Enhanced star formation in $z\sim6$ quasar companions

TL;DR

This study uses cosmological simulations to show that quasar feedback at z~6 can enhance star formation and increase the brightness and number of observable companion galaxies, aligning with recent ALMA survey findings.

Contribution

It provides the first quantitative analysis of how quasar outflows influence the properties and detectability of companion galaxies at high redshift.

Findings

Quasar feedback increases companion SFR by 2-3 times.

Feedback makes companions more massive and brighter in [CII] and UV.

Feedback slightly boosts the observable quasar bias and companion number density.

Abstract

Quasars powered by supermassive black holes (MBH, $>10^8~M_{\odot}$) at $z\sim 6$ are predicted to reside in cosmic over-dense regions. However, observations so far could not confirm this expectation due to limited statistics. The picture is further complicated by the possible effects of quasar outflows (i.e. feedback) that could either suppress or stimulate the star formation rate (SFR) of companion galaxies, thus modifying the expected bias. Here we quantify feedback effects on the properties and detectability of companions by comparing cosmological zoom-in simulations of a quasar in which feedback is either included or turned-off. With respect to the no-feedback case, companions (a) directly impacted by the outflow have their SFR increased by a factor $2-3$, and (b) tend to be more massive. Both effects shift the [CII]158$\mu$m and UV luminosity functions toward brighter magnitudes. This leads us to conclude that quasar feedback slightly increases the effective quasar bias, boosting the number density of observable quasar companions, in agreement with what has been found around the brightest quasars of recent ALMA [CII] surveys. Deeper observations performed with JWST and/or ALMA will improve the statistical significance of this result by detecting a larger number of fainter quasar companions.