The origin of molecular hydrogen emission in cooling-flow filaments

TL;DR

This paper investigates the origin of molecular hydrogen emission in galaxy cluster cooling-flow filaments, showing that collisional processes in shielded dusty gas energized by cosmic rays or MHD waves can produce observed strong H2 lines.

Contribution

It demonstrates that collisional excitation in shielded dusty gas can account for the molecular hydrogen emission, challenging previous models based solely on photoionization.

Findings

Strong H2 lines can be produced by collisional excitation in shielded gas.

Selection effects cause a correlation between H2 line energy and population temperature.

Purely collisional processes can mimic starlight-pumped H2 emission with higher intensities.

Abstract

The optical filaments found in many cooling flows in galaxy clusters consist of low density ($\sim 10^3 \pcc$) cool ($\sim 10^3$ 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 MHD 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 \htwo line upper level energy and the population temperature. These selection effects allow a purely collisional gas to produce \htwo emission that masquerades as starlight-pumped \htwo 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.