The Structure of Dark Molecular Gas in the Galaxy. II. Physical State of “CO-dark” Gas in the Perseus Arm

Busch, Michael P.; Allen, R. J.; Engelke, Philip D.; Hogg, D. E.; Neufeld, David A.; Wolfire, M. G.

doi:10.3847/1538-4357/ab3a4b

Cited by 19 publications

(25 citation statements)

References 48 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…While these average volume densities are not high enough to be observationally probed by CO-bright molecular gas manifesting at n 200 cm −3 (see e.g. Busch et al 2019), they are consistent with densities of so-called "CO-dark" gas, a phase associated with warmer, more diffuse molecular hydrogen which is not detectable in CO or HI. While invisible in both CO and HI, γ ray observations -capable of tracing a cloud's gas mass independent of its chemical state -show that diffuse molecular gas surrounds all nearby CO-detected molecular clouds targeted in this study.…”

Section: Chemical Transition Between Atomic and Molecular Gasmentioning

confidence: 62%

On the Three-Dimensional Structure of Local Molecular Clouds

Zucker,

Goodman,

Alves

et al. 2021

Preprint

View full text Add to dashboard Cite

We leverage the 1 pc spatial resolution of the 3D dust map to characterize the three-dimensional structure of nearby molecular clouds (d 400 pc). We start by "skeletonizing" the clouds in 3D volume density space to determine their "spines," which we project on the sky to constrain cloud distances with ≈ 1% uncertainty. For each cloud, we determine an average radial volume density profile around its 3D spine and fit the profiles using Gaussian and Plummer functions. The radial volume density profiles are well-described by a two-component Gaussian function, consistent with clouds having broad, lower-density outer envelopes and narrow, higher-density inner layers. The ratio of the outer to inner envelope widths is ≈ 3 : 1. We hypothesize that these two components may be tracing a transition between atomic and diffuse molecular gas or between the unstable and cold neutral medium. Plummer-like models can also provide a good fit, with molecular clouds exhibiting shallow power-law wings with density, n, falling off like n −2 at large radii. Using Bayesian model selection, we find that parameterizing the clouds' profiles using a single Gaussian is disfavored. We compare our results with 2D dust extinction maps, finding that the 3D dust recovers the total cloud mass from integrated approaches with fidelity, deviating only at higher levels of extinction (A V 2 − 3 mag). The 3D cloud structure described here will enable comparisons with synthetic clouds generated in simulations, offering unprecedented insight into the origins and fates of molecular clouds in the interstellar medium.

show abstract

Section: Chemical Transition Between Atomic and Molecular Gasmentioning

confidence: 62%

On the Three-Dimensional Structure of Local Molecular Clouds

Zucker,

Goodman,

Alves

et al. 2021

Preprint

View full text Add to dashboard Cite

show abstract

“…In addition to the inferred H2 fractions, observations have directly detected molecules in diffuse molecular clouds in absorption e.g., HCO + , HCN (Hogerheijde et al 1995a, HF, H2O (Flagey et al 2013, Sonnentrucker et al 2015, OH + , H2O + (e.g., Wyrowski et al 2010, Gerin et al 2010, ArH + (Schilke et al 2014, Jacob et al 2020, CH Sheffer et al (2008), SH Neufeld et al (2015), and in both absorption and emission e.g., OH , Busch et al 2019, with CH, HF and H2O being particularly good (linear) tracers of the H2 fraction. Although not strictly CO-dark in the original sense since the bulk of the gas may be detectable in H I, nevertheless these observations detect a portion of molecular gas that is not seen in CO emission.…”

Section: Diffuse Gasmentioning

confidence: 99%

Photodissociation and X-Ray Dominated Regions

Wolfire¹,

Vallini²,

Chevance³

2022

Preprint

Self Cite

View full text Add to dashboard Cite

The radiation from stars and active galactic nuclei (AGN) creates photodissociation regions (PDRs) and X-ray dominated regions (XDRs), where the chemistry or heating are dominated by far-ultraviolet (FUV) radiation or X-ray radiation, respectively. PDRs include a wide range of environments from the diffuse interstellar medium to dense star-forming regions. XDRs are found in the center of galaxies hosting AGN, in protostellar disks, and in the vicinity of X-ray binaries. In this review, we describe the dominant thermal, chemical, and radiation transfer processes in PDRs and XDRs, as well as a brief description of models and their use to analyze observations. We then present recent results from Milky Way, nearby extragalactic, and high-redshift observations. Several important results are:

show abstract

“…This excess molecular gas points to the inability of CO to act as a proxy for H 2 , in regions of low dust column densities (or visual extinction) where CO readily undergoes photodissociation. In recent years, through observations of their radio and FIR rotational transitions, hydrides such as CH (Gerin et al 2010a;Wiesemeyer et al 2018;Jacob et al 2019), OH (Allen et al 2015;Engelke & Allen 2018;Rugel et al 2018;Busch et al 2019Busch et al , 2021 and HF (Neufeld et al 2010b;Sonnentrucker et al 2015) have been established as important tracers of CO-dark H 2 gas in addition to the [C II] 158 µm line (Langer et al 2014) and the J = 1 − 0 rotational transition of HCO + (Lucas & Liszt 1996;Gerin et al 2010b). We note that the CH-H 2 relationship was first established by Federman (1982) and later by Sheffer et al (2008) and then Weselak (2019) using optical observations of the A 2 ∆ -X 2 Π system of CH at 4300 Å and the (2−0), (3−0), and (4−0) bands of the Lyman B-X transitions of H 2 toward stars located in the local diffuse ISM (10 19 < N (H 2 ) < 10 21 cm −2 ).…”

Section: Hygal: Key Science Goalsmentioning

confidence: 99%

HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules -- I. Survey description and a first look toward W3(OH), W3 IRS5 and NGC 7538 IRS1

Jacob,

Neufeld,

Schilke

et al. 2022

Preprint

View full text Add to dashboard Cite

The HyGAL SOFIA legacy program surveys six hydride molecules -ArH + , OH + , H 2 O + , SH, OH, and CH -and two atomic constituents -C + and O -within the diffuse interstellar medium (ISM) by means of absorption-line spectroscopy toward 25 bright Galactic background continuum sources. This detailed spectroscopic study is designed to exploit the unique value of specific hydrides as tracers and probes of different phases of the ISM, as demonstrated by recent studies with the Herschel Space Observatory. The observations performed under the HyGAL program will allow us to address several questions related to the lifecycle of molecular material in the ISM and the physical processes that impact the phase transition from atomic to molecular gas, such as: (1) What is the distribution function of the H 2 fraction in the ISM? (2) How does the ionization rate due to low-energy cosmicrays vary within the Galaxy? (3) What is the nature of interstellar turbulence (e.g., typical shear or shock velocities), and what mechanisms lead to its dissipation? In this overview, we discuss the observing strategy, the synergies with ancillary and archival observations of other small molecules, and the data reduction and analysis schemes we adopted; and we present the first results obtained toward three of the survey targets, W3(OH), W3 IRS5 and NGC 7538 IRS1. Robust measurements of the column densities of these hydrides -obtained through widespread observations of absorption lines-help address the questions raised, and there is a very timely synergy between these observations and the development of theoretical models, particularly pertaining to the formation of H 2 within the turbulent ISM. The provision of enhanced HyGAL data products will therefore serve as a legacy for future ISM studies.

show abstract

The Structure of Dark Molecular Gas in the Galaxy. II. Physical State of “CO-dark” Gas in the Perseus Arm

Cited by 19 publications

References 48 publications

On the Three-Dimensional Structure of Local Molecular Clouds

On the Three-Dimensional Structure of Local Molecular Clouds

Photodissociation and X-Ray Dominated Regions

HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules -- I. Survey description and a first look toward W3(OH), W3 IRS5 and NGC 7538 IRS1

Contact Info

Product

Resources

About