Astronomy Tea Talk

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.

Refreshments served at 4:00pm