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Article Contents

ABSTRACT

1.A VERY BRIEF HISTORY

2.GALAXIES IN GASEOUS HALOS

2.1.The Major Problems of Galaxy Evolution

2.1.1.How do galaxies sustain their star formation?

2.1.2.What quenches galaxies and what keeps them that way?

2.1.3.Why do galaxies lack their fair share of baryons?

2.1.4.Where are the metals?

2.2.Our Point of View

3.HOW WE STUDY THE CGM

3.1.Transverse Absorption-Line Studies

3.2.Stacking Analyses

3.3.Down the Barrel

3.4.Emission-line maps

3.5.Hydrodynamic Simulations

4.THE PHYSICAL STATE OF THE CGM

4.1.The Complex, Multiphase CGM

4.2.From Basic Observables to Physical Properties

4.3.Line Profiles and Gas Kinematics

4.4.Challenges in Characterizing the Multiphase CGM

4.5.Gastrophysical Models

5.THE BARYONIC MASS DISTRIBUTION OF THE CGM

5.1.The Missing Baryons Budget

5.2.CGM Masses by Phase

5.2.1.Cold Gas, T < 10⁴ K

5.2.2.UV Absorption Lines and the Cool 10⁴⁻⁵ K CGM

5.2.3.UV Absorption Lines and the Warm 10⁵⁻⁶ K CGM

5.2.4.The Hot T > 10⁶ K Phase

5.2.5.Theoretical Considerations

6.METALS: NATURE'S TRACER PARTICLES

6.1.The Metals Census

6.2.Metals Observed as Gas

6.3.Metals Observed as Dust

7.INFLOWS, OUTFLOWS, AND RECYCLING

7.1.The Problems: Galaxy Fueling and "Missing" Metals

7.2.Empirical Signs of Fueling and Inflows

7.3.The Preeminence of Outflows

7.4.Following the Metals: The Role of Recycling

8.THE PARADOX OF QUENCHING

8.1.The Fate of Cold Accretion and The Problem with Recycling

8.2.The CGM of AGN and Quasars

9.OPEN PROBLEMS, FUTURE PROSPECTS, AND FINAL THOUGHTS

9.1.Progress and Problems

9.1.1.The Scale Problem

9.1.2.Mass Flows and the Fate Problem

9.2.Future Prospects for Data

9.3.Final Thoughts

REFERENCES