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 Ly \alpha -emitting galaxies ( LAEs ) .
To this end , we have undertaken a deep narrow-band imaging survey sampling Ly \alpha emission at this redshift .
Of the 93 LAE candidates within a 36′ \times 36′Â ( 70 \times 70Â Mpc ^ { 2 } ) field , 21 galaxies form a significant surface overdensity ( \delta _ { \Sigma, { LAE } } = 3.3 \pm 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 ( \approx 10 ^ { 15 } M _ { \odot } ) 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 Ly \alpha - and UV luminosity is 30–70 % higher for the protocluster LAEs relative to the field .
This difference can not 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 Ly \alpha blob and an ultramassive galaxy found in this region paint a picture consistent with the expected early growth of galaxies in clusters .