Implications of Coronal Line Emission in NGC 4696

TL;DR

This paper introduces a new spectral modeling capability in CLOUDY, revisits coronal line observations in NGC 4696, and suggests the coronal gas is an interface rather than condensed intracluster medium, challenging previous cooling flow models.

Contribution

It provides updated predictions for coronal line ratios, analyzes observational data with these models, and proposes a new interpretation of the coronal gas origin in galaxy clusters.

Findings

Predicted [Fe XIV]/[Fe X] ratio ~3, higher than previous estimates.

Observations are inconsistent with simple cooling flow models.

Coronal gas likely forms an interface rather than condensing from the intracluster medium.

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] {\lambda}5303 / [Fe X] {\lambda}6375 luminosity ratio, a critical diagnostic of a cooling plasma, to slightly less than unity. By contrast, our calculations predict a ratio ~3. We revisit recent optical coronal line observations along the X-ray cool arc around NGC 4696 by Canning et al. (2011), which detected [Fe X] {\lambda}6375, but not [Fe XIV] {\lambda}5303. 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 (A_V > 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 solar masses. 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.