Contents of: VI/111/./abstract/AFRANCES_DUST_PGN.abs

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==> In this proposal, more time is being requested for AFRANCES.DUST_PG
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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 of either the long-standing paradigm according to which
spheroidal galaxies have underwent a huge bust of Star-Formation (SF) at high
redshift, making up the bulk of stars, or its current compeossiting scenario
envisaging galaxy formation as a more gradual process of merging of smaller
fragments taking place at more recent cosmic epochs. Recently detected high-z
sources in the optical band invariably display low SF rates, more typical of
forming disks than of young E/S0's. We have recently discussed two concepts (see
Franceschini et al., 1994, ApJ427,140; Granato et al. 1996) that could help to
solve this enigma. One is that our inability to detect forming high-redshift
luminous spheroids might be due to the effect of extinction of the optical-UV
emission by a dust-enriched ISM, in which case the optical emission would be
deprived and most of the energy output would occurr in the IR. Furthermore, all
forming spheroids probably harbor a (radio or radio-quiet) quasar, which would
complicate further the issue (e.g. Franceschini and Gratton, 1996, MNRAS), but
would also ease the target selection.
Thanks to its ability to extend the optical exploration to substantially longer
wavelengths (3-20 micron), ISO could substantially contribute to solve this
enigmas. Two approaches will be followed to test these possibilities. One is
to survey blindly entire portions of the sky to the faintest possible flux
levels. The alternative one, adopted here, is to concentrate the efforts to
study a limited number of candidates, including a few BAL QSO's, dusty
radio-galaxies and dusty gravitational lenses. We propose to spend a few hours
of the ISO mission to perform photometric imaging with the very sensitive CAM
filters LW3 (or LW10) of 11 targets. This will also provide invaluable
information on some mysterious objects in the distant universe.

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