We present a search for companion [ CII ] 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 _ { [ CII ] } \sim 10 ^ { 8 } at z \sim 6 .
We identify four robust line detections from a blind search of five deep fields centered on ultra-luminous infrared galaxies and QSOs , over an order of magnitude more than expected based on current observations and predictions , suggesting that these objects 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 20 – 70 kpc ( projected ) from their central source , providing a constraint on their halo masses of the central galaxies ranging from 2.5 \times 10 ^ { 12 } M _ { \odot } to 4 \times 10 ^ { 13 } M _ { \odot } .
To place these discoveries in context , we employ a mock galaxy catalog to estimate the luminosity function for [ CII ] during reionization and compare to our observations .
The simulations support this result by showing a similar level of elevated counts found around such luminous sources .