Large-Scale Filamentary Structure around the Protocluster at Redshift z = 3.1

We report the discovery of a large-scale coherent filamentary structure of Lya emitters in the redshift space at z = 3.1. We carried out spectroscopic observations to map the three-dimensional structure of the beltlike feature of the Lya emitters discovered by our previous narrowband imaging observations centered on the protocluster at z = 3.1. The feature was found to consist of at least three physical filaments connecting with each other. The result is in qualitative agreement with the prediction of the "biased" galaxy formation theories that galaxies preferentially formed in large-scale filamentary or sheetlike mass overdensities in the early universe. We also found that the two known giant Lya emission-line nebulae showing high star formation activities are located near the intersection of these filaments, which presumably evolves into a massive cluster of galaxies in the local universe. This may suggest that massive galaxy formation occurs at a characteristic place in the surrounding large-scale structure at high redshift.