbles, diffuse emission, and a plethora of point sources. By contrast, most of the MIR emission in WLM is pointlike, with a small amount of extended emission. Compared to solar-metallicity galaxies, the ratio of 7.7μm intensity ( $I_{\nu }^{\mathrm{F770W}}$ ), tracing polycyclic aromatic hydrocarbons (PAHs), to 21μm intensity ( $I_{\nu }^{\mathrm{F2100W}}$ ), tracing small, warm dust grain emission, is suppressed in these low-metallicity dwarfs. Using Atacama Large Millimeter/submillimeter Array CO(2–1) observations, we find that detected CO intensity versus $I_{\nu }^{\mathrm{F770W}}$ at ≈2 pc resolution in dwarfs follows a similar relationship to that at solar metallicity and lower resolution, while the CO versus $I_{\nu }^{\mathrm{F2100W}}$ relationship in dwarfs lies significantly below that derived from solar-metallicity galaxies at lower resolution, suggesting more pronounced destruction of CO molecules at low metallicity. Finally, adding in Local Group L-Band Survey 21 cm Hiobservations from the Very Large Array, we find that $I_{\nu }^{\mathrm{F2100W}}$ and $I_{\nu }^{\mathrm{F770W}}$ versus total gas ratios are suppressed in NGC 6822 and WLM compared to solar-metallicity galaxies. In agreement with dust models, the level of suppression appears to be at least partly accounted for by the reduced galaxy-averaged dust-to-gas and PAH-to-dust mass ratios in the dwarfs. Remaining differences are likely due to spatial variations in dust model parameters, which should be an exciting direction for future work in local dwarf galaxies. 

Abstract Measuring the properties of the cold neutral medium (CNM) in low-metallicity galaxies provides insights into heating and cooling mechanisms in early Universe-like environments. We report detections of two localized atomic neutral hydrogen (Hi) absorption features in NGC 6822, a low-metallicity (0.2Z_⊙) dwarf galaxy in the Local Group. These are the first unambiguous CNM detections in a low-metallicity dwarf galaxy outside the Magellanic Clouds. The Local GroupL-band Survey (LGLBS) enabled these detections, due to its high spatial (15 pc for Hiemission) and spectral (0.4 km s⁻¹) resolution. We introduce LGLBS and describe a custom pipeline for searching for Hiabsorption at high angular resolution and extracting associated Hiemission. A detailed Gaussian decomposition and radiative transfer analysis of the NGC 6822 detections reveals five CNM components, with key properties: a mean spin temperature of 32 ± 6 K, a mean CNM column density of 3.1 × 10²⁰cm⁻², and CNM mass fractions of 0.33 and 0.12 for the two sightlines. Stacking nondetections does not reveal low-level signals below our median optical depth sensitivity of 0.05. One detection intercepts a star-forming region, with the Hiabsorption profile encompassing the CO (2−1) emission, indicating coincident molecular gas and a depression in high-resolution Hiemission. We also analyze a nearby sightline with deep, narrow Hiself-absorption dips, where the background warm neutral medium is attenuated by intervening CNM. The association of CNM, CO, and Hαemissions suggests a close link between the colder, denser Hiphase and star formation in NGC 6822. 

Abstract Type Ia supernovae (SNe Ia) are important cosmological tools, probes of binary star evolution, and contributors to cosmic metal enrichment; yet, a definitive understanding of the binary star systems that produce them remains elusive. Of particular interest is the identity of the mass-donor companion to the exploding carbon–oxygen white dwarf (CO WD). In this work, we present early-time (first observation within 10 days post-explosion) radio observations of six nearby (within 40 Mpc) SNe Ia taken by the Jansky Very Large Array, which are used to constrain the presence of synchrotron emission from the interaction between ejecta and circumstellar material (CSM). The two motivations for these early-time observations are: (1) to constrain the presence of low-density winds and (2) to provide an additional avenue of investigation for those SNe Ia observed to have early-time optical/UV excesses that may be due to CSM interaction. We detect no radio emission from any of our targets. Toward our first aim, these non-detections further increase the sample of SNe Ia that rule out winds from symbiotic binaries and strongly accreting white dwarfs. and discuss the dependence on underlying model assumptions and how our observations represent a large increase in the sample of SNe Ia with low-density wind constraints. For the second aim, we present a radiation hydrodynamics simulation to explore radio emission from an SN Ia interacting with a compact shell of CSM, and find that relativistic electrons cannot survive to produce radio emission despite the rapid expansion of the shocked shell after shock breakout. The effects of model assumptions are discussed for both the wind and compact shell conclusions. 

ABSTRACT We present radio observations of the symbiotic recurrent nova V3890 Sagitarii following the 2019 August eruption obtained with the MeerKAT radio telescope at 1.28 GHz and Karl G. Janksy Very Large Array (VLA) at 1.26−35 GHz. The radio light curves span from day 1 to 540 days after eruption and are dominated by synchrotron emission produced by the expanding nova ejecta interacting with the dense wind from an evolved companion in the binary system. The radio emission is detected early on (day 6) and increases rapidly to a peak on day 15. The radio luminosity increases due to a decrease in the opacity of the circumstellar material in front of the shocked material and fades as the density of the surrounding medium decreases and the velocity of the shock decelerates. Modelling the light curve provides an estimated mass-loss rate of $${\overset{\hbox{$$\bullet $$}}{M}}_{\textrm {wind}} \approx 10^{-8}\, {\textrm {M}}_\odot ~{\textrm {yr}}^{-1}$$ from the red giant star and ejecta mass in the range of Mej = 10−5––10−6 M⊙ from the surface of the white dwarf. V3890 Sgr likely hosts a massive white dwarf similar to other symbiotic recurrent novae, thus considered a candidate for supernovae type Ia (SNe Ia) progenitor. However, its radio flux densities compared to upper limits for SNe Ia have ruled it out as a progenitor for SN 2011fe like supernovae. 

Measuring the properties of the cold neutral medium (CNM) in low-metallicity galaxies provides insight into heating and cooling mechanisms in early Universe-like environments. We report detections of two localized atomic neutral hydrogen (Hi) absorption features in NGC 6822, a low-metallicity (0.2 Z⊙) dwarf galaxy in the Local Group. These are the first unambiguous CNM detections in a low-metallicity dwarf galaxy outside the Magellanic Clouds. The Local Group L-Band Survey (LGLBS) enabled these detections due to its high spatial (15 pc for Hi emission) and spectral (0.4 km s−1) resolution. We introduce LGLBS and describe a custom pipeline to search for Hi absorption at high angular resolution and extract associated Hi emission. A detailed Gaussian decomposition and radiative transfer analysis of the NGC 6822 detections reveals five CNM components, with key properties: a mean spin temperature of 32±6 K, a mean CNM column density of 3.1×1020 cm−2, and CNM mass fractions of 0.33 and 0.12 for the two sightlines. Stacking non-detections does not reveal low-level signals below our median optical depth sensitivity of 0.05. One detection intercepts a star-forming region, with the Hi absorption profile encompassing the CO (2−1) emission, indicating coincident molecular gas and a depression in high-resolution Hi emission. We also analyze a nearby sightline with deep, narrow Hi self-absorption dips, where the background warm neutral medium is attenuated by intervening CNM. The association of CNM, CO, and Hα emissions suggests a close link between the colder, denser Hi phase and star formation in NGC 6822. 

ABSTRACT Young stellar objects (YSOs) are the gold standard for tracing star formation in galaxies but have been unobservable beyond the Milky Way and Magellanic Clouds. But that all changed when the JWST was launched, which we use to identify YSOs in the Local Group galaxy M33, marking the first time that individual YSOs have been identified at these large distances. We present Mid-Infrared Instrument (MIRI) imaging mosaics at 5.6 and 21 $$\mu$$m that cover a significant portion of one of M33’s spiral arms that has existing panchromatic imaging from the Hubble Space Telescope and deep Atacama Large Millimeter/submillimeter Array CO measurements. Using these MIRI and Hubble Space Telescope images, we identify point sources using the new dolphot MIRI module. We identify 793 candidate YSOs from cuts based on colour, proximity to giant molecular clouds (GMCs), and visual inspection. Similar to Milky Way GMCs, we find that higher mass GMCs contain more YSOs and YSO emission, which further show YSOs identify star formation better than most tracers that cannot capture this relationship at cloud scales. We find evidence of enhanced star formation efficiency in the southern spiral arm by comparing the YSOs to the molecular gas mass. 

Abstract We present the Local GroupL-Band Survey, a Karl G. Jansky Very Large Array (VLA) survey producing the highest-quality 21 cm and 1–2 GHz radio continuum images to date, for the six VLA-accessible, star-forming, Local Group galaxies. Leveraging the VLA’s spectral multiplexing power, we simultaneously survey the 21 cm line at high 0.4 km s⁻¹velocity resolution, the 1–2 GHz polarized continuum, and four OH lines. For the massive spiral M31, the dwarf spiral M33, and the dwarf irregular galaxies NGC 6822, IC 10, IC 1613, and the Wolf–Lundmark–Melotte Galaxy, we use all four VLA configurations and the Green Bank Telescope to reach angular resolutions of <5″ (10–20 pc) for the 21 cm line with <10²⁰cm⁻²column density sensitivity, and even sharper views (<2″; 5–10 pc) of the continuum. Targeting these nearby galaxies (D ≲ 1 Mpc) reveals a sharp, resolved view of the atomic gas, including 21 cm absorption, and continuum emission from supernova remnants and Hiiregions. These data sets can be used to test theories of the abundance and formation of cold clouds, the driving and dissipation of interstellar turbulence, and the impact of feedback from massive stars and supernovae. Here, we describe the survey design and execution, scientific motivation, data processing, and quality assurance. We provide a first look at and publicly release the wide-field 21 cm Hidata products for M31, M33, and four dwarf irregular targets in the survey, which represent some of the highest-physical-resolution 21 cm observations of any external galaxies beyond the LMC and SMC. 

Abstract Momentum feedback from isolated supernova remnants (SNRs) have been increasingly recognized by modern cosmological simulations as a resolution-independent means to implement the effects of feedback in galaxies, such as turbulence and winds. However, the integrated momentum yield from SNRs is uncertain due to the effects of SN clustering and interstellar medium (ISM) inhomogeneities. In this paper, we use spatially resolved observations of the prominent 10 kpc star-forming ring of M31 to test models of mass-weighted ISM turbulence driven by momentum feedback from isolated, nonoverlapping SNRs. We use a detailed stellar age distribution (SAD) map from the Panchromatic Hubble Andromeda Treasury survey, observationally constrained SN delay-time distributions, and maps of the atomic and molecular hydrogen to estimate the mass-weighted velocity dispersion using the Martizzi et al. ISM turbulence model. Our estimates are within a factor of two of the observed mass-weighted velocity dispersion in most of the ring, but exceed observations at densities ≲0.2 cm −3 and SN rates >2.1 × 10 −4 SN yr −1 kpc −2 , even after accounting for plausible variations in SAD models and ISM scale height assumptions. We conclude that at high SN rates the momentum deposited is most likely suppressed by the nonlinear effects of SN clustering, while at low densities, SNRs reach pressure equilibrium before the cooling phase. These corrections should be introduced in models of momentum-driven feedback and ISM turbulence.