2023A&A...674L...5B

The OH⁺(1₁ - 1₀) absorption line is a powerful tracer of inflowing and outflowing gas in the predominantly atomic diffuse and turbulent halo surrounding galaxies. In this Letter, we present observations of OH⁺(1₁ - 1₀), CO(9-8) and the underlying dust continuum in five strongly lensed z ∼ 2 - 4 quasi-stellar objects (QSOs), using the Atacama Large Millimeter/submillimeter Array (ALMA) to detect outflowing neutral gas. Blue-shifted OH⁺(1₁ - 1₀) absorption is detected in three out of five QSOs and tentatively detected in a fourth. Absorption at systemic velocities is also detected in one source also displaying blue-shifted absorption. OH⁺(1₁ - 1₀) emission is observed in three out of five QSOs at systemic velocities and the OH⁺(2₁ - 1₀) transition is also detected in one source. CO(9-8) is detected in all five QSOs at high S/N, providing information on the dense molecular gas within the host galaxy. We compare our sample to high-z far-infrared (FIR) luminous star-forming and active galaxies from the literature. We find no difference in OH⁺ absorption line properties between active and star-forming galaxies with both samples roughly following the same optical depth-dust temperature relation. This suggests that these observables are driven by the same mechanism in both samples. Similarly, star-forming and active galaxies both follow the same OH⁺ emission-FIR relation. Obscured QSOs display broader (> 800 km s^–1) emission than the unobscured QSOs and all but one of the high-z star-forming galaxies (likely caused by the warm molecular gas reservoir obscuring the accreting nucleus). Broader CO(9-8) emission (> 500 km s^–1) is found in obscured versus unobscured QSOs, but overall they cover a similar range in line widths as the star-forming galaxies and follow the CO(9-8)-FIR luminosity relation found in low-z galaxies. We find that outflows traced by OH⁺ are only detected in extreme star-forming galaxies (indicated by broad CO(9-8) emission) and in both types of QSOs, which, in turn, display no red-shifted absorption. This suggests that diffuse neutral outflows in galaxy halos may be associated with the most energetic evolutionary phases leading up to and following the obscured QSO phase.