The sudden appearance of dust in the early Universe

Observations suggest that high-redshift galaxies are eith er very dusty or essentially dust free. The evolution from one regime to the other must also be very fast, since evolved and dusty galaxies show up at redshifts corresponding to a Universe which is only about 500 Myr old. In the present paper models which predicts the existence of an apparent dichotomy between dusty and dust-free galaxies at high redshift are considere d. Galaxies become dusty as soon as they reach an evolved state and the transition is very rapid. A special case suggests that while stellar dust production is overall relatively insignifican t ‐ contrary to what has been argued recently ‐ it can at the same time be consistent with effi cient dust production in supernovae in the local Universe. Special attention will be given to the re cent discovery of a dusty normal galaxy (A1689-zD1) at a very high redshift z = 7.5± 0.2.