Book contents
- Frontmatter
- Contents
- Preface
- Conference participants
- Conference photograph / poster
- 1 Physics of H2 and HD
- 2 Formation - Destruction
- 3 Observations and Models
- 4 Extragalactic and Cosmology
- The Role of H2 Molecules in Cosmological Structure Formation
- The Role of H2 Molecules in Primordial Star Formation
- Evolution of Primordial H2 for Different Cosmological Models
- Dynamics of H2 Cool Fronts in the Primordial Gas
- Is Reionization Regulated by H2 in the Early Universe?
- H2 in Galaxies
- Transformation of Galaxies within the Hubble Sequence
- Extragalactic H2 and its Variable Relation to CO
- The Galactic Dark Matter Halo: Is it H2?
- Observations of H2 in Quasar Absorbers
- H2 Emission as a Diagnostic of Physical Processes in Starforming Galaxies
- 5 Outlook
- Author index
Extragalactic H2 and its Variable Relation to CO
from 4 - Extragalactic and Cosmology
Published online by Cambridge University Press: 04 August 2010
- Frontmatter
- Contents
- Preface
- Conference participants
- Conference photograph / poster
- 1 Physics of H2 and HD
- 2 Formation - Destruction
- 3 Observations and Models
- 4 Extragalactic and Cosmology
- The Role of H2 Molecules in Cosmological Structure Formation
- The Role of H2 Molecules in Primordial Star Formation
- Evolution of Primordial H2 for Different Cosmological Models
- Dynamics of H2 Cool Fronts in the Primordial Gas
- Is Reionization Regulated by H2 in the Early Universe?
- H2 in Galaxies
- Transformation of Galaxies within the Hubble Sequence
- Extragalactic H2 and its Variable Relation to CO
- The Galactic Dark Matter Halo: Is it H2?
- Observations of H2 in Quasar Absorbers
- H2 Emission as a Diagnostic of Physical Processes in Starforming Galaxies
- 5 Outlook
- Author index
Summary
We derive and discuss the strong dependence on metallicity of the CO to H2 conversion factor X = N(H2)/Ico = 12.2 – 2.5log[O]/[H] appropriate to extragalactic objects, as well as the weaker dependence found for such objects from interferometer measurements.
Introduction
The difficulty of directly observing molecular hydrogen (H2), the major constituent of the interstellar medium in galaxies, and ways of doing so indirectly are reviewed elsewhere in this volume (Combes 2000). Usually, H2 cloud properties are derived by extrapolation from more easily conducted CO observations. For instance, observed CO cloud sizes and velocity widths yield total molecular gas masses under the assumption of virial equilibrium. However, in extragalactic systems especially, this method is beset by pitfalls (see Israel, 1997, hereafter Is97) and requires high linear resolutions (i.e. use of interferometer arrays). More seriously, the fundamental assumption of virialization appears to be false. As individual components (‘clumps’) have velocities of only a few km s−1 and CO complex sizes are 50–100 pc, crossing times are comparable to CO complex lifetimes of only a few times 107 years or less (Leisawitz et al. 1989; Fukui et al. 2000; see also Elmegreen 2000). As equilibrium cannot be reached in a single crossing time or less, the virial theorem is not applicable to such complexes. Indeed, the elongated and interconnected filamentary appearance of many large CO cloud complexes do not suggest virialized systems (see also Maloney 1990).
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- Information
- Molecular Hydrogen in Space , pp. 293 - 296Publisher: Cambridge University PressPrint publication year: 2000
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