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Star formation and stellar feedback are interlinked processes that redistribute energy and matter throughout galaxies. When young, massive stars form in spatially clustered environments, they create pockets of expanding gas termed superbubbles. As these processes play a critical role in shaping galaxy discs and regulating the baryon cycle, measuring the properties of superbubbles provides important input for galaxy evolution models. With wide coverage and high angular resolution (50-150 pc) of the PHANGS-ALMA $^{12}$CO (2-1) survey, we can now resolve and identify a statistically representative number of superbubbles with molecular gas in nearby galaxies. We identify superbubbles by requiring spatial correspondence between shells in CO with stellar populations identified in PHANGS-HST, and combine the properties of the stellar populations with CO to constrain feedback models and quantify their energetics. We visually identify 325 cavities across 18 PHANGS-ALMA galaxies, 88 of which have clear superbubble signatures (unbroken shells, central clusters, kinematic signatures of expansion). We measure their radii and expansion velocities using CO to dynamically derive their ages and the mechanical power driving the bubbles, which we use to compute the expected properties of the parent stellar populations driving the bubbles. We find consistency between the predicted and derived stellar ages and masses of the stellar populations if we use a supernova blast wave model that injects energy with a coupling efficiency of 10%, whereas continuous models fail to explain stellar ages we measure. Not only does this confirm molecular gas accurately traces superbubble properties, but it also provides key observational constraints for superbubble models. We also find evidence that the bubbles sweep up gas as they expand and speculate that these sites have the potential to host new generations of stars., Comment: 21 pages, 15 figures, 3 tables. Accepted to A&A. Abstract abridged for arXiv

Low-metallicity dwarf galaxies often show no or little CO emission, despite the intense star formation observed in local samples. Both simulations and resolved observations indicate that molecular gas in low-metallicity galaxies may reside in small dense clumps, surrounded by a substantial amount of more diffuse gas, not traced by CO. Constraining the relative importance of CO-bright versus CO-dark H2 star-forming reservoirs is crucial to understand how star formation proceeds at low metallicity. We put to the test classically used single component radiative transfer models and compare their results to those obtained assuming an increasingly complex structure of the interstellar gas, mimicking an inhomogeneous distribution of clouds with various physical properties. We compute representative models of the interstellar medium as combinations of several gas components, each with a specific set of physical parameters. We introduce physically-motivated models assuming power-law distributions for the density, ionization parameter, and the depth of molecular clouds. We confirm the presence of a predominantly CO-dark molecular reservoir in low-metallicity galaxies. The predicted total H2 mass is best traced by [C II]158um and, to a lesser extent, by [CI] 609um, rather than by CO(1-0). We examine the CO-to-H2 conversion factor vs. metallicity relation and find that its dispersion increases significantly when different geometries of the gas are considered. We define a clumpiness parameter that anti-correlates with [CII]/CO and explains the dispersion of the CO-to-H2 conversion factor vs. metallicity relation. We find that low-metallicity galaxies with high clumpiness may have CO-to-H2 conversion factor as low as the Galactic value. We identify the clumpiness of molecular gas as a key parameter to understand variations of geometry-sensitive quantities, such as CO-to-H2 conversion factor., Comment: Submitted to A&A, comments welcome

The complex physical, kinematic, and chemical properties of galaxy centres make them interesting environments to examine with molecular line emission. We present new $2-4$" (${\sim}75{-}150$ pc at $7.7$ Mpc) observations at 2 and 3 mm covering the central $50$" (${\sim}1.9$ 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 1mm CO observations and 33 GHz continuum observations to explore the star formation rate surface density ${\Sigma_{\mathrm{SFR}}}$ 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 ${\Sigma_{\mathrm{SFR}}}$, but the dense gas fraction ($f_{\mathrm{dense}}$) and star formation efficiency of the dense gas (${\mathrm{SFE_{dense}}}$) fits show different behaviours than expected from large-scale disc observations.The SBE has a higher ${\Sigma_{\mathrm{SFR}}}$, $f_{\mathrm{dense}}$, 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., Comment: 38 pages, 23 figures, accepted for publication in A&A

There exists some consensus that stellar mass surface density ($Σ_{*}$) and molecular gas mass surface density ($Σ_{\rm mol}$) are the main quantities responsible for locally setting the star formation rate. This regulation is inferred from locally resolved scaling relations between these two quantities and the star formation rate surface density ($Σ_{\rm SFR}$). However, the universality of these relations is debated. Here, we probe the interplay between these three quantities across different galactic environments at a spatial resolution of 150 pc. We perform a hierarchical Bayesian linear regression to find the best set of parameters $C_{*}$, $C_{\rm mol}$, and $C_{\rm norm}$ that describe the star-forming plane conformed by these quantities, such that $\log Σ_{\rm SFR} = C_{*} \log Σ_{*} + C_{\rm mol} \log Σ_{\rm mol} + C_{\rm norm}$, and explore variations in the determined parameters across galactic environments, focusing our analysis on the $C_{*}$ and $C_{\rm mol}$ slopes. We find signs of variations in the posterior distributions of $C_{*}$ and $C_{\rm mol}$ across different galactic environments. Bars show the most negative value of $C_{*}$, a sign of longer depletion times, while spiral arms show the highest $C_{*}$ among all environments. We conclude that systematic variations in the interplay of $Σ_{*}$, $Σ_{\rm mol}$ and $Σ_{\rm SFR}$ across galactic environments exist at a spatial resolution of 150 pc, and we interpret these variations as produced by an additional mechanism regulating the formation of stars that is not captured by either $Σ_{*}$ or $Σ_{\rm mol}$. We find that these variations correlate with changes in the star formation efficiency across environments, which could be linked to the dynamical state of the gas that prevents it from collapsing and forming stars, or to changes in the molecular gas fraction., 21 pages, 16 figures, accepted for publication in A&A

Aims. There exists some consensus that the stellar mass surface density (Σ⋆) and molecular gas mass surface density (Σmol) are the main quantities responsible for locally setting the star formation rate. This regulation is inferred from locally resolved scaling relations between these two quantities and the star formation rate surface density (ΣSFR), which have been extensively studied in a wide variety of works. However, the universality of these relations is debated. Here, we probe the interplay between these three quantities across different galactic environments at a spatial resolution of 150 pc. Methods. We performed a hierarchical Bayesian linear regression to find the best set of parameters C⋆, Cmol, and Cnorm that describe the star-forming plane conformed by Σ⋆, Σmol, and ΣSFR, such that logΣSFR = C⋆logΣ⋆ + CmollogΣmol + Cnorm. We also explored variations in the determined parameters across galactic environments, focusing our analysis on the C⋆ and Cmol slopes. Results. We find signs of variations in the posterior distributions of C⋆ and Cmol across different galactic environments. The dependence of ΣSFR on Σ⋆ spans a wide range of slopes, with negative and positive values, while the dependence of ΣSFR on Σmol is always positive. Bars show the most negative value of C⋆ (−0.41), which is a sign of longer depletion times, while spiral arms show the highest C⋆ among all environments (0.45). Variations in Cmol also exist, although they are more subtle than those found for C⋆. Conclusions. We conclude that systematic variations in the interplay of Σ⋆, Σmol, and ΣSFR across different galactic environments exist at a spatial resolution of 150 pc, and we interpret these variations to be produced by an additional mechanism regulating the formation of stars that is not captured by either Σ⋆ or Σmol. Studying environmental variations in single galaxies, we find that these variations correlate with changes in the star formation efficiency across environments, which could be linked to the dynamical state of the gas that prevents it from collapsing and forming stars, or to changes in the molecular gas fraction.

(abridged) The dominant thermal mechanisms in the neutral interstellar medium, which acts as a star-forming gas reservoir, are uncertain in extremely metal-poor galaxies. Our objective is to identify the heating mechanisms in one such galaxy, IZw18, and assess the diagnostic value of fine-structure cooling lines. We also seek to constrain the mass of H$_2$, which, despite being an important catalyst and tracer of star formation, remains elusive in this object. Building on a previous photoionization model within a multi-sector topology, we provide additional constraints from the [CII] and [OI] lines and the dust mass recently measured with Herschel. The heating of the HI region appears to be mainly due to photoionization by radiation from a bright X-ray binary source, while photoelectric effect (PE) is negligible. The [CII] and [OI] lines imply an average X-ray luminosity of $4\times10^{40}$ erg s$^{-1}$, while the [NeV] upper limits bring strong constraints to the soft X-ray flux arising from the binary. A negligible amount of H$_2$ is predicted, but $\lesssim10^7$ M$_\odot$ of H$_2$ may be hidden in sufficiently dense clouds of order $\lesssim10$ pc in size. Regardless of the presence of significant amounts of H$_2$, [CII] and [OI] do not trace the so-called CO-dark gas, but the almost purely atomic medium. Although the [CII]+[OI]/TIR ratio is close to values found in more metal-rich sources, it cannot be safely used as a PE heating efficiency proxy. This ratio seems to be kept stable due to a correlation between the X-ray luminosity and the star-formation rate. We propose that X-ray heating is an important process in extremely metal-poor sources. The weak PE heating due to the low dust-to-gas ratio could be compensated for by the larger occurrence and power of X-ray binaries in low-metallicity galaxies. We speculate that X-ray heating may quench star formation., Comment: Accepted for publication in A&A. Language editing completed in version 2