Molecular hydrogen emission in cooling flows

We have carried out K-band spectroscopy both of central cluster galaxies in cooling flows and of radio galaxies with comparable radio powers and distances that are not in strong cooling flows. We detected H-2(1-0)S(1)-line emission from the inner few kpc of the cooling flow galaxies, but not in the galaxies outside of strong cooling flows. It is therefore very likely that the emission is related to the presence of the cooling flow. The strength of the emission requires that there be a source of reexcitation or reheating for the H-2 molecules; the emission does not simply arise from material cooling out of the flow and emitting H-2 photons as it passes through similar to 2000 K. The relative strengths of H alpha and H-2(1-0)S(1) emission rule out fast shocks or other bulk ionizing mechanisms as the source of excitation. Photoionization of cold clouds by the surrounding X-ray-emitting material plausibly explains the relative line strengths and total luminosity if excitation of H-2 by suprathermal secondary electrons is taken into acccount.