2019ApJ...879....9S

In the hierarchical theory of galaxy formation, a galaxy overdensity is a hallmark of a massive cosmic structure. However, it is less well understood how different types of galaxies trace the underlying large-scale structure. Motivated by the discovery of a z = 3.13 protocluster, we examine how the same structure is populated by Lyα-emitting galaxies (LAEs). To this end, we have undertaken a deep narrowband imaging survey sampling Lyα emission at this redshift. Of the 93 LAE candidates within a 36' x 36' (70 x 70 Mpc²) field, 21 galaxies form a significant surface overdensity (δ_Σ, LAE = 3.3 ± 0.9), which is spatially segregated from the Lyman break galaxy (LBG) overdensity. One possible interpretation is that they trace two separate structures of comparable masses (≃10¹⁵ M_☉) where the latter is hosted by a halo assembled at an earlier time. We speculate that the dearth of LAEs in the LBG overdensity region may signal the role of halo assembly bias in galaxy formation, which would suggest that different search techniques may be biased accordingly to the formation age or dynamical state of the host halo. The median Lyα and UV luminosity is 30%-70% higher for the protocluster LAEs relative to the field. This difference cannot be explained by the galaxy overdensity alone and may require a top-heavy mass function, higher star formation efficiency for protocluster halos, or suppression of galaxy formation in low-mass halos. A luminous Lyα blob and an ultramassive galaxy found in this region paint a picture consistent with the expected early growth of galaxies in clusters.