Investigating overdensities around z>6 galaxies through ALMA
  observations of [CII]
  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$_{\rm [CII]} \sim 10^8$ at z $\sim6$. 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.
