Chatzikos, M. ; Williams, R. J. R. ; Ferland, G. J. ; Canning, R. E. A. ; Fabian, A. C. ; Sanders, J. S. ; van Hoof, P. A. M. ; Johnstone, R. M. ; Lykins, M. ; Porter, R. L.

published in Monthly Notices of the Royal Astronomical Society, 446, pp. 1234-1244 (2015)

Abstract: We announce a new facility in the spectral code CLOUDY that enables tracking the evolution of a cooling parcel of gas with time. For gas cooling from temperatures relevant to galaxy clusters, earlier calculations estimated the [Fe XIV] λ5303/[Fe X] λ6375 luminosity ratio, a critical diagnostic of a cooling plasma, to slightly less than unity. By contrast, our calculations predict a ratio of ˜3. We revisit recent optical coronal line observations along the X-ray cool arc around NGC 4696 by Canning et al., which detected [Fe X] λ6375, but not [Fe XIV] λ5303. We show that these observations are not consistent with predictions of cooling flow models. Differential extinction could in principle account for the observations, but it requires extinction levels (AV > 3.625) incompatible with previous observations. The non-detection of [Fe XIV] implies a temperature ceiling of 2.1 million K. Assuming cylindrical geometry and transonic turbulent pressure support, we estimate the gas mass at ˜1 million M⊙. The coronal gas is cooling isochorically. We propose that the coronal gas has not condensed out of the intracluster medium, but instead is the conductive or mixing interface between the X-ray plume and the optical filaments. We present a number of emission lines that may be pursued to test this hypothesis and constrain the amount of intermediate-temperature gas in the system.

DOI: 10.1093/mnras/stu2173
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