Constraining galaxy overdensities around three z~6.5 quasars with ALMA and MUSE

TL;DR

This study investigates galaxy overdensities around three high-redshift quasars using ALMA and MUSE, revealing that large-scale structures are best detected with wide-field observations, and finds no [CII] emission from some submillimeter galaxies at quasar redshifts.

Contribution

It provides new observational constraints on the distribution of Lyman-$\alpha$ and [CII] emitters around z~6.5 quasars, emphasizing the importance of wide-field surveys for understanding early universe structures.

Findings

Detected 2 LAEs near one quasar, indicating overdensity.

Most submillimeter galaxies are not at quasar redshifts.

Wide-field observations are essential for mapping large-scale structures.

Abstract

We quantify galaxy overdensities around three high-redshift quasars with known [CII] 158um companions: PJ231-20 (z=6.59), PJ308-21 (z=6.24) and J0305-3150 (z=6.61). Recent SCUBA2 imaging revealed the presence of 17 submillimeter galaxies (SMG) with sky separations $0.7'< \theta < 2.4'$ from these three quasars. We present ALMA Band 6 follow-up observations of these SCUBA2-selected SMGs to confirm their nature and redshift. We also search for continuum-undetected [CII] emitters in the ALMA pointings and make use of archival MUSE observations to search for Lyman-$\alpha$ Emitters (LAE) associated with the quasars. While most of the SCUBA2-selected sources are detected with ALMA in the continuum, no [CII] line emission could be detected, indicating that they are not at the quasar redshifts. Based on the serendipitous detection of CO 7-6 and [CI] emission lines, we find that four SMGs in the field of PJ231-20 are at z~2.4, which is coincident with the redshift of a Mg II absorber in the quasar rest-frame UV spectrum. We report the discovery of 2 LAEs within <0.6 cMpc of PJ231-20 at the same redshift, indicating a LAE overdensity around this quasar. Taken together, these observations provide new constraints on the large-scale excess of Lyman-$\alpha$- and [CII]-emitting galaxies around z>6 quasars and suggest that only wide-field observations, such as MUSE, ALMA or JWST mosaics, can reveal a comprehensive picture of large-scale structure around quasars in the first billion years of the Universe.