Enhanced H ₂O formation through dust grain chemistry in X-ray exposed environments

Context. The ultraluminous infrared galaxy Mrk 231, which shows signs of both black hole accretion and star formation, exhibits very strong water rotational lines between λ = 200-670 μm, comparable to the strength of the CO rotational lines. High-redshift quasars also show similar CO and H ₂O line properties, while starburst galaxies, such as M 82, lack these very strong H ₂O lines in the same wavelength range, but do show strong CO lines. Aims. We explore the possibility of enhancing the gas phase H ₂O abundance in X-ray exposed environments, using bare interstellar carbonaceous dust grains as a catalyst. Cloud-cloud collisions cause C and J shocks, and strip the grains of their ice layers. The internal UV field created by X-rays from the accreting black hole does not allow reforming the ice. Methods. We determined the formation rates of both OH and H ₂O on dust grains, with temperatures T _dust = 10-60 K, using both Monte Carlo and rate equation method simulations. The acquired formation rates were added to our X-ray chemistry code, which allowed us to calculate the thermal and chemical structure of the interstellar medium near an active galactic nucleus. Results. We derive analytic expressions for the formation of OH and H ₂O on bare dust grains as a catalyst. Oxygen atoms arriving on the dust are released into the gas phase in the form of OH and H ₂O. The efficiencies of this conversion due to the chemistry occurring on dust are near 30 percent for oxygen converted into OH and 60 percent for oxygen converted into H ₂O between T _dust = 15-40 K. At higher temperatures, the efficiencies decline rapidly. When the gas is mostly atomic, molecule formation on dust is dominant over the gas-phase route, which is then quenched by the low H ₂ abundance. Here, it is possible to enhance the warm (T > 200 K) water abundance by an order of magnitude in X-ray exposed environments. This helps explain the observed bright water lines in nearby and high-redshift ULIRGs and quasars.