2016MNRAS.459.2806A

Non-thermal pressure from turbulence and bulk flows is a fundamental ingredient in hot gaseous haloes, and in the intracluster medium, it will be measured through emission line kinematics with calorimeters on future X-ray spacecraft. In this paper, we present a complementary method for measuring these effects, using forbidden FUV emission lines of highly ionized Iron which trace 10⁷ K gas. The brightest of these is [Fe xxi] λ1354.1. We search for these lines in archival Hubble Space Telescope (HST)-Cosmic Origins Spectrograph (COS) spectra from the well-known elliptical galaxies M87 and NGC4696, which harbor large reservoirs of 10⁷ K gas. We report a 2.2σ feature which we attribute to [Fe xxi] from a filament in M87, and positive residuals in the nuclei of M87 and NGC4696, for which the 90 per cent upper limits on the line flux are close to the predicted fluxes based on X-ray observations. In a newer reduction of the data from the Hubble Spectroscopic Legacy Archive, these limits become tighter and the [Fe xxi] feature reaches a formal significance of 5.3σ, neglecting uncertainty in fitting the continuum. Using our constraints, we perform emission measure analysis, constraining the characteristic path length and column density of the ∼ 10⁷ K gas. We also examine several sightlines towards filaments or cooling flows in other galaxy clusters, for which the fraction of gas at 10⁷ K is unknown, and place upper limits on its emission measure in each case. A medium-resolution HST-COS observation of the M87 filament for ∼ 10 orbits would confirm our detection of [Fe xxi] and measure its width.