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

¹ Max-Planck Institute for Astrophysics, Garching bei Muenchen, Germany
e-mail: michevan@mpa-garching.mpg.de
² Space Research Institute (IKI), Russian Academy of Sciences, Profsoyuznaya 84/32, Moscow 117997, Russia
³ Institute for Advanced Study, 1 Einstein Drive, Princeton, NJ, USA

Received: 20 December 2017
Accepted: 1 June 2018

Abstract

We present new Hubble Space Telescope – Cosmic Origins Spectrograph (HST-COS) G130M spectroscopy which we have obtained for a sightline toward a filament projected 1.9 kpc from the nucleus of M87, near the edge of the inner radio lobe to the east of the nucleus. The combination of the sensitivity of COS and the proximity of M87 allows us to study the structure of this filament in unparalleled detail. We propose that the filament is composed of many cold clumps, each surrounded by an FUV-emitting boundary layer, with the filament having a radius r_c ~ 10 pc and the clumps filling the cylinder with a low volume filling factor. The observed velocity dispersion in emission lines from the filament results from the random motions of these clumps within the filament. We have measured fluxes and kinematics for emission lines of Lyα, C II λ1335, and N V λ1238, finding v_r = 147 ± 2 km s⁻¹, 138 ± 18 km s⁻¹, and 148₋₁₆⁺¹⁴ km s⁻¹ relative to M87, and line broadenings σ_r = 171 ± 2 km s⁻¹, 189₋₁₁⁺¹² km s⁻¹, and 128₋₁₇⁺²³ km s⁻¹ respectively. We associate these three lines, as well as archival measurements of Hα, C IV λ1549, and He II λ1640, with a multitemperature boundary layer around clumps which are moving with supersonic random motions in the filament. This boundary layer is a significant coolant of the hot gas. We show that the [C II] λ158 μm flux observed by Herschel-PACS from this region implies the existence of a massive cold (T ~ 10³ K) component in the filament which contains significantly more mass (M ~ 8000 M_⊙ within our r ≈ 100 pc sightline) than the FUV-emitting boundary layer. It has about the same bulk velocity and velocity dispersion as the boundary layer. We also detect [Fe XXI] λ1354 in emission at 4−5σ. This line is emitted from 1 keV (T ≈ 10⁷ K) plasma, and we use it to measure the bulk radial velocity v_r = −92₋₂₂⁺³⁴ km s⁻¹ and velocity dispersion σ_r = 69₋₂₇⁺⁷⁹ km s⁻¹ of the plasma at this temperature. In contrast to the intermediate-temperature FUV lines, [Fe XXI] is blueshifted relative to M87 and matches the bulk velocity of a nearby filament to the south. We hypothesize that this line arises from the approaching face of the radio bubble expanding through this sightline, while the filament lies on the receding side of the bubble. A byproduct of our observations is the detection of absorption from interstellar gas in our Galaxy, observed in C II λ1335 and Lyα.