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Non-thermal pressure from turbulence and bulk flows is a fundamentalingredient in hot gaseous halos, and in the intracluster medium it will bemeasured through emission line kinematics with calorimeters on future X-rayspacecraft. In this paper we present a complementary method for measuring theseeffects, using forbidden FUV emission lines of highly ionized Iron which trace$10^7$ K gas. The brightest of these is [Fe XXI] $\lambda$1354.1. We search forthese lines in archival HST-COS spectra from the well-known elliptical galaxiesM87 and NGC4696, which harbor large reservoirs of $10^7$ K gas. We report a2.2$\sigma$ feature which we attribute to [Fe XXI] from a filament in M87, andpositive residuals in the nuclei of M87 and NGC4696, for which the 90\% upperlimits on the line flux are close to the predicted fluxes based on X-rayobservations. In a newer reduction of the data from the Hubble SpectroscopicLegacy Archive, these limits become tighter and the [Fe XXI] feature reaches aformal significance of 5.3$\sigma$, neglecting uncertainty in fitting thecontinuum. Using our constraints, we perform emission measure analysis,constraining the characteristic path length and column density of the$\sim10^7$ K gas. We also examine several sightlines towards filaments orcooling flows in other galaxy clusters, for which the fraction of gas at $10^7$K is unknown, and place upper limits on its emission measure in each case. Amedium-resolution HST-COS observation of the M87 filament for $\sim$10 orbitswould confirm our detection of [Fe XXI] and measure its width.

Searching for FUV line emission from $10^7$ K gas in massive elliptical galaxies and galaxy clusters as a tracer of turbulent velocities