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Article Contents

ABSTRACT

1.INTRODUCTION
1.1.Brief Historical Perspective
1.2.CO Excitation

2.THEORETICAL BASIS
2.1.Giant Molecular Clouds
2.2.Galaxies
2.3.Other Sources of Velocity Dispersion
2.4.Optically Thin Limit
2.5.Insights from Cloud Models

3.A MODERN THEORETICAL PERSPECTIVE
3.1.Photodissociation Regions
3.2.Numerical Simulations

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

5.XCO IN NORMAL GALAXIES
5.1.Extragalactic Virial Mass Estimates
5.2.Extragalactic Dust-Based Estimates of XCO
5.3.Extragalactic Spectral Line Modeling
5.4.Synthesis: XCO in Normal Galaxies

6.XCO AT LOW METALLICITIES
6.1.Theoretical Expectations for Low Metallicity Gas
6.2.Metallicity-Dependent Calibrations of XCO

7.XCO IN STARBURSTS AND OTHER LUMINOUS GALAXIES
7.1.Luminous Infrared Galaxies
7.2.Ultraluminous Infrared Galaxies
7.3.Synthesis: XCO in Starbursts

8.XCO AT HIGH REDSHIFTS
8.1.Observed CO Line Ratios
8.2.Estimates of XCO in High Redshift Systems
8.3.Synthesis: XCO at High Redshift

9.CONCLUSIONS AND OPEN PROBLEMS
9.1.Toward a Single Prescription

REFERENCES