The following document lists the file abstract/AFRANCES_DUST_PG.abs
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How did galaxies form and evolve is still a mystery to a large extent. Conflicting observational results and theoretical predictions did not allow yet to decide in favour either of the long-standing paradigm according to which spheroidal galaxies have ended their main formation phase just after one or two Gyrs after the Big-Bang, or of its current competiting scenario envisaging galaxy formation as a more gradual process of merging of smaller fragments taking place at recent cosmic epochs. A crucial observational result lies in the fact that current deep surveys in all e.m. bands have failed to reveal any populations of luminous starbursts corresponding to the main formation epoch. Because of its improvement in sensitivity with respect to previous surveys, there is a widespread feeling that ISO will substantially contribute to solve this enigma. This is based on the concept (see Franceschini et al., 1994, ApJ427,140) that our inability to detect forming galaxies might be due to the effect of extinction of the optical-UV emission by a dust-enriched ISM. If this is the case, the main energy output of primeval objects is expected to come out in the IR. Two approaches will be followed to explore this possibility. One is to survey blindly entire portions of the sky to the faintest possible flux levels. The alternative is to concentrate the efforts on a limited number of candidates discovered through direct (i.e. energy likage in the radio, mm, or near-IR) or indirect (e.g. gravitational lensing effects, spectral absorption in the direction of high-z QSOs) criteria. We propose to spend a few hours of the ISO lifetime to perform photometric imaging at both mid- and far-IR wavelengths on the currently best PG candidates (11 targets in Aut. and 12 in Spring). This will contribute to provide not only a decisive test of current galaxy formation scenarios, but also invaluable information on some mysterious objects in the distant universe.