Investigating Overdensities around z > 6 Galaxies through ALMA Observations of [C II]

We present a search for companion [C II] emitters to known luminous sources at 6 < z < 6.5 in deep, archival ALMA observations. The observations are deep enough to detect sources with ${L}_{[{\rm{C}}{\rm{II}}]}\sim {10}^{8}\ {L}_{\odot }$ at z ∼ 6. We identify three new robust line detections from a blind search of five deep fields centered on ultraluminous infrared galaxies and QSOs. We calculate the volume density of companions and find a relative overdensity of ${6}_{-3}^{+4}$ and ${86}_{-37}^{+60}$ when comparing to current observational constraints and theoretical predictions, respectively. These results suggest that the central sources may be highly biased tracers of mass in the early universe. We find these companion lines to have comparable properties to other known galaxies at the same epoch. All companions lie less than 650 km s^-1 and between 25 and 60 kpc (projected) from their central source. To place these discoveries in context, we employ a mock galaxy catalog to estimate the luminosity function for [C II] during reionization and compare to our observations. The simulations support this result by showing a similar level of elevated counts found around such luminous [C II] sources.