Searching for FUV line emission from 10⁷ K gas in massive elliptical galaxies and galaxy clusters as a tracer of turbulent velocities

Abstract

Non-thermal pressure from turbulence and bulk flows is a fundamental ingredient in hot gaseous halos, 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 10 7 K gas. The brightest of these is [Fe xxi] λ1354.1. We search for these lines in archival HST-COS spectra from the well-known elliptical galaxies M87 and NGC4696, which harbor large reservoirs of 10 7 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% 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 ∼ 10 7 K gas. We also examine several sightlines towards filaments or cooling 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. A mediumresolution HST-COS observation of the M87 filament for ∼10 orbits would confirm our detection of [Fe xxi] and measure its width.

Far-Uv Emission Properties of Fr1 Radio Galaxies

“…Weak N V, C II, and Si IV emission lines are also seen at the redshift of M 87 and we confirm the very weak [Fe XXI] feature published by Anderson & Sunyaev (2016) at a 5σ level.…”

FUV line emission, gas kinematics, and discovery of [Fe XXI] λ1354.1 in the sightline toward a filament in M87

“…For the first issue, as we review in Anderson & Sunyaev (2016), the FUV excess continuum spectrum does not typically produce line emission, and indeed it would be very surprising to see FUV line emission from such an old stellar population.…”

“…The [Fe xxi] line merits some additional discussion. As we showed in Anderson & Sunyaev (2016), there are several for-bidden lines in the FUV from various species of highly ionized Iron, but in emission [Fe xxi] is the brightest, and in collisional excitation equilibrium (CIE) its emissivity peaks at log T = 7.05 K. This line is well-studied in solar flare spectra, and its ionization potential is 1.7 keV so photoionization is not expected to be important outside of AGN environments. While T ∼ 10 7 K plasma generally radiates primarily in X-rays, the ability to study its emission in the FUV allows us to measure kinematics of the hot plasma, which is generally impossible without microcalorimeters or mm wavelength observations of the kinematic Sunyaev-Zel'dovich effect from bulk motions and turbulence inside clusters of galaxies (Inogamov & Sunyaev 2003).…”

“…In Anderson & Sunyaev (2016) we used archival HST-COS data to provide tentative evidence for [Fe xxi] emission in several sightlines toward galaxy clusters with known reservoirs of 10 7 K plasma, including the nucleus of M87 and the filament region studied here. Danforth et al (2016) were able to confirm our suggestion of [Fe xxi] in the M87 nucleus, and here we will confirm its presence in the filaments.…”

Gentle Heating by Mixing in Cooling Flow Clusters

“…A by product of this process is that the vortices induce turbulence in the ICM, accounting for the finding of turbulence in some cooling flows (e.g., Zhuravleva et al 2014Zhuravleva et al , 2015Arévalo et al 2016;Anderson & Sunyaev 2016;Hofmann et al 2016). Although the vortices are very efficient in mixing hot bubble's gas with the ICM, the mixing does not necessary destroy the bubbles.…”