Investigating overdensities around z>6 galaxies through ALMA observations of [CII]

TL;DR

This study uses deep ALMA observations to identify an unexpectedly high number of [CII] emitters around z>6 galaxies, suggesting these luminous sources are highly biased tracers of early Universe mass structures.

Contribution

It provides the first robust detection of excess [CII] emitters near luminous z>6 galaxies, indicating a potential bias in early Universe mass distribution and offering new constraints on halo masses.

Findings

Detected four robust [CII] emitters around luminous z>6 galaxies.

Found the number of companions exceeds expectations by an order of magnitude.

Supported by simulations showing similar elevated counts around luminous sources.

Abstract

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.