Abstract

Galaxy formation occurs along the densest part of the filamentary
Intergalactic Medium where the gas can collapse and form stars. The
proto-galactic phases preceding substantial star-formation are however
unobservable - in emission - in most of the studies conducted so far
and thus poorly constrained. Recent theoretical models have suggested
that gas-rich, low-mass haloes at high redshift may have indeed very
low or absent star formation, as a consequence of lower gas
metallicity, H2 self-regulation effects or reduced cooling accretion
due photoionization. These "dark" galaxies may represent an important
gas baryonic reservoir for later star-formation in more massive
systems and a crucial ingredient in order to reproduce the properties
of current galaxy population. In this talk, I will present the result
of a an ongoing, successful program that uses a new approach to
directly detect and study these "dark" phases of galaxy formation: the
key idea is to use an external ``source of illumination'', a bright
quasar, to light up with fluorescent Ly-alpha emission dark, proto-
galactic clouds and dense streams around galaxies in a large
cosmological volume. In the first part of the talk, I will discuss our
pilot project based on deep narrow-band imaging on VLT/FORS centered
on a z=2.4 hyper-luminous quasar: how we identified and characterized
the physical properties of the first sample of high-redshift, "dark"
galaxy candidates. In the second part of the talk, I will present the
detection of fluorescent emission from the Circumgalactic Medium of
star forming galaxies and very recent, spectacular results obtained
with Keck/LRIS of the detection of hundred-kpc scale filaments
surrounding bright quasars.

Presentation

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