Detections of Molecular Hydrogen in the Outer Filaments of NGC 1275

Abstract

We present clear spectroscopic detections of molecular hydrogen in the outer filaments of the H-alpha nebula surrounding the central galaxy of the Perseus cluster, NGC 1275. This implies the presence of warm molecular gas clouds at projected distances up to 24 kpc from the nucleus of the host galaxy, embedded in the hot intracluster medium. The emission-line intensity ratios reveal that the molecular hydrogen is predominately thermally excited with excitation temperatures of 1600-2200K, suggesting a lack of pressure balance between the molecular component and its surrounding medium. Excitation by stellar UV or by the central AGN is shown to be unlikely, whilst thermal excitation by X-rays or conduction from the ICM or shocks are all possibilities. Evidence for a non-thermal component is found in the spectra of some regions based on a low ortho-to-para ratio.