Enhanced H2O formation through dust grain chemistry in X-ray exposed environments

Abstract

The ULIRG Mrk 231 exhibits very strong water rotational lines between \lambda = 200-670\mu m, comparable to the strength of the CO rotational lines. High redshift quasars also show similar CO and H2O line properties, while starburst galaxies, such as M82, lack these very strong H2O lines in the same wavelength range, but do show strong CO lines. We explore the possibility of enhancing the gas phase H2O 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 to reform the ice. We determine formation rates of both OH and H2O on dust grains, having temperature T_dust=10-60 K, using both Monte Carlo as well as rate equation method simulations. The acquired formation rates are added to our X-ray chemistry code, that allows us to calculate the thermal and chemical structure of the interstellar medium near an active galactic nucleus. We derive analytic expressions for the formation of OH and H2O on bare dust grains as a catalyst. Oxygen atoms arriving on the dust are released into the gas phase under the form of OH and H2O. The efficiencies of this conversion due to the chemistry occurring on dust are of order 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 rapidly decline. When the gas is mostly atomic, molecule formation on dust is dominant over the gas-phase route, which is then quenched by the low H2 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 to explain the observed bright water lines in nearby and high-redshift ULIRGs and Quasars.