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Context. 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 that is not traced by CO. Constraining the relative importance of CO-bright versus CO-dark H2 star-forming reservoirs is crucial to understanding how star formation proceeds at low metallicity. Aims. We test classically used single component radiative transfer models and compare their results to those obtained on the assumption of an increasingly complex structure of the interstellar gas, mimicking an inhomogeneous distribution of clouds with various physical properties. Methods. Using the Bayesian code MULTIGRIS, we computed representative models of the interstellar medium as combinations of several gas components, each with a specific set of physical parameters. We introduced physically motivated models assuming power-law distributions for the density, ionization parameter, and the depth of molecular clouds. Results. This new modeling framework allows for the simultaneous reproduction of the spectral constraints from the ionized gas, neutral atomic gas, and molecular gas in 18 galaxies from the Dwarf Galaxy Survey. 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]158 μm and, to a lesser extent, by [C I] 609 μm, rather than by CO(1–0). We examine the CO-to-H2 conversion factor (αCO) versus metallicity relation and find that its dispersion increases significantly when different geometries of the gas are considered. We define a “clumpiness” parameter that is anti-correlated with [C II]/CO and explains the dispersion of the αCO versus metallicity relation. We find that low-metallicity galaxies with high clumpiness parameters may have αCO values as low as the Galactic value, even at low metallicity. Conclusions. We identify the clumpiness of molecular gas as a key parameter for understanding variations of geometry-sensitive quantities, such as αCO. This new modeling framework enables the derivation of constraints on the internal cloud distribution of unresolved galaxies, based solely on their integrated spectra.
Context. 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 that is not traced by CO. Constraining the relative importance of CO-bright versus CO-dark H2 star-forming reservoirs is crucial to understanding how star formation proceeds at low metallicity. Aims. We test classically used single component radiative transfer models and compare their results to those obtained on the assumption of an increasingly complex structure of the interstellar gas, mimicking an inhomogeneous distribution of clouds with various physical properties. Methods. Using the Bayesian code MULTIGRIS, we computed representative models of the interstellar medium as combinations of several gas components, each with a specific set of physical parameters. We introduced physically motivated models assuming power-law distributions for the density, ionization parameter, and the depth of molecular clouds. Results. This new modeling framework allows for the simultaneous reproduction of the spectral constraints from the ionized gas, neutral atomic gas, and molecular gas in 18 galaxies from the Dwarf Galaxy Survey. 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]158 μm and, to a lesser extent, by [C I] 609 μm, rather than by CO(1–0). We examine the CO-to-H2 conversion factor (αCO) versus metallicity relation and find that its dispersion increases significantly when different geometries of the gas are considered. We define a “clumpiness” parameter that is anti-correlated with [C II]/CO and explains the dispersion of the αCO versus metallicity relation. We find that low-metallicity galaxies with high clumpiness parameters may have αCO values as low as the Galactic value, even at low metallicity. Conclusions. We identify the clumpiness of molecular gas as a key parameter for understanding variations of geometry-sensitive quantities, such as αCO. This new modeling framework enables the derivation of constraints on the internal cloud distribution of unresolved galaxies, based solely on their integrated spectra.
UGC 2885 ($z = 0.01935$) is one of the largest and most massive galaxies in the local Universe, yet it has an undisturbed spiral structure, which is unexpected for such an object and is not predicted by cosmological simulations. Understanding the detailed properties of extreme systems such as UGC 2885 can provide insight into the limits of scaling relations and the physical processes driving galaxy evolution. Our goal is to understand whether UGC 2885 has followed a similar evolutionary path as other high-mass galaxies by examining its place in the fundamental metallicity relation and on the star-forming main sequence. We present new observations of UGC 2885 with the Canada-France-Hawaii Telescope and the Institut de radioastronomie millimétrique 30 m telescope. We used these novel data to calculate metallicity and molecular hydrogen mass values, respectively. We estimated the stellar mass (M$_ star $) and star formation rate (SFR) based on mid-infrared observations with the Wide-field Infrared Survey Explorer. We find global metallicities $Z = 9.28$, 9.08, and 8.74 at the 25 kpc ellipsoid from the N2O2, R23, and O3N2 indices, respectively. This puts UGC 2885 at the high end of the galaxy metallicity distribution. We find a molecular hydrogen mass of M$_ $ M$_ odot $, a SFR of $1.63 odot $ yr$^ $, and a stellar mass of $4.83 $ M$_ odot $, which gives a star formation efficiency ($ SFR $) of $8.67 $ yr$^ $. This indicates that UGC 2885 has an extremely high molecular gas content compared to known samples of star-forming galaxies ($ times more) and a relatively low SFR for its current gas content. We conclude that UGC 2885 has gone through cycles of star formation periods, which increased its stellar mass and metallicity to its current state. The mechanisms that are fuelling the current molecular gas reservoir and keeping the galaxy from producing stars remain uncertain. We discuss the possibility that a molecular bar is quenching star-forming activity.
Our ability to trace the star-forming molecular gas is important to our understanding of the Universe. We can trace this gas using CO emission, converting the observed CO intensity into the H2 gas mass of the region using the CO-to-H2 conversion factor ($X_{\rm {\small {CO}}}$). In this paper, we use simulations to study the conversion factor and the molecular gas within galaxies. We analysed a suite of simulations of isolated disc galaxies, ranging from dwarfs to Milky Way-mass galaxies, that were run using the FIRE-2 subgrid models coupled to the CHIMES non-equilibrium chemistry solver. We use the non-equilibrium abundances from the simulations, and we also compare to results using abundances assuming equilibrium, which we calculate from the simulation in post-processing. Our non-equilibrium simulations are able to reproduce the relation between CO and H2 column densities, and the relation between $X_{\rm {\small {CO}}}$ and metallicity, seen within observations of the Milky Way. We also compare to the xCOLD GASS survey, and find agreement with their data to our predicted CO luminosities at fixed star formation rate. We also find the multivariate function used by xCOLD GASS overpredicts the H2 mass for our simulations, motivating us to suggest an alternative multivariate function of our fitting, though we caution that this fitting is uncertain due to the limited range of galaxy conditions covered by our simulations. We also find that the non-equilibrium chemistry has little effect on the conversion factor (<5%) for our high-mass galaxies, though still affects the H2 mass and $L_{\rm {\small {CO}}}$ by ≈25%.
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