Invited review, Book chapter in "Outskirts of Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and Space Science Library, Springer, in press.
https://arxiv.org/abs/1612.05275v1

For a PDF version of the article, click here.

MOLECULAR GAS IN THE OUTSKIRTS OF GALAXIES

Linda C. Watson 1 and Jin Koda 2


1 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
2 NAOJ Chile Observatory, Joaquín Montero 3000 Oficina 702, Vitacura, Santiago, Chile;
Joint ALMA Office, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile;
Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794, USA


Abstract: The outskirts of galaxies offer extreme environments where we can test our understanding of the formation, evolution, and destruction of molecules and their relationship with star formation and galaxy evolution. We review the basic equations that are used in normal environments to estimate physical parameters like the molecular gas mass from CO line emission and dust continuum emission. Then we discuss how those estimates may be affected when applied to the outskirts, where the average gas density, metallicity, stellar radiation field, and temperature may be lower. We focus on observations of molecular gas in the outskirts of the Milky Way, extragalactic disk galaxies, early-type galaxies, groups, and clusters. The scientific results show the versatility of molecular gas, as it has been used to trace Milky Way spiral arms out to a galactocentric radius of 15 kpc, to study star formation in extended ultraviolet disk galaxies, to probe galaxy interactions in polar ring S0 galaxies, and to investigate ram pressure stripping in clusters. Throughout the Chapter, we highlight the physical stimuli that accelerate the formation of molecular gas, including internal processes such as spiral arm compression and external processes such as interactions.


Table of Contents

INTRODUCTION

MOLECULAR GAS FROM THE INNER TO THE OUTER REGIONS OF GALAXIES

MOLECULAR ISM MASSES: BASIC EQUATIONS
Brightness Temperature, Flux Density and Luminosity
Observations of the Molecular ISM using CO Line Emission
CO(J = 1-0) Line Emission
CO(J = 2-1) Line Emission
Observations of the Molecular ISM using Dust Continuum Emission
The ISM in Extreme Environments Such as the Outskirts
Variations of α10 (or XCO)
Variations of R21/10
Variations of Dust Properties and Temperature

MOLECULAR GAS OBSERVATIONS IN THE OUTSKIRTS OF DISK GALAXIES
The Milky Way
Properties of Molecular Clouds in the Outer Milky Way
Extragalactic Disk Galaxies
Molecular Gas Detections
Star Formation in Extragalactic Disk Galaxies
Theory

MOLECULAR GAS OBSERVATIONS IN THE OUTSKIRTS OF EARLY-TYPE GALAXIES

MOLECULAR GAS OBSERVATIONS IN GALAXY GROUPS AND CLUSTERS

CONCLUSIONS AND FUTURE DIRECTIONS

REFERENCES

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