Galaxy properties from the outskirts to the core of a protocluster at <i>z</i> = 3.70

ABSTRACT We study the dependence of galaxy properties on their spatial position within a protocluster by carrying out follow-up spectroscopy on a protocluster candidate selected from the wide-field imaging of the Hyper SuprimeCam Subaru Strategic Programme. In the $4.5\sigma$ surface-overdense region of g-dropout galaxies, we now spectroscopically confirm that 13 galaxies, at least, are clustering in a narrow redshift range of $\Delta z\lt 0.05$ around $z=3.70$. Next, we investigate the physical properties and three-dimensional (3D) distribution of the protocluster’s member galaxies. Based on spectroscopically confirmed g-dropout galaxies, we find an overabundance of rest-frame ultraviolet (UV) bright galaxies in the protocluster. The UV brightest protocluster member turns out to be an active galactic nucleus, and the other UV brighter members tend to show smaller Ly$\alpha$ equivalent widths than field counterparts. The member galaxies tend to densely populate near the 3D centre of the protocluster, but the separation from the nearest neighbour rather than the distance from the centre of the protocluster is more tightly correlated to galaxy properties, implying that the protocluster is still in an early phase of cluster formation and only close neighbours have a significant impact on the physical properties of protocluster members. The number density of massive galaxies, selected from an archival photometric-redshift catalogue, is higher near the centre of the protocluster, while dusty starburst galaxies are distributed on the outskirts. The protocluster thus appears to consist of multiple galaxy populations, whose spatial distributions reveal the developmental stage of the galaxy cluster.