In Rep. Prog. Phys. 70 (2007) 1099-1176
astro-ph/0511534

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MOLECULES IN GALAXIES

Alain Omont


Institut d'Astrophysique de Paris, CNRS and Université Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France omont@iap.fr


Abstract. The main achievements, current developments and prospects of molecular studies in external galaxies are reviewed. They are put in the context of the results of several decades of studies of molecules in local interstellar medium, their chemistry and their importance for star formation. CO observations have revealed the gross structure of molecular gas in galaxies. Together with other molecules, they are among the best tracers of star formation at galactic scales. Our knowledge about molecular abundances in various local galactic environments is progressing. They trace physical conditions and metallicity, and they are closely related to dust processes and large aromatic molecules. Major recent developments include mega-masers, and molecules in Active Galactic Nuclei; millimetre emission of molecules at very high redshift; and infrared H2 emission as tracer of warm molecular gas, shocks and photodissociation regions. The advent of sensitive giant interferometers from the centimetre to sub-millimetre range, especially ALMA in the near future in the mm/submm range, will open a new area for molecular studies in galaxies and their use to trace star formation at all distances.


Table of Contents

GENERAL INTRODUCTION
Central role of the interstellar medium (ISM) in the physics, structure, evolution and multi-lambda emission of galaxies
Molecular gas is a major component of the ISM
Landmarks of the history of the discovery and studies of interstellar molecules in the Milky Way (MW)
Main achievements of molecular studies in external galaxies

COSMIC MOLECULES AND LOCAL INTERSTELLAR MEDIUM
Physics and various components of the local interstellar molecular gas
Observational techniques
Modelling (millimetre) molecular line formation, and diagnostic of physical conditions and molecular abundances in the ISM
Uniqueness of basic processes of interstellar chemistry
Relation between interstellar and other cosmic molecules

CO OBSERVATIONS AND GROSS STRUCTURE OF MOLECULAR GAS IN THE MILKY WAY
Introduction: atomic and molecular interstellar gas
Molecular gas in the Milky Way at galactic scale

CO OBSERVATIONS AND GROSS STRUCTURE OF MOLECULAR GAS IN OTHER GALAXIES
Molecular gas in spiral galaxies from CO surveys of local galaxies
Molecular gas in other types of galaxies

MOLECULES AS TRACERS OF STAR FORMATION AT GALACTIC SCALES
Introduction. Star formation rate
Star formation and molecular clouds in non-starburst galaxies
Molecular gas and starbursts in luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs)
OH mega-masers.

MOLECULAR ABUNDANCES IN VARIOUS LOCAL GALACTIC ENVIRONMENTS
Summary of standard molecular abundances in various regions of the Milky Way, and models of interstellar chemistry
Observed abundance variations in local galaxies: I. the Magellanic Clouds
Observed abundance variations in local galaxies: II. Nearby starbursts and other galaxies
Abundance ratios of isotopic varieties, and inferences for chemical evolution of galaxies

TRENDS IN MOLECULAR ABUNDANCES FROM ABSORPTION LINES AT HIGH REDSHIFT
H2 UV absorption lines in Damped Lyman-alpha systems of quasars
Millimetre and radio absorption lines in lensing galaxies and radio-sources

MOLECULES, DUST AND PAHS (LARGE AROMATIC MOLECULES)
Introduction. Interplay between dust and molecules in galaxies
Mid-infrared emission of PAHs in galaxies
Diffuse Interstellar Bands (DIBs) in galaxies
Molecular infrared spectral features in extragalactic dust

MILLIMETRE EMISSION OF MOLECULES AT VERY HIGH REDSHIFT
Star formation and ULIRGs at high redshift
CO studies
Molecules in the host galaxies of high-z AGN
Other species and detailed studies through strong gravitational lensing

INFRARED H2 EMISSION, TRACER OF WARM MOLECULAR GAS, SHOCKS AND PHOTODISSOCIATION REGIONS
Basic features and physics of H2 emission, and Milky Way observations
The 2µm H2 emission in galaxies
Mid-IR H2 pure-rotational lines in galaxies
Prospects for detecting H2 in forming galaxies

MOLECULES AND ACTIVE GALACTIC NUCLEI (AGN)
Interplay between super-massive black holes and their host galaxy
Molecules in the central regions and fueling the AGN
H2O mega-masers and AGN molecular disks

PROSPECTS
Waiting for ALMA: ongoing studies and submillimetre breakthroughs
The ALMA revolution
Accompanying- and post-ALMA: JWST, extremely large telescopes and SKA

GENERAL CONCLUSION

REFERENCES

List of Main Abbreviations

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