Back to the article index

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.X_CO IN THE MILKY WAY

4.1.X_CO Based on Virial Techniques

4.2.Column Density Determinations Using Dust and Optically Thin Lines

4.3.X_CO Based on Gamma-Ray Observations

4.4.Synthesis: Value and Systematic Variations of X_CO in the Milky Way

5.X_CO IN NORMAL GALAXIES

5.1.Extragalactic Virial Mass Estimates

5.2.Extragalactic Dust-Based Estimates of X_CO

5.3.Extragalactic Spectral Line Modeling

5.4.Synthesis: X_CO in Normal Galaxies

6.X_CO AT LOW METALLICITIES

6.1.Theoretical Expectations for Low Metallicity Gas

6.2.Metallicity-Dependent Calibrations of X_CO

7.X_CO IN STARBURSTS AND OTHER LUMINOUS GALAXIES

7.1.Luminous Infrared Galaxies

7.2.Ultraluminous Infrared Galaxies

7.3.Synthesis: X_CO in Starbursts

8.X_CO AT HIGH REDSHIFTS

8.1.Observed CO Line Ratios

8.2.Estimates of X_CO in High Redshift Systems

8.3.Synthesis: X_CO at High Redshift

9.CONCLUSIONS AND OPEN PROBLEMS

9.1.Toward a Single Prescription

REFERENCES