Quasar radiation transforms the gas in a merging companion galaxy

TL;DR

This study shows that quasar radiation at high redshift can significantly alter the gas in a merging galaxy, disrupting molecular clouds and potentially suppressing star formation, highlighting a form of negative feedback during galaxy mergers.

Contribution

It provides direct observational evidence that quasar radiation can transform gas properties in merging galaxies, revealing a new aspect of quasar feedback mechanisms.

Findings

Quasar radiation excites and confines molecular gas into tiny cloudlets.

Dense molecular clouds are disrupted, hindering star formation.

The merging system is at z ≈ 2.7, with galaxies separated by a few kiloparsecs.

Abstract

Quasars, powered by gas accretion onto supermassive black holes, rank among the most energetic objects of the Universe. While they are thought to be ignited by galaxy mergers and affect the surrounding gas, observational constraints on both processes remain scarce. Here we unveil a major merging system at redshift $z \approx 2.7$, and demonstrate that radiation from the quasar in one galaxy directly alters the gas properties in the other galaxy. Our findings reveal that the galaxies, with centroids separated by only a few kiloparsecs and approaching each other at speed $\approx550\,$km$\,$s$^{-1}$, are massive, form stars, and contain a substantial molecular mass. Yet, dusty molecular gas seen in absorption against the quasar nucleus is highly excited and confined within cloudlets with densities $\sim 10^5$ - $10^6$ cm$^{-3}$ and sizes $<$0.02 pc, several orders of magnitude more compact than those observed in intervening (non-quasar) environments. This is also approximately 10$^5$ times smaller than currently resolvable through molecular-line emission at high redshifts. We infer that, wherever exposed to the quasar radiation, molecular gas is disrupted, leaving behind surviving dense clouds too small to give birth to new stars. Our results not only underscore the role of major galaxy mergers in triggering quasar activity, but also reveal localized negative feedback as a profound alteration of internal gas structure which likely hampers star formation.