Fermi-LAT γ-Ray Study of the Interstellar Medium and Cosmic Rays in the Chamaeleon Molecular Cloud Complex: A Look at the Dark Gas as Optically Thick H i

Hayashi, Katsuhiro; Mizuno, T.; Fukui, Y.; Okamoto, Ryuji; Yamamoto, Hiroaki; Hidaka, N.; Okumura, A.; Tajima, H.; Sano, Hidetoshi

doi:10.3847/1538-4357/ab4351

Cited by 18 publications

(11 citation statements)

References 55 publications

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“…As γ rays interact with hydrogen nuclei, they can trace the total interstellar gas. Comparison of γ ray observations in our Galaxy with CO and H 0 has uncovered an important reservoir of neutral dark gas that is not traced by CO or H i (Grenier et al 2005;Ackermann et al 2012;Hayashi et al 2019). While γ ray emission, in principle, can be one of the most accurate methods possible to get at the total gas mass, its use to measure the total interstellar gas mass in other galaxies is limited for now because of the need for relatively high-resolution observations.…”

Section: Co Dark Gas Studiesmentioning

confidence: 78%

Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas

Madden

Cormier

Hony

et al. 2020

A&A

119

128

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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 H 2 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 H 2 mass (M H 2) 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], L TIR , and the total M H 2. We provide recipes based on these models to derive total M H 2 mass estimates from observations. We apply the models to the Herschel Dwarf Galaxy Survey, extracting the total M H 2 for each galaxy, and compare this to the H 2 determined from the observed CO (1−0) line. This allows us to quantify the reservoir of H 2 that is CO-dark and traced by the [C ii]λ158 µm. Results. We demonstrate that while the H 2 traced by CO (1−0) can be negligible, the [C ii]λ158 µm can trace the total H 2. We find 70 to 100% of the total H 2 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] /L CO(1−0) ratio. A conversion factor for [C ii]λ158 µm to total H 2 and a new CO-to-total-M H 2 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 H 2 in galaxies, taking into account the CO and [C ii] observations. Accounting for this CO-dark H 2 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 H 2 compared to the small amount of H 2 in the CO-emitting region.

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Section: Co Dark Gas Studiesmentioning

confidence: 78%

Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas

Madden

Cormier

Hony

et al. 2020

A&A

119

128

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“…The detection of CO-dark gas in our Galaxy by comparing γray observations (as a tracer of the total hydrogen gas, molecular and atomic) to CO and H I observations (Grenier et al 2005;Ackermann et al 2012;Hayashi et al 2019) demonstrated that this is not only relevant for the lowmetallicity interstellar medium. The presence of CO-dark gas in our Galaxy was later confirmed, e.g., from the Herschel GOT-C + observations (Langer et al 2010, 2014; Pineda et al…”

Section: The Spatially Resolved α Co Conversion Factor and Codark Molmentioning

confidence: 81%

SOFIA/FIFI-LS Full-disk [C ii] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946

Bigiel

Krabbe

Cormier

et al. 2020

ApJ

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We present SOFIA/FIFI-LS observations of the [C II] 158 μm cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO, and H I data to compare [C II] emission to dust properties, star formation rate (SFR), H 2 , and H I at 560 pc scales via stacking by environment (spiral arms, interarm, and center), radial profiles, and individual, beam-sized measurements. We attribute 73% of the [C II] luminosity to arms, and 19% and 8% to the center and interarm region, respectively. [C II]/TIR, [C II]/CO, and [C II]/PAH radial profiles are largely constant, but rise at large radii ( 8 kpc) and drop in the center ("[C II] deficit"). This increase at large radii and the observed decline with the 70 μm/100 μm dust color are likely driven by radiation field hardness. We find a near proportional [C II]-SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. [C II] also becomes increasingly luminous relative to CO at low SFR (interarm or large radii), likely indicating more efficient photodissociation of CO and emphasizing the importance of [C II] as an H 2 and SFR tracer in such regimes. Finally, based on the observed [C II] and CO radial profiles and different models, we find α CO to increase with radius, in line with the observed metallicity gradient. The low α CO (galaxy average  2 M e pc −2 (K km s −1) −1) and low [C II]/CO ratios (∼400 on average) imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way.

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“…where X d is the dust-to-gas ratio, d 0 is a global offset, and α accounts for possible non-linearity in the conversion from gas to dust. This form has been used before in analysis of γ-ray data where the best fit value was α ∼1.4 (Hayashi et al 2019). For this analysis, the value of α is chosen to be one of 1.0, 1.2, and 1.4 and the values of X d and d 0 are determined using a maximum likelihood fit.…”

Section: Modelmentioning

confidence: 99%

Signatures of Recent Cosmic-Ray Acceleration in the High-Latitude $γ$-Ray Sky

Jóhannesson,

Porter

2021

Preprint

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Cosmic-ray (CR) sources temporarily enhance the relativistic particle density in their vicinity over the background distribution accumulated from the Galaxy-wide past injection activity and propagation. If individual sources are close enough to the solar system, their localised enhancements may present as features in the measured spectra of the CRs and in the associated secondary electromagnetic emissions. Large scale loop like structures visible in the radio sky are possible signatures of such nearby CR sources. If so, these loops may also have counterparts in the high-latitude γ-ray sky. Using ∼10 years of data from the Fermi Large Area Telescope, applying Bayesian analysis including Gaussian Processes, we search for extended enhanced emission associated with putative nearby CR sources in the energy range from 1 GeV to 1 TeV for the sky region |b| > 30 • . We carefully control the systematic uncertainty due to imperfect knowledge of the interstellar gas distribution. Radio Loop IV is identified for the first time as a γ-ray emitter and we also find significant emission from Loop I. Strong evidence is found for asymmetric features about the Galactic l = 0 • meridian that may be associated with parts of the so-called "Fermi Bubbles", and some evidence is also found for γ-ray emission from other radio loops. Implications for the CRs producing the features and possible locations of the sources of the emissions are discussed.

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Fermi-LAT γ-Ray Study of the Interstellar Medium and Cosmic Rays in the Chamaeleon Molecular Cloud Complex: A Look at the Dark Gas as Optically Thick H i

Cited by 18 publications

References 55 publications

Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas

Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas

SOFIA/FIFI-LS Full-disk [C ii] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946

Signatures of Recent Cosmic-Ray Acceleration in the High-Latitude $γ$-Ray Sky

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