Abstract
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
Highlights
The massive central galaxy in many clusters is often surrounded by an extensive emission-line nebulosity (e.g. Cowie et al 1983; Johnstone, Fabian & Nulsen 1987; Heckman et al 1989; Crawford et al 1999)
The nebulosity was studied in the optical spectral region but strong ultraviolet (UV) and infrared (IR) lines have since been seen, together with dust which is inferred from the depletion of calcium, Balmer line ratios and dust lanes which are present in some cases
In the case of the NGC 4696 short-wavelength data, there is additional power in the spectra not accounted for in the uncertainty arrays which may be due to the presence of fringes that have not been completely removed in the pipleline software
Summary
The massive central galaxy in many clusters is often surrounded by an extensive emission-line nebulosity (e.g. Cowie et al 1983; Johnstone, Fabian & Nulsen 1987; Heckman et al 1989; Crawford et al 1999). They probe a new region in temperature space (∼300−400 K) which lies between the very cold regions observed in transitions of the CO molecule and the warmer regions seen though ro-vibrational transitions of the hydrogen molecule These emission-line filaments seem to be a key marker of feedback in cooling core clusters (Hu, Cowie & Wang 1985; Crawford et al 1999) even though they do not contain a significant fraction of the total gas mass. It is by studying regions located away from the nucleus of the galaxies that we expect to be able to make progress in the understanding of the heating and ionization mechanisms of these filaments as they offer a simpler environment than is found close to the nuclei of the galaxies
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