Searching for Galaxy Overdensities in the Fields of 10 z>6 Quasars

The highest-redshift quasars (z>6) host supermassive black holes (M_BH > 1e9 M_⊙) and presumably reside in massive host galaxies located in some of the largest galaxy overdensities at early cosmic epochs. However, optical searches for such overdensities have so far been inconclusive. One caveat is that the sources could be too faint in optical wavelengths, so while overdensities may be present, they must be detected at a longer wavelength regime. The Atacama Large Millimeter Array (ALMA) now provides the sensitivity and resolution required to detect and resolve faint sources at very high redshift (z>5-6). Instead of blind surveys, the data we present are observations of known bright quasars from the ALMA archive. Examining the sidelines of these quasars and comparing them with the number count of sources in blind surveys enables us to learn whether quasars are present in galaxy overdensities or if their environments are indistinguishable from a blank field. We use ALMA cycle 0, 1 and 2 data to map the vicinity of ten quasars at z>6 in the continuum at ~1.2mm, tracing the far infrared dust emission, to perform an independent search for companions around the quasars. We also examine the presence of the [CII] line in these fields. We compare the number density of such sources to 'blank field' studies to see if there is evidence for an overdensity of sources in the immediate vicinity of the quasars. Either outcome ('overdensity' or 'no overdensity') would have important implications for early structure formation. Preliminary results show there is an excess of positive flux in these fields, and there is a total of a few (<10) +5σ detections in the ten fields, but further work to estimate the number of spurious detections is necessary.