To be published in "The Spectral Energy Distribution of Gas-Rich Galaxies: Confronting Models with Data", Proceedings of the International Workshop held 4-8 October 2004 in Heidelberg, 4-8 Oct. 2004, Edited by Cristina C. Popescu and Richard J. Tuffs, AIP Conf. Ser.

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INTERSTELLAR DUST: WHAT IS IT, HOW DOES IT EVOLVE, AND WHAT ARE ITS OBSERVATIONAL CONSEQUENCES?

Eli Dwek


Laboratory for Astronomy and Solar Physics
NASA Goddard Space Flight Center
Greenbelt, MD 20771, U.S.A.
e-mail: eli.dwek@nasa.gov


Abstract. The presence of dust in the interstellar medium of galaxies dramatically affects their spectral appearance, and the interpretation of their underlying physical properties. Consequently, the diagnostic of galaxy spectra depends crucially on our understanding of the nature and properties of these dust particles. Models of interstellar dust particles must be able to reproduce a basic set of observational constraints, including the general interstellar extinction and infrared emission observed in the diffuse interstellar medium (ISM). Recent analysis of the solar spectrum have resulted in a convergence between the solar and B star abundances. This development, and the steadiness in the solar abundance determination of the primary refractory elements Mg, Si, and Fe, strongly suggest that any viable dust model must also obey interstellar abundances constraints. Fifteen dust models that differ in composition and size distribution and that simultaneously satisfy the local extinction, infrared emission, and abundances constraints have been shown to exist. This multitude of viable dust models provides us with an increased flexibility in understanding dust evolution and the many variations in dust properties in different phases of the ISM and stellar environments.

The evolution of stars, the primary sources of dust, and the general ISM, in which the dust is processed, change the abundance of dust as a function of time. In particular, the delayed injection of dust from low mass stars, which are the primary sources of carbon dust, into the ISM gives rise to a changing silicate-to-carbon dust mass ratio, which will affect the UV-visual extinction in galaxies. If PAHs are only produced in AGB stars, the delayed injection of these molecules into the ISM may be in part responsible for the absence of PAH features in young star forming regions, or for the existence of a metallicity threshold below which PAHs have not yet formed.

On a cosmological scale, an epoch of rapid and efficient dust formation is needed to account for the presence of dust-enshrouded star forming regions at high redshift. We show that the first opacity is probably produce by SN-condensed carbon dust that formed in less than ~ 100 Myr after the onset of galaxy formation.


Table of Contents

INTRODUCTION

INTERSTELLAR DUST MODELS
What constitutes an interstellar dust model?
Observational constraints in the local ISM
Interstellar dust models with cosmic abundances constraints
Results and astrophysical implications
Should there be a universal dust model?

THE EVOLUTION OF DUST
Dust sources and relative contributions
Dust processing in the ISM
The destruction of SN condensates by the reverse shock
Putting it all together in an idealized evolutionary model

WHEN DO GALAXIES BECOME FIRST OPAQUE?

SUMMARY

REFERENCES

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