The origin of molecular hydrogen emission in cooling-flow filaments

The optical filaments found in many cooling flows in galaxy clusters consist of low-density (~10³ cm^-3) cool (~10³ K) gas surrounded by significant amounts of cosmic-ray and magnetic field energy. Their spectra show anomalously strong low-ionization and molecular emission lines when compared with Galactic molecular clouds exposed to ionizing radiation such as the Orion complex. Previous studies have shown that the spectra cannot be produced by O-star photoionization. Here, we calculate the physical conditions in dusty gas that is well shielded from external sources of ionizing photons and is energized either by cosmic rays or dissipative magnetohydrodynamics waves. Strong molecular hydrogen lines, with relative intensities similar to those observed, are produced. Selection effects introduced by the microphysics produce a correlation between the H₂ line upper level energy and the population temperature. These selection effects allow a purely collisional gas to produce H₂ emission that masquerades as starlight-pumped H₂ but with intensities that are far stronger. This physics may find application to any environment where a broad range of gas densities or heating rates occur.