There has been a growing body of evidence that galaxy overdense structures being detected at the location of deep Lyα absorption. We found an interesting protocluster candidate, which may locate in front of a background bright quasar at z=3.09, whose spectrum exhibits three possible large Lyα absorption lines with high effective optical depth. However, a definitive conclusion cannot be drawn from the currently available shallow SDSS spectrum alone. The photo-z distribution of the region clearly skewed toward the low-z side (z<3.09) compared to the distribution outside the region, implying the presence of a protocluster in front of the quasar. We propose deep DEIMOS spectroscopy for both the foreground protocluster candidate at z∼3 and the background quasar and galaxies. The purpose of this observation is to determine the membership of the protocluster, and at the same time to determine the Hi distribution in this overdense region by observing a background quasar and several bright background galaxy candidates, whose LOSs penetrate this region. We will relate galaxy properties with the Hi gas properties to find the signatures of an efficient pristine gas supply through inflows from the surrounding IGM, or the photoionization or AGN feedback from galaxies.
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