Dust formation in early galaxies

We investigate the sources and amount of dust in early galaxies. We discuss dust nucleation in stellar atmospheres using published extended atmosphere models, stellar evolution tracks and nucleation conditions. The thermally pulsating asymptotic giant branch phase of intermediate-mass stars is likely to be the most promising site for dust formation in stellar winds. We present an elementary model including dust formation time-scales in which the amount of dust in the interstellar medium is governed by chemical evolution. The implications of the model for high-redshift galaxies are investigated and we show there is no difficulty in producing dusty galaxies at redshifts above 5 if supernovae are a dominant source of interstellar dust. If dust does not condense efficiently in supernovae then significant dust masses can only be generated at z > 5 by galaxies with a high star formation efficiency. This is consistent with the high star formation rates implied by submillimetre sources found in deep Submillimetre Common User Bolometric Array surveys. We find the visual optical depth for individual star-forming clouds can reach values greater than 1 at very low metallicity (1/100 solar) provided that the mass-radius exponent of molecular clouds is less than 2. Most of the radiation from star formation will emerge at infrared wavelengths in the early Universe provided that dust is present. The (patchy) visual optical depth through a typical early galaxy will, however, remain less than 1 on average until a metallicity of 1/10 solar is reached.