The energy source of the filaments around the giant galaxy NGC1275

The brightest galaxy in the nearby Perseus cluster, NGC1275, is surrounded by a network of filaments. These were first observed through their Hα emission but are now known to have a large molecular component with a total mass approaching 10¹¹M_⊙ of gas. The filaments are embedded in hot intracluster gas and stretch over 80kpc. They have an unusually low excitation spectrum which is well modelled by collisional heating and ionization by secondary electrons. Here we note that the surface radiative flux from the outer filaments is close to the energy flux impacting on them from particles in the hot gas. We propose that the secondary electrons within the cold filaments, which excite the observed submillimetre through ultraviolet emission, are due to the hot surrounding gas efficiently penetrating the cold gas through reconnection diffusion. Some of the soft X-ray emission seen from the filaments is then due to charge exchange, although this is insufficient to account for all the observed X-ray flux. The filaments are complex with multiphase gas. Interpenetration of hot and cold gases leads to the filaments growing in mass, at a rate of up to 100M_⊙yr^-1. The lack of soft X-ray cooling emission in cool core clusters is then due to the non-radiative cooling of hot gas on mixing with cold gas around and within the central galaxy.