Extremely weak CO emission in IZw 18

Zhou, Luwenjia; Shi, Yong; Zhang, Zhi-Yu; Wang, Junzhi

doi:10.1051/0004-6361/202039033

Cited by 7 publications

(10 citation statements)

References 59 publications

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“…The red data points and upper limits are fromMadden et al (2020). We also show on top the most likely solution (empty black circle with the gray circle indicating the standard deviation) with M(H 2 ) = 10 5.4 M and CO 3500 times fainter than inZhou et al (2021) and on bottom the secondary, less likely, solution with 10 7 M and CO in agreement withZhou et al (2021).…”

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confidence: 89%

“…We now add molecular gas observations as constraints for model M NM . For CO, we use the recently claimed CO(2-1) detection of Zhou et al (2021), corresponding to a CO(1-0) luminosity of 0.18 L (≈ 3.5σ detection). Even though the signal is quite uncertain, the new observation is much deeper than previous studies and provides at the very least a useful upper limit.…”

Section: Neutral Molecular Gas: Model M Nmmentioning

confidence: 99%

“…M NA adds neutral atomic gas observations ([C ii], [O i], [Si ii],and [Fe ii]). M NM adds the CO marginal detection ofZhou et al (2021) and upper limits on near-IR and mid-IR H 2 . The escape fractions are given for several energy ranges in Rydberg.…”

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confidence: 95%

“…Properties of the molecular sector in model M NM . From top to bottom we show as a function of the total predicted H 2 mass, the mixing weight of the molecular sector, the mass of H 0 , of dust, the CO(1-0) intensity (scaled to 4.56 × 10 36 erg s −1 ; with the horizontal lines showing the observed range inZhou et al 2021), and the fraction of [C ii] originating from the molecular sector.…”

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confidence: 99%

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Topological models to infer multiphase interstellar medium properties

Lebouteiller

Ramambason

2022

A&A

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Context. Spectroscopic observations of high-redshift galaxies slowly reveal the same complexity of the interstellar medium (ISM) as expected from resolved observations in nearby galaxies. While providing, in principle, a wealth of diagnostics concerning galaxy evolution, star formation, or the nature and influence of compact objects, such high-z spectra are often spatially and spectrally unresolved, and inferring reliable diagnostics represents a major obstacle. Bright, nearby, unresolved galaxies observed in the optical and infrared domains provide many constraints to design methods to infer ISM properties, but they have so far been limited to deterministic methods and/or with simple topological assumptions (e.g., single 1D model). Aims. It is urgent to build upon previous ISM multiphase and multicomponent methods by using a probabilistic approach that makes it possible to derive probability density functions for relevant parameters while also enabling a large number of free parameters with potential priors. The goal is to provide a flexible statistical framework that is agnostic to the model grid and that considers either a few discrete components defined by their parameter values and/or statistical distributions of parameters. In this paper, we present a first application with the objective to infer probability distributions of several physical parameters (e.g., the mass of H 0 , H 2 , escape fraction of ionizing photons, and metallicity) for the star-forming regions of the metal-poor dwarf galaxy I Zw 18 in order to confirm the low molecular gas content and high escape fraction of ionizing photons from H ii regions. Methods. We present a Bayesian approach to model a suite of spectral lines using a sequential Monte Carlo method provided by the Python package PyMC which combines several concepts such as tempered likelihoods, importance sampling, and independent Metropolis-Hastings chains. The algorithm, provided by the associated code MULTIGRIS, accepts a few components which can be represented as sectors around one or several stellar clusters, or continuous (e.g., power-law, normal) distributions for any given parameter. We applied this approach to a grid of models calculated with the photoionization and photodissociation code Cloudy in order to produce topological models of I Zw 18.Results. The statistical framework we present makes it possible to consider a large number of spectroscopic tracers, with the extinction and systematic uncertainties as potential additional random variables. We applied this technique to the galaxy I Zw 18 in order to reproduce and go beyond previous topological models specifically tailored to this object. While our grid is designed for global properties of low-metallicity star-forming galaxies, we were able to calculate accurate values for the metallicity, number of ionizing photons, masses of ionized and neutral hydrogen, as well as the dust mass and the dust-to-gas mass ratio in I Zw 18. We find a relatively modest amount of H 2 (∼ 10 5 M ) which is predominantly CO-dark and trac...

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confidence: 89%

Section: Neutral Molecular Gas: Model M Nmmentioning

confidence: 99%

mentioning

confidence: 95%

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confidence: 99%

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Topological models to infer multiphase interstellar medium properties

Lebouteiller

Ramambason

2022

A&A

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“…It also hosts HII regions of extremely low metallicity, 12 + log(O/H) = 7.17 ± 0.03 in the northwestern and 7.18 ± 0.03 in the southeastern component [174]. Showing extremely weak CO emission [175], the galaxy may still be in the process of formation [176]. IZw 18 is with D ≈ 18 Mpc (M V ≈ −15 M .…”

Section: Some Outstanding Targetsmentioning

confidence: 99%

The Interstellar Medium of Dwarf Galaxies

2022

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Dwarf galaxies are by far the most numerous galaxies in the Universe, showing properties that are quite different from those of their larger and more luminous cousins. This review focuses on the physical and chemical properties of the interstellar medium of those dwarfs that are known to host significant amounts of gas and dust. The neutral and ionized gas components and the impact of the dust will be discussed, as well as first indications for the existence of active nuclei in these sources. Cosmological implications are also addressed, considering the primordial helium abundance and the similarity of local Green Pea galaxies with young, sometimes protogalactic sources in the early Universe.

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Modeling the molecular gas content and CO-to-H₂ conversion factors in low-metallicity star-forming dwarf galaxies

Ramambason,

Lebouteiller,

Madden

et al. 2023

A&A

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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.

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Extremely weak CO emission in IZw 18

Cited by 7 publications

References 59 publications

Topological models to infer multiphase interstellar medium properties

Topological models to infer multiphase interstellar medium properties

The Interstellar Medium of Dwarf Galaxies

Modeling the molecular gas content and CO-to-H₂ conversion factors in low-metallicity star-forming dwarf galaxies

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Extremely weak CO emission in IZw 18

Cited by 7 publications

References 59 publications

Topological models to infer multiphase interstellar medium properties

Topological models to infer multiphase interstellar medium properties

The Interstellar Medium of Dwarf Galaxies

Modeling the molecular gas content and CO-to-H2 conversion factors in low-metallicity star-forming dwarf galaxies

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Modeling the molecular gas content and CO-to-H₂ conversion factors in low-metallicity star-forming dwarf galaxies