In "H2 in Space", Cambridge University Press, ed. F. Combes and G. Pineau des Forets. (astro-ph/9910296)


H2 IN GALAXIES

F. Combes


DEMIRM, Observatoire de Paris, 61 Av. de l'Observatoire, F-75 014, Paris, France


Abstract. The bulk of the molecular gas in spiral galaxies is under the form of cold H2, that does not radiate and is only suspected through tracer molecules, such as CO. All tracers are biased, and in particular H2 could be highly underestimated in low metallicity regions. Our knowledge is reviewed of the H2 content of galaxies, according to their types, environment, or star-forming activities. The HI and CO components are generally well-mixed (spiral arms, vertical distribution), although their radial distributions are radically different, certainly due to radial abundance gradients. The hypothesis of H2 as dark matter is discussed, as well as the implications on galaxy dynamics, or the best perspectives for observational tests.


Table of Contents

HOW TO OBSERVE H2 IN GALAXIES?
The H2 / CO conversion ratio
Dust as a tracer
Gamma-rays
Direct H2 observations

CO AND H2 CONTENT OF GALAXIES
Variation with morphological type
Dwarf and LSB galaxies
Ultra-luminous IRAS galaxies

SPATIAL DISTRIBUTION
Radial Distributions
Large-scale Structure
Vertical Structure
Small scale structure of clouds

H2 AS A DARK MATTER CANDIDATE
Baryonic mass fraction
The smallest fragments
Gamma-rays
Various models of H2 as dark matter
Detection possibilities

CONCLUSIONS

REFERENCES

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