Quasar radiation dramatically transforms the gas in a merging companion galaxy

Quasars, powered by gas accretion onto supermassive black holes at the centers of massive galaxies, rank among the Universe’s most energetic objects. Galaxy mergers have been proposed as pivotal events in fueling these supermassive black holes, igniting quasars into their active, luminous phase. The subsequent tremendous energy release is then anticipated to impact the quasar’s surroundings, influencing morphology, gas and star-formation properties, not only in the quasar host but also in companion galaxies. However, evidence supporting merging galaxies as primary feeding mechanism remains scarce and feedback effects on gas properties lack observational constraints, in particular on its small-scale structure. Here we unveil a major merging system at redshift 𝑧 ≈ 2.7 (approximately 2.5 billion years post Big Bang), and demonstrate that the gas properties in the merging companion galaxy are directly and dramatically altered by the quasar radiation. Our findings reveal that the quasar-host and companion galaxies, with centroids separated by only a few kiloparsecs and approaching each other at speed ≈ 550 km s ⁻¹ , are both 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 of sizes <0.01 pc and densities ∼ 10 ⁵ - 10 ⁶ cm ⁻³ , several orders of magnitude more compact than those observed in non-quasar environments and approximately 10 ⁵ times smaller than the scales attainable in emission. 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 and star-formation activity, but also reveal localized negative feedback as a profound alteration of internal gas structure which hampers star formation.