THE DARK MOLECULAR GAS

By M. Wolfire, D. Hollenbach, C. McKee. 2010

455 citations recorded by the source corpus.

The mass of molecular gas in an interstellar cloud is often measured using line emission from low rotational levels of CO, which are sensitive to the CO mass, and then scaling to the assumed molecular hydrogen H2 mass. However, a significant H2 mass may lie outside the CO region, in the outer regions of the molecular cloud where the gas-phase carbon resides in C or C+. Here, H2 self-shields or is shielded by dust from UV photodissociation, whereas CO is photodissociated. This H2 gas is “dark” in molecular transitions because of the absence of CO and other trace molecules, and because H2 emits so weakly at temperatures 10 K <T < 100 K typical of this molecular component. This component has been indirectly observed through other tracers of mass such as gamma rays produced in cosmic-ray collisions with the gas and far-infrared/submillimeter wavelength dust continuum radiation. In this paper, we theoretically model this dark mass and find that the fraction of the molecular mass in this dark component is remarkably constant (∼0.3 for average visual extinction through the cloud ) and insensitive to the incident ultraviolet radiation field strength, the internal density distribution, and the mass of the molecular cloud as long as , or equivalently, the product of the average hydrogen nucleus column and the metallicity through the cloud, is constant. We also find that the dark mass fraction increases with decreasing , since relatively more molecular H2 material lies outside the CO region in this case.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/28aa46058e262d14f9d0cddcaa86ff4ced851035

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Citations in the corpus

Turbulent molecular clouds (2012, background): “This CO–quiet H2 gas lies at the periphery of known bright molecular complexes, a fact consistent with model predictions (Wolfire et al. 2010).”
Feedback-regulated star formation in molecular clouds and galactic discs (2013, background): “…Small Magellanic Cloud (Blitz et al. 2007) and for high-redshift galaxies below a critical stellar mass (Genzel et al. 2012), probably because CO molecules are photo-dissociated by the UV radiation field as the attenuation by dust decreases (e.g., van Dishoeck & Black 1986; Wolfire et al. 2010).”
A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES (2011, background): “Low mass systems often have lower metallicities and correspondingly less dust, which is required to shield CO (e.g., Maloney & Black 1988; Bolatto et al. 1999; Glover & Mac Low 2010; Wolfire et al. 2010).”
Star Formation in the Milky Way and Nearby Galaxies (2012, background): “The dark gas appears around AV = 0.4 mag, and is presumably the CO-poor, but H2-containing, outer parts of clouds (e.g., Wolfire et al. 2010), which can be seen in [CII] emission at 158 µm and in fluorescent H2 emission at 2 µm (e.g., Pak et al. 1998).”
THE CO-TO-H2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES (2012, methodology): “The fraction of gas mass in the layer where CO is photodissociated is predicted to be ∼ 30% at Milky Way metallicity/DGR (Wolfire et al. 2010).”
Cold gas properties of the Herschel Reference Survey - II. Molecular and total gas scaling relations (2014, methodology): “…% M(gas) = 1.3[M(Hi) + M(H2)] [M ⊙] (2) The constant conversion factor used to infer the total molecular gas mass of the HRS galaxies has been determined using gamma-ray data observed byCOS-B (Strong et al. 1988), and thus includes the contribution of the dark molecular gas (Wolfire et al. 2010).”
OBSERVATIONS OF Arp 220 USING HERSCHEL-SPIRE: AN UNPRECEDENTED VIEW OF THE MOLECULAR GAS IN AN EXTREME STAR FORMATION ENVIRONMENT (2011, background): “…a large range in density ( 101 − 107 cm−3) and incident FUV flux, Log G0 (-0.5 – 6.5), where G0 is the FUV flux in units of the local interstellar value also sometimes referred to as the Habing interstellar radiation field i.e., FUV flux = 1.3 × 10−4 × G0 erg cm −2 s−1 sr−1 (Wolfire et al. 2010).”
The Herschel Dwarf Galaxy Survey - I. Properties of the low-metallicity ISM from PACS spectroscopy (2015, background): “This H2 gas not traced by CO is known as the CO-dark gas (e.g., Wolfire et al. 2010).”
REGULARITY UNDERLYING COMPLEXITY: A REDSHIFT-INDEPENDENT DESCRIPTION OF THE CONTINUOUS VARIATION OF GALAXY-SCALE MOLECULAR GAS PROPERTIES IN THE MASS-STAR FORMATION RATE PLANE (2013, unknown): “For this reason, we assign αCO values to those 122 galaxies lacking an observational constraint with the following prescription from Bolatto et al. (2013), which is based on modeling in Wolfire et al. (2010): αCO(Z ′) αCO(Z′=1) = exp ( +4.0∆AV Z′ AV,MW ) exp ( −4.0∆AV AV,MW ) .”
DEVELOPMENT OF THE MODEL OF GALACTIC INTERSTELLAR EMISSION FOR STANDARD POINT-SOURCE ANALYSIS OF FERMI LARGE AREA TELESCOPE DATA (2016, background): “In addition to metallicity variations, the CO molecule is strongly affected by UV photo-dissociation in the outer regions of molecular clouds where H2 can exist without CO or where CO is only weakly excited (Wolfire et al. 2010; Glover & Low 2011).”
Very extended cold gas, star formation and outflows in the halo of a bright QSO at z>6 (2014, background): “…it can also be an important component of some unshielded dense molecular clouds with large envelopes where the formation of CO is inhibited and where most of the gas-phase carbon is found in C0 or C+ atoms (Wolfire et al. 2010; Langer et al. 2010, 2014; Velusamy et al. 2010; Pineda et al. 2013).”
Terahertz Spectroscopy for Astronomy: From Comets to Cosmology (2011, result): “of diffuse clouds in a sample characterized by C and HI emission but without CO demonstrates the presence of a dignificant diffuse warm, dark H component [39]–[41] that had been suggested by previous work [42], [43].”
STARS, GAS, AND DARK MATTER IN THE SOLAR NEIGHBORHOOD (2015, background): “…show is not associated with either CO emission or 21 cm emission (Grenier et al. 2005), consists primarily of opaque HI; by contrast, other observations (Lee et al. 2015 and references therein) and theory (Wolfire et al. 2010) are consistent with this gas consisting primarily of H2 and atomic C.”
REGULATION OF STAR FORMATION RATES IN MULTIPHASE GALACTIC DISKS: A THERMAL/DYNAMICAL EQUILIBRIUM MODEL (2010, background): “CO is present only in the portions of clouds where AV >∼ 1 (Wolfire et al. 2010; Glover & Mac Low 2010), so that it can become a poor tracer of molecular mass in low-metallicity systems.”
A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, background): “In these cases, we expect a larger fraction of CO-dark clouds and a rise in the CO–H2 conversion factor (Maloney & Black 1988; Wolfire et al. 2010; Shetty et al. 2011b; Feldmann et al. 2011).”
COMBINED CO AND DUST SCALING RELATIONS OF DEPLETION TIME AND MOLECULAR GAS FRACTIONS WITH COSMIC TIME, SPECIFIC STAR-FORMATION RATE, AND STELLAR MASS (2014, background): “Motivated by the theoretical work of Wolfire et al. (2010) on the photo-dissociation of clouds with a range of hydrogen densities and UV radiation field intensities, but with a constant hydrogen column, Bolatto et al. (2013) have proposed the following fitting function for χ(Z), (12 log( / ) 8.67)…”
THE STATE OF THE GAS AND THE RELATION BETWEEN GAS AND STAR FORMATION AT LOW METALLICITY: THE SMALL MAGELLANIC CLOUD (2011, background): “Such envelopes are expected at all metallicities (Glover & Mac Low 2010), and at the low metallicity of the SMC they likely constitute the dominant reservoir of molecular gas (Wolfire et al. 2010).”