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The low- J rotational transitions of 12CO are commonly used to trace the distribution of molecular gas in galaxies. Their ratios are sensitive to excitation and physical conditions in the molecular gas. Spatially resolved studies of CO ratios are still sparse and affected by flux calibration uncertainties, especially since most do not have high angular resolution or do not have short-spacing information and hence miss any diffuse emission. We compare the low- J CO ratios across the disc of two massive, star-forming spiral galaxies NGC 2903 and NGC 3627 to investigate whether and how local environments drive excitation variations at GMC scales. We use Atacama Large Millimeter Array (ALMA) observations of the three lowest- J CO transitions at a common angular resolution of 4 arcsec (190 pc). We measure median line ratios of |$R_{21}=0.67^{+0.13}_{-0.11}$|⁠ , |$R_{32}=0.33^{+0.09}_{-0.08}$|⁠ , and |$R_{31}=0.24^{+0.10}_{-0.09}$| across the full disc of NGC 3627. We see clear CO line ratio variation across the galaxy consistent with changes in temperature and density of the molecular gas. In particular, towards the centre, R 21, R 32, and R 31 increase by 35  per cent, 50  per cent, and 66  per cent, respectively, compared to their average disc values. The overall line ratio trends suggest that CO(3–2) is more sensitive to changes in the excitation conditions than the two lower J transitions. Furthermore, we find a similar radial R 32 trend in NGC 2903, albeit a larger disc-wide average of |$\langle R_{32}\rangle =0.47^{+0.14}_{-0.08}$|⁠. We conclude that the CO low- J line ratios vary across environments in such a way that they can trace changes in the molecular gas conditions, with the main driver being changes in temperature. [ABSTRACT FROM AUTHOR]

We present new HCN and HCO+ (J = 3-2) images of the nearby star-forming galaxies (SFGs) NGC 3351, NGC 3627, and NGC 4321. The observations, obtained with the Morita ALMA Compact Array, have a spatial resolution of ~290-440 pc and resolve the inner Rgal. 0:6-1 kpc of the targets, as well as the southern bar end of NGC 3627. We complement this data set with publicly available images of lower excitation lines of HCN, HCO+, and CO and analyse the behaviour of a representative set of line ratios: HCN(3-2)/HCN(1-0), HCN(3-2)/HCO+(3-2), HCN(1-0)/CO(2-1), and HCN(3-2)/CO(2-1). Most of these ratios peak at the galaxy centres and decrease outwards. We compare the HCN and HCO+ observations with a grid of one-phase, non-local thermodynamic equilibrium (non-LTE) radiative transfer models and find them compatible with models that predict subthermally excited and optically thick lines. We study the systematic variations of the line ratios across the targets as a function of the stellar surface density (±star), the intensity-weighted CO(2-1) (hICOi), and the star formation rate surface density (±SFR). We find no apparent correlation with ±SFR, but positive correlations with the other two parameters, which are stronger in the case of hICOi. The HCN/CO-hICOi relations show .0.3 dex galaxy-to-galaxy offsets, with HCN(3-2)/CO(2-1)-hICOi being ~2 times steeper than HCN(1-0)/CO(2-1). In contrast, the HCN(3-2)/HCN(1-0)-hICOi relation exhibits a tighter alignment between galaxies. We conclude that the overall behaviour of the line ratios cannot be ascribed to variations in a single excitation parameter (e.g., density or temperature). [ABSTRACT FROM AUTHOR]

Context. Molecular gas is a necessary fuel for star formation. The CO (1−0) transition is often used to deduce the total molecular hydrogen but is challenging to detect in low-metallicity galaxies in spite of the star formation taking place. In contrast, the [C II]λ158 μm is relatively bright, highlighting a potentially important reservoir of H2 that is not traced by CO (1−0) but is residing in the C+-emitting regions. Aims. Here we aim to explore a method to quantify the total H2 mass (MH2) in galaxies and to decipher what parameters control the CO-dark reservoir. Methods. We present Cloudy grids of density, radiation field, and metallicity in terms of observed quantities, such as [O I], [C I], CO (1−0), [C II], LTIR, and the total MH2. We provide recipes based on these models to derive total MH2 mass estimates from observations. We apply the models to the Herschel Dwarf Galaxy Survey, extracting the total MH2 for each galaxy, and compare this to the H2 determined from the observed CO (1−0) line. This allows us to quantify the reservoir of H2 that is CO-dark and traced by the [C II]λ158 μm. Results. We demonstrate that while the H2 traced by CO (1−0) can be negligible, the [C II]λ158 μm can trace the total H2. We find 70 to 100% of the total H2 mass is not traced by CO (1−0) in the dwarf galaxies, but is well-traced by [C II]λ158 μm. The CO-dark gas mass fraction correlates with the observed L[C II]/LCO(1−0) ratio. A conversion factor for [C II]λ158 μm to total H2 and a new CO-to-total-MH2 conversion factor as a function of metallicity are presented. Conclusions. While low-metallicity galaxies may have a feeble molecular reservoir as surmised from CO observations, the presence of an important reservoir of molecular gas that is not detected by CO can exist. We suggest a general recipe to quantify the total mass of H2 in galaxies, taking into account the CO and [C II] observations. Accounting for this CO-dark H2 gas, we find that the star-forming dwarf galaxies now fall on the Schmidt–Kennicutt relation. Their star-forming efficiency is rather normal because the reservoir from which they form stars is now more massive when introducing the [C II] measures of the total H2 compared to the small amount of H2 in the CO-emitting region. [ABSTRACT FROM AUTHOR]

Context. The neutral interstellar medium of galaxies acts as a reservoir to fuel star formation. The dominant heating and cooling mechanisms in this phase are uncertain in extremely metal-poor star-forming galaxies. The low dust-to-gas mass ratio and low polycyclic aromatic hydrocarbon abundance in such objects suggest that the traditional photoelectric effect heating may not be effective. Aims. Our objective is to identify the dominant thermal mechanisms in one such galaxy, I Zw 18 (1=30 Z⊙, assess the diagnostic value of fine-structure cooling lines, and estimate the molecular gas content. Even though molecular gas is an important catalyst and tracer of star formation, constraints on the molecular gas mass remain elusive in the most metal-poor galaxies. Methods. Building on a previous photoionization model describing the giant H ii region of I Zw 18-NW within a multi-sector topology, we provide additional constraints using, in particular, the [C II] 157 μm and [O I] 63 μm lines and the dust mass recently measured with the Herschel Space Telescope. Results. The heating of the H I region appears to be mainly due to photoionization by radiation from a bright X-ray binary source, while the photoelectric effect is negligible. Significant cosmic ray heating is not excluded. Inasmuch as X-ray heating dominates in the H I gas, the infrared fine-structure lines provide an average X-ray luminosity of order 4 × 1040 erg s-1 over the last few 104 yr in the galaxy. The upper limits to the [Ne v] lines provide strong constraints on the soft X-ray flux arising from the binary. A negligible mass of H2 is predicted. Nonetheless, up to ~107 M⊙ of H2 may be hidden in a few sufficiently dense clouds of order .5 pc (.0.0500) in size. Regardless of the presence of significant amounts of H2 gas, [C II] and [O I] do not trace the so-called "CO-dark gas", but they trace the almost purely atomic medium. Although the [C II]+[O I] to total infrared ratio in IZw 18 is similar to values in more metal-rich sources (~1%), it cannot be safely used as a photoelectric heating effciency proxy. This ratio seems to be kept stable owing to a correlation between the X-ray luminosity and the star formation rate. Conclusions. X-ray heating could be an important process in extremely metal-poor sources. The lack of photoelectric heating due to the low dust-to-gas ratio tends to 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. [ABSTRACT FROM AUTHOR]