Powering mechanism of bright optical emission lines in brightest cluster galaxies

Ascertain the dominant ionization and excitation mechanism responsible for the bright optical emission lines (e.g., Hα, [N II], [S II]) observed in the filamentary gas of brightest cluster galaxies in cool-core clusters, distinguishing among reprocessed EUV/soft X-ray radiation from cooling plasma, turbulent mixing layers, shocks, cosmic ray heating, and AGN photoionization.

Background

This paper finds nearly constant ratios among X-ray/Hα and other tracers along filaments, which disfavor mechanisms strongly dependent on distance from the central AGN. Models invoking reprocessing of EUV/soft X-ray radiation from cooling plasma, possibly with mild turbulent heating, can reproduce the observed ratios, while turbulent mixing layers and shocks are also plausible.

Despite these clues, the principal mechanism powering the emission lines remains unresolved, motivating targeted diagnostics to isolate the dominant heating and ionization sources.