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Session 95 - Cosmological Parameters and Large Scale Structure Formation.
Oral session, Friday, January 09
International Ballroom Center,
We report on the discoveries of several protogalaxies associated with known quasars at z > 4, corresponding to epochs of < 1 Gyr after the big bang. These objects have typical magnitudes R \sim 25^m, implying restframe L \sim L_*, a relatively modest Ly\alpha line emission, and no high-ionization lines, suggesting a stellar, rather than an AGN origin of the observed luminosity. The inferred star formation rates, both from the Ly\alpha emission and the continuum luminosity at \lambda_rest = 1500Åare a few M_ødot/yr. Their properties are very similar to those of Lyman-break galaxies at z \sim 3. In addition, we detected QSO-ionized Ly\alpha emission nebulæ\ in at least two cases; their properties are quite distinct from those of the companion galaxies, which are likely powered by star formation.
Projected median separations of these objects from the QSOs are \sim 100 h^-1 comoving kpc, an order of magnitude less than the comoving r.m.s. of L_* galaxies today, implying a number density of objects comparable to that in rich cluster cores. The implied comoving star formation density is also \sim 1000 times higher than the upper limits in this redshift range for general field galxies in the HDF. The frequency of these QSO companion galaxies at z > 4 is also at least an order of magnitude higher than in the comparable samples at z \sim 2 - 3, the peak of the quasar era. These must be very special regions in the early universe.
A generic prediction of biased galaxy formation models is that the first massive structures should be forming at the very highest peaks of the primordial density field, and that the first protogalaxies should be strongly clustered. Several independent arguments suggest that quasars at z> 4 are likely situated in future giant ellipticals, perhaps in the cores of future rich clusters, marking the sites of early galaxy formation. Our discoveries of their companions galaxies support this idea.
