Exciting molecular hydrogen in the central galaxies of cooling flows

Abstract

The origin of rovibrational H 2 emission in the central galaxies of cooling flow clusters is poorly understood. Here we address this issue using data from our near-infrared spectroscopic survey of 32 of the most line-luminous such systems, presented in the companion paper by Edge et al. We consider excitation by X-rays from the surrounding intracluster medium (ICM), ultraviolet (UV) radiation from young stars, and shocks. The v = 1-0 K-band lines with upper levels within 10 4 K of the ground state appear to be mostly thermalized (implying gas densities ≥ 10 5 cm - 3 ), with the excitation temperature typically exceeding 2000 K, as found earlier by Jaffe, Bremer & van der Werf. Together with the lack of strong v = 2-0 lines in the H-band, this rules out UV radiative fluorescence. Using the CLOUDY photoionization code, we deduce that the H 2 lines can originate in a population of dense clouds, exposed to the same hot (T ∼ 50 000 K) stellar continuum as the lower density gas which produces the bulk of the forbidden optical line emission in the Ha-luminous systems. This dense gas may be in the form of self-gravitating clouds deposited directly by the cooling flow, or may instead be produced in the high-pressure zones behind strong shocks. Furthermore, the shocked gas is likely to be gravitationally unstable, so collisions between the larger clouds may lead to the formation of globular clusters.