ARlogo Annu. Rev. Astron. Astrophys. 2013. 51: 207-268
Copyright © 2013 by Annual Reviews. All rights reserved

Reprinted with kind permission from Annual Reviews, 4139 El Camino Way, Palo Alto, California, USA

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THE CO-TO-H2 CONVERSION FACTOR

Alberto D. Bolatto 1, Mark Wolfire 1 and Adam K. Leroy 2


1 Department of Astronomy, University of Maryland, College Park, MD 20742
2 National Radio Astronomy Observatory, Charlottesville, VA 22903


Abstract: CO line emission represents the most accessible and widely used tracer of the molecular interstellar medium. This renders the translation of observed CO intensity into total H2 gas mass critical to understand star formation and the interstellar medium in our Galaxy and beyond. We review the theoretical underpinning, techniques, and results of efforts to estimate this CO-to-H2 "conversion factor," XCO, in different environments. In the Milky Way disk, we recommend a conversion factor XCO = 2 × 1020 cm-2(K km s-1)-1 with ± 30% uncertainty. Studies of other "normal galaxies" return similar values in Milky Way-like disks, but with greater scatter and systematic uncertainty. Departures from this Galactic conversion factor are both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that XCO increases with decreasing metallicity, turning up sharply below metallicity approx 1/3 - 1/2 solar in a manner consistent with model predictions that identify shielding as a key parameter. Based on spectral line modeling and dust observations, XCO appears to drop in the central, bright regions of some but not all galaxies, often coincident with regions of bright CO emission and high stellar surface density. This lower XCO is also present in the overwhelmingly molecular interstellar medium of starburst galaxies, where several lines of evidence point to a lower CO-to-H2 conversion factor. At high redshift, direct evidence regarding the conversion factor remains scarce; we review what is known based on dynamical modeling and other arguments.


Keywords: ISM: general — ISM: molecules — galaxies: ISM — radio lines: ISM


Table of Contents

INTRODUCTION
Brief Historical Perspective
CO Excitation

THEORETICAL BASIS
Giant Molecular Clouds
Galaxies
Other Sources of Velocity Dispersion
Optically Thin Limit
Insights from Cloud Models

A MODERN THEORETICAL PERSPECTIVE
Photodissociation Regions
Numerical Simulations

XCO IN THE MILKY WAY
XCO Based on Virial Techniques
Column Density Determinations Using Dust and Optically Thin Lines
XCO Based on Gamma-Ray Observations
Synthesis: Value and Systematic Variations of XCO in the Milky Way

XCO IN NORMAL GALAXIES
Extragalactic Virial Mass Estimates
Extragalactic Dust-Based Estimates of XCO
Extragalactic Spectral Line Modeling
Synthesis: XCO in Normal Galaxies

XCO AT LOW METALLICITIES
Theoretical Expectations for Low Metallicity Gas
Metallicity-Dependent Calibrations of XCO

XCO IN STARBURSTS AND OTHER LUMINOUS GALAXIES
Luminous Infrared Galaxies
Ultraluminous Infrared Galaxies
Synthesis: XCO in Starbursts

XCO AT HIGH REDSHIFTS
Observed CO Line Ratios
Estimates of XCO in High Redshift Systems
Synthesis: XCO at High Redshift

CONCLUSIONS AND OPEN PROBLEMS
Toward a Single Prescription

REFERENCES

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