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

Abstract

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 2 O line properties, while starburst galaxies, such as M 82, lack these very strong H 2 O lines in the same wavelength range, but do show strong CO lines. Aims. We explore the possibility of enhancing the gas phase H 2 O 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 2 O 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 2 O 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 2 O. 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 2 O 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.

How micron-sized dust particles determine the chemistry of our Universe

“…Therefore, the rates we derive here overestimate the actual rates at temperatures greater than 30 K24 (when the rates decrease exponentially). Note that the value of T dust for the validity of our estimates is higher at higher densities.…”

“…To estimate the impact of chemical desorption on the gas phase composition of astrophysical environments, we use the chemical network presented in previous studies1024, but include the chemical desorption yields derived from the experiments presented in this work (Table 1). We solve this chemical network using the rate equations method.…”

New Compton-thick AGN in the circumnuclear H2O maser hosts UGC 3789 and NGC 6264

“…Indeed, when the interstellar medium is mostly atomic the gasphase formation of H2O is quenched by the low H2 abundance. However, Neufeld et al (1994) take into account only the formation of water via the gas-phase route, while recent models suggest that, in X-ray exposed environments, the formation of water on dust grains (which dominates over the gas-phase route when the gas is mostly atomic) might be very efficient (Meijerink, Cazaux & Spaans 2012). In order to inspect whether or not the dust may survive at 0.028 pc from the black hole, we calculate the inner radius of the dust distribution in NGC 2273.…”

Surface chemistry in photodissociation regions

“…In environments powered by high radiation fields, the dust grains are mainly bare, since no ice mantles can form on their surfaces owing to radiation (Meijerink et al 2012). Dust grains, when not covered by ice, provide an ideal place for chemical reactions to occur that directly enrich the gas phase.…”