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

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: meijerink@astro.rug.nl

Received: 14 July 2011
Accepted: 6 November 2011

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 H₂O line properties, while starburst galaxies, such as M 82, lack these very strong H₂O lines in the same wavelength range, but do show strong CO lines.

Aims. 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 reforming the ice.

Methods. We determined the formation rates of both OH and H₂O 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 H₂O 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 H₂O. 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 H₂O 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 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 explain the observed bright water lines in nearby and high-redshift ULIRGs and quasars.