The CO-H2 Conversion Factor in Disc Galaxies and Mergers

By D. Narayanan, Mark R. Krumholz, E. Ostriker, L. Hernquist. 2011

134 citations recorded by the source corpus.

Relating the observed CO emission from giant molecular clouds (GMCs) to the underlying H2 column density is a long-standing problem in astrophysics. While the Galactic CO–H2 conversion factor (XCO) appears to be reasonably constant, observations indicate that XCO may be depressed in high surface density starburst environments. Using a multiscale approach, we investigate the dependence of XCO on the galactic environment in numerical simulations of disc galaxies and galaxy mergers. XCO is proportional to the GMC surface density divided by the integrated CO intensity, WCO, and WCO is related to the kinetic temperature and velocity dispersion in the cloud. In disc galaxies (except within the central ∼ kpc), the galactic environment is largely unimportant in setting the physical properties of GMCs provided they are gravitationally bound. The temperatures are roughly constant at ∼10 K due to the balance of CO cooling and cosmic ray heating, giving a nearly constant CO–H2 conversion factor in discs. In mergers, the velocity dispersion of the gas rises dramatically during coalescence. The gas temperature also rises as it couples well to the warm (∼50 K) dust at high densities (n > 104 cm−3). The rise in velocity dispersion and temperature combine to offset the rise in surface density in mergers, causing XCO to drop by a factor of ∼2–10 compared to the disc simulation. This model predicts that high-resolution Atacama Large Millimeter/submillimeter Array observations of nearby ultraluminous infrared galaxies should show velocity dispersions of 101–102 km s−1, and brightness temperatures comparable to the dust temperatures.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/9ac235c75ff36a1d3f5a72a082fa60dea1198209

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In Astronomy and Astrophysics

1. P. Hopkins
2. L. Hernquist
3. D. Narayanan
4. F. Walter
5. Mark R. Krumholz
6. R. Somerville
7. R. Dav'e
8. A. Dekel

Citations in the corpus

• 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…”
• THE MOLECULAR GAS IN LUMINOUS INFRARED GALAXIES. II. EXTREME PHYSICAL CONDITIONS AND THEIR EFFECTS ON THE Xco FACTOR (2012, methodology): “Galaxy-sized numerical studies of Xco in disks and mergers recently presented by Narayanan et al. 2011 address the aforementioned important point of coupled GMC-galaxy evolution by tracking the evolution of GMC boundary conditions, but do so by necessarily adopting – 15 – some subresolution methods…”
• Star formation in galaxy mergers with realistic models of stellar feedback and the interstellar medium (2012, result): “What we see is a continuum between these systems, which is completely consistent with the observations (see e.g. Narayanan et al. 2011).”
• 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): “Our subsequent fit to the data showed that – for reasonable assumptions about the relative spatial distribution of optical and CO emission, see Figure 4(a) – the Narayanan et al. (2011) prescription leads to a SF law that is a bit shallower than our preferred integrated S-K relation (Mmol.”
• THE IMPACT OF INTERACTIONS, BARS, BULGES, AND ACTIVE GALACTIC NUCLEI ON STAR FORMATION EFFICIENCY IN LOCAL MASSIVE GALAXIES (2012, background): “However, the high densities found in mergers may lead to both elevated dust temperatures and lower values of αCO (Narayanan et al. 2011).”
• A survey of molecular gas in luminous sub-millimetre galaxies (2012, background): “Prior to the start of the survey, only two SMGs had been detected in 12CO, SMM J02399−0136 (z = 2.81; Frayer et al. 1998; Ivison et al. 1998; Genzel et al. 2003) and SMM J14011+0252 (z = 2.56; Frayer et al. 1999; Ivison et al. 2001).”
• A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, methodology): “Beyond this, in Narayanan et al. (2011b), we argued that a Schmidt index of unity is unlikely to describe the starburst environments modelled here as this choice of power-law index does not allow mergers to undergo a starburst.”
• The CO-to-H2 Conversion Factor (2013, methodology): “Global models ofXCO using hydrodynamic simulations to investigate the variation due to galactic environment are carried out by Feldmann, Gnedin & Kravtsov (2012) and Narayanan et al. (2011, 2012).”
• 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): “From conversion ‘factor’ to conversion ‘function’: From theoretical considerations the CO conversion factor in equation (4) is expected to be a function of several physical parameters (Narayanan et al. 2011, 2012, Feldmann et al. 2012 a,b).”
• A fundamental relation between the metallicity, gas content, and stellar mass of local galaxies (2013, background): “…been several attempts to model and to observationally constrain the relation between the CO/H2 conversion factor and gas-phase metallicity (e.g. Narayanan et al. 2011; Genzel et al. 2012; Bolatto et al. 2013), such a dependence is so strong that uncertainties in the metallicity and/or…”
• LOW CO LUMINOSITIES IN DWARF GALAXIES (2012, background): “…al. 2010; Glover & Mac Low 2011). αCO is therefore assumed to be a strong function of metallicity and radiation field strength, although robust functional parametrizations of these dependences are still lacking (but see Shetty et al. 2011a,b and Narayanan et al. 2011 for recent theoretical works).”
• Dusty Star Forming Galaxies at High Redshift (2014, background): “Narayanan et al. (2011b) and Narayanan et al. (2012a) coupled 3D non-local thermodynamic equilibrium (LTE) radiative transfer calculations and dust radiative transfer simulations with smoothed particle hydrodynamic (SPH) models of disk galaxies and galaxy mergers to derive a functional form for the…”
• THE EVOLVING INTERSTELLAR MEDIUM OF STAR-FORMING GALAXIES SINCE z = 2 AS PROBED BY THEIR INFRARED SPECTRAL ENERGY DISTRIBUTIONS (2012, methodology): “According to Narayanan et al. (2011) : αCO = 6.75× 1020〈WCO〉 −0.32 6.3× 1019 × (Z/Z⊙)0.65 (10) where 〈WCO〉, is the CO surface brightness of the galaxy in K Km s−1.”