Kiloparsec-scale ALMA Imaging of [CII] and Dust Continuum Emission of 27 Quasar Host Galaxies at z~6

TL;DR

This study uses high-resolution ALMA imaging to analyze the [CII] line and dust emission in 27 quasar host galaxies at z~6, revealing their structure, star formation, and black hole positioning within early universe galaxies.

Contribution

First detailed kpc-scale ALMA imaging of [CII] and dust in a large sample of z~6 quasar hosts, showing their morphology, companion presence, and star formation distribution.

Findings

Half of the quasars have companion galaxies within 3-90 kpc.

Dust emission is more compact than [CII] emission.

Star formation rate densities peak at galaxy centers, below Eddington limit.

Abstract

We present a study of the [CII] 158micron line and underlying far-infrared (FIR) continuum emission of 27 quasar host galaxies at z~6, traced by the Atacama Large Millimeter/submillimeter Array at a spatial resolution of ~1 physical kpc. The [CII] emission in the bright, central regions of the quasars have sizes of 1.0-4.8kpc. The dust continuum emission is typically more compact than [CII]. We find that 13/27 quasars (approximately one-half) have companion galaxies in the field, at projected separations of 3-90kpc. The position of dust emission and the Gaia-corrected positions of the central accreting black holes are cospatial (typical offsets <0.1"). This suggests that the central black holes are located at the bottom of the gravitational wells of the dark matter halos in which the z>6 quasar hosts reside. Some outliers with offsets of ~500pc can be linked to disturbed morphologies, most likely due to ongoing or recent mergers. We find no correlation between the central brightness of the FIR emission and the bolometric luminosity of the accreting black hole. The FIR-derived star-formation rate densities (SFRDs) in the host galaxies peak at the galaxies' centers, at typical values between 100 and 1000 M_sun/yr/kpc^2. These values are below the Eddington limit for star formation, but similar to those found in local ultraluminous infrared galaxies. The SFRDs drop toward larger radii by an order of magnitude. Likewise, the [CII]/FIR luminosity ratios of the quasar hosts are lowest in their centers (few x10^-4) and increase by a factor of a few toward the galaxies' outskirts, consistent with resolved studies of lower-redshift sources.