Modelling CO emission – I. CO as a column density tracer and the X factor in molecular clouds

By R. Shetty, S. Glover, C. Dullemond, Ralf Klessen. 2010

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Theoretical and observational investigations have indicated that the abundance of carbon monoxide (CO) is very sensitive to intrinsic properties of the gaseous medium, such as density, metallicity and the background radiation field. CO observations are often employed to study the properties of molecular clouds (MCs), such as mass, morphology and kinematics. It is thus important to understand how well CO traces the total mass, which in MCs is predominantly due to molecular hydrogen (H 2 ). Recent hydrodynamic simulations by Glover & Mac Low have explicitly followed the formation and destruction of molecules in model MCs under varying conditions. These models have confirmed that CO formation strongly depends on the cloud properties. Conversely, the formation of H 2 is primarily determined by the amount of time available for its formation. We apply radiative transfer calculations to these MC models in order to investigate the properties of CO line emission. We focus on integrated CO (J = 1-0) intensities emerging from individual clouds, including its relationship to the total, H 2 and CO column densities, as well as the 'X factor,' the ratio of H 2 column density to CO intensity. Models with high CO abundances have a threshold CO intensity of ≈65 K km s ―1 at sufficiently large extinctions (or column densities). Clouds with low CO abundances show no such intensity thresholds. The distributions of total and H 2 column densities are well described as lognormal functions, though the distributions of CO intensities and column densities are usually not lognormal. In general, the probability distribution functions of the integrated intensity do not follow the distribution functions of CO column densities. In the model with Milky Way-like conditions, the X factor is in agreement with the near-constant value determined from observations. In clouds with lower metallicity, lower density or a higher background UV radiation field, the CO abundances are in general lower, and hence the X factor can vary appreciably - sometimes by up to 4 orders of magnitude. In models with high densities, the CO line is fully saturated, so that the X factor is directly proportional to the molecular column density.

Subfields: Applied Mathematics, Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/8970d138205f61ff50630fb0aa06889657f5f308

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Citations in the corpus

Molecular hydrogen abundances of galaxies in the EAGLE simulations (2015, background): “…of gas metallicity and the ISRF, such that X increases in low metallicity and/or intense ISRF environments (e.g. Bell, Viti & Williams 2007; Meijerink, Spaans & Israel 2007; Bayet et al. 2009; Shetty et al. 2011; Narayanan et al. 2012; Feldmann et al., 2012; Bisbas, Papadopoulos & Viti 2015).”
THE STAR FORMATION RATE OF TURBULENT MAGNETIZED CLOUDS: COMPARING THEORY, SIMULATIONS, AND OBSERVATIONS (2012, methodology): “…when a detailed, fully coupled, chemical, and radiative cooling and heating model is used instead of a polytropic EOS, the PDF of the main molecular gas component, H2, follows a lognormal distribution very well (Glover & Mac Low 2007a; Glover et al. 2010; Shetty et al. 2011; Micic et al. 2012).”
The CO-to-H2 Conversion Factor (2013, methodology): “Shetty et al. (2011b) investigate the results of varying the temperature, CO abundance, and turbulent line width.”
Cold gas properties of the Herschel Reference Survey - II. Molecular and total gas scaling relations (2014, background): “…field, is still far from being settled both observationally and theoretically (Boselli et al. 200, Bell et al. 2007, Bolatto et al. 2008, Liszt et al. 2010, Leroy et al. 2011, Shetty et al. 2011a,b, Narayanan et al. 2012, Kennicutt & Evans 2012, Bolatto et al. 2013, Sandstrom et al. 2013).”
Star Formation in the Milky Way and Nearby Galaxies (2012, background): “…the picture at this point is that measuring the extent of the brick wall will not tell you anything about particularly deep or dense extensions of the building, but if the buildings are similar, the extent of the wall is a rough guide to the total mass of the building (cf. Shetty et al. 2011b).”
Turbulent molecular clouds (2012, background): “…CO formation in turbulent simulations have been compared by Glover & Clark (2012): they find that, not unexpectedly, the gas dynamics is not sensitive to the details of the chemistry models, but all the adopted models so far fail to reproduce the CO abundances in such regions (Shetty et al. 2011).”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “Both models (Wolfire et al. 2010; Glover & Mac Low 2011; Krumholz et al. 2011b; Narayanan et al. 2011, 2012; Shetty et al. 2011a,b; Feldmann et al. 2012a,b) and observations (Bolatto et al. 2011; Leroy et al. 2011; Genzel et al. 2012; Leroy et al. 2013; Bolatto et al. 2013) suggest that, at low…”
A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, methodology): “As shown by Shetty et al. (2011a), GMCs with physical parameters comparable to those observed in Galactic GMCs exhibit XCO values close to the Galactic mean, XCO ≈ 2–4 × 1020cm−2/K-kms−1 [αCO ≈ 3– 6 M pc−2(K − kms−1)−1].”
LOW CO LUMINOSITIES IN DWARF GALAXIES (2012, background): “This bases on the idea that with decreasing metallicity the CO-bright cores shrink to (much) smaller size than the surrounding H2 clouds (e.g., Bolatto et al. 1999; Wolfire et al. 2010; Shetty et al. 2011a,b).”
Dusty Star Forming Galaxies at High Redshift (2014, background): “At low metallicities, the relative size of the CO emitting region in a molecular cloud shrinks, thereby driving XCO up (Maloney andWolfire, 1997; Wolfire et al., 2010; Feldmann et al., 2012a; Shetty et al., 2011b; Krumholz et al., 2011; Lagos et al., 2012; Narayanan et al., 2012b).”
21 cm cosmology in the 21st century (2011, background): “CO emission in local galaxies is known to originate in giant molecular clouds (GMCs) whose typical size is ∼10 pc.”
VALIDATION OF THE EQUILIBRIUM MODEL FOR GALAXY EVOLUTION TO z ∼ 3 THROUGH MOLECULAR GAS AND DUST OBSERVATIONS OF LENSED STAR-FORMING GALAXIES (2013, background): “…observations and models suggest that the value of αCO increases, as the CO molecule becomes a poor proxy for H2 (e.g. Israel 1997; Dame et al. 2001; Rosolowsky et al. 2003; Blitz et al. 2007; Leroy et al. 2011; Glover & Mac Low 2011; Shetty et al. 2011; Feldmann et al. 2012; Genzel et al. 2012).”
THE MOLECULAR GAS IN LUMINOUS INFRARED GALAXIES. II. EXTREME PHYSICAL CONDITIONS AND THEIR EFFECTS ON THE Xco FACTOR (2012, methodology): “…simulations and the Xco factor: important caveats, and ways forward Recently GMC-sized numerical simulations that include H2 and CO formation/destruction and radiative transfer in MHD turbulent models where used to explore Xco and its dependance on average ISM conditions (Shetty et al. 2011a,b).”
The CO-H2 Conversion Factor in Disc Galaxies and Mergers (2011, background): “In the context of the models presentedhere, such a trend is plausible (see also Shetty et al. 2011b).”