Discovery of Atomic and Molecular Mid-Infrared Emission Lines in Off-Nuclear Regions of NGC 1275 and NGC 4696 with the Spitzer Space Telescope

ABSTRACT We present Spitzer high-resolution spectra of off-nuclearregions in the central cluster galax-ies NGC1275and NGC4696in the Perseus and Centaurus clusters, respectively.Both objectsare surrounded by extensive optical emission-line filamentary nebulae, bright outer parts ofwhich are the targets of our observations. The 10–37 µm spectra show strong pure rotationallines from molecular hydrogen revealing a molecular component to the filaments which hasan excitation temperature of ∼ 300−400K. The flux in the 0-0S(1) molecular hydrogen linecorrelates well with the strength of the optical lines, having about 3 per cent of the Hα+[NII]emission. The 11.3µm PAH feature is seen in some spectra. Emission is also seen from bothlow and high ionization fine structure lines. Molecular hydr ogen cooler than ∼400K domi-nates the mass of the outer filaments; the nebulae are predomi nantly molecular.Key words: galaxies: clusters: general – galaxies: clusters: individual: NGC 1275 – galaxies:clusters: individual: NGC 4696 – – intergalactic medium – in frared: galaxies