2002MNRAS.337...63W

The origin of rovibrational H₂emission 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), ultra-violet (UV) radiation from young stars, and shocks. The v = 1-0 K -band lines with upper levels within 10⁴ K of the ground state appear to be mostly thermalized (implying gas densities ≳10⁵ 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 H₂lines 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 Hα-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.