The origin of cold gas in giant elliptical galaxies and its role in fuelling radio-mode AGN feedback

Werner, Norbert and Oonk, J. B. R. and Sun, M. and Nulsen, P. E. J. and Allen, S. W. and Canning, R. E. A. and Simionescu, A. and Hoffer, A. and Connor, T. and Donahue, M. and Edge, A. C. and Fabian, A. C. and von der Linden, A. and Reynolds, C. S. and Ruszkowski, M. (2014) The origin of cold gas in giant elliptical galaxies and its role in fuelling radio-mode AGN feedback. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 439 (3). pp. 2291-2306. ISSN 0035-8711

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Abstract

The nature and origin of the cold interstellar medium (ISM) in early type galaxies are still a matter of debate, and understanding the role of this component in galaxy evolution and in fueling the central supermassive black holes requires more observational constraints. Here, we present a multi-wavelength study of the ISM in eight nearby, X-ray and optically bright, giant elliptical galaxies, all central dominant members of relatively low mass groups. Using far-infrared spectral imaging with the Herschel Photodetector Array Camera & Spectrometer (PACS), we map the emission of cold gas in the cooling lines of [C ii]λ157μm, [O i]λ63μm, and [O ib]λ145μm. Additionally, we present Hα+[N ii] imaging of warm ionized gas with the Southern Astrophysical Research (SOAR) telescope, and a study of the thermodynamic struc- ture of the hot X-ray emitting plasma with Chandra. All systems with extended Hα emission in our sample (6/8 galaxies) display significant [C ii] line emission indicating the presence of reservoirs of cold gas. This emission is co-spatial with the optical Hα+[N ii] emitting nebu- lae and the lowest entropy soft X-ray emitting plasma. The entropy profiles of the hot galactic atmospheres show a clear dichotomy, with the systems displaying extended emission line neb- ulae having lower entropies beyond r & 1 kpc than the cold-gas-poor systems. We show that while the hot atmospheres of the cold-gas-poor galaxies are thermally stable outside of their innermost cores, the atmospheres of the cold-gas-rich systems are prone to cooling instabili- ties. This provides considerable weight to the argument that cold gas in giant ellipticals is pro- duced chiefly by cooling from the hot phase. We show that cooling instabilities may develop more easily in rotating systems and discuss an alternative condition for thermal instability for this case. The hot atmospheres of cold-gas-rich galaxies display disturbed morphologies indicating that the accretion of clumpy multiphase gas in these systems may result in variable power output of the AGN jets, potentially triggering sporadic, larger outbursts. In the two cold-gas-poor, X-ray morphologically relaxed galaxies of our sample, NGC 1399 and NGC 4472, powerful AGN outbursts may have destroyed or removed most of the cold gas from the cores, allowing the jets to propagate and deposit most of their energy further out, increasing the entropy of the hot galactic atmospheres and leaving their cores relatively undisturbed.

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