Large amounts of dust (> 10 8 M ⊙ ) have recently been discovered in high redshifts quasars 1,2 and galaxies 3−5 , corresponding to a time when the Universe was less than one-tenth of its present age. The stellar winds produced by stars in the late stages of their evolution (on the asymptotic giant branch of the Hertzsprung-Russell diagram) are though to be the main source of dust in galaxies, but they cannot produce that dust on a short-enough timescale 6 (< 1 Gyr) to explain the results in the high-redshift galaxies.Supernova explosions of massive stars (type II) are also a potential source, with models predicting 0.2-4 M ⊙ of dust 7−10 . As massive stars evolve rapidly, on timescales of a few Myr, these supernovae could be responsible for the high-redshift dust. Observations 11−13 of supernova remnants in the Milky Way, however, have hitherto revealed only 10 −7 − 10 −3 M ⊙ of dust each, which is insufficient to explain the high-redshift data. Here we report the detection of ∼ 2 − 4 M ⊙ of cold dust in the youngest known Galactic remnant, Cassiopeia A. This observation implies that supernovae are at least as important as stellar winds in producing dust in our Galaxy and would have been the dominant source of dust at high redshifts.Over the past three decades, many searches for dust in supernova remnants (SNR) have been made in the mid and far-infrared (6-100µm). Remnants must be studied when they are young, before they have swept up large masses of interstellar material which makes it difficult to distinguish dust formed in the ejecta from that present in the ISM prior to the explosion. The handful of Galactic remnants which are both young and close enough (Cas A, Kepler and Tycho) have been studied with the Infrared Astronomical Satellite (IRAS) and the Infrared Space Observatory (ISO), but although dust at 100-200 K has been detected, the dust mass deduced is only 10 −7 − 10 −3 M ⊙ , many orders of magnitude lower than the solar mass quantities predicted. 11−13 The formation of dust in the recent supernova 1987A has been implied indirectly from the fading of the silicate line and increase in 10µm emission, 14 although the quantity is heavily dependant on the assumptions made 1

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.

The timescales to replenish dust from the cool, dense winds of Asymptotic Giant Branch stars are believed to be greater than the timescales for dust destruction. In high redshift galaxies, this problem is further compounded as the stars take longer than the age of the Universe to evolve into the dust production stages. To explain these discrepancies, dust formation in supernovae (SNe) is required to be an important process but until very recently dust in supernova remnants has only been detected in very small quantities. We present the first submillimeter observations of cold dust in Kepler's supernova remnant (SNR) using SCUBA. A two component dust temperature model is required to fit the Spectral Energy Distribution (SED) with $T_{warm} \sim 102$K and $T_{cold} \sim 17$K. The total mass of dust implied for Kepler is $\sim 1M_{\odot}$ - 1000 times greater than previous estimates. Thus SNe, or their progenitors may be important dust formation sites.Comment: 12 pages, 2 figures, accepted to ApJL, corrected proof

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