FUV line emission, gas kinematics, and discovery of [Fe XXI] lambda 1354.1 in the sightline toward a filament in M87

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) similar to 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 Ly alpha, C II lambda 1335, and NV lambda 1238, finding upsilon(r) = 147 +/- 2 km s(-1), 138 +/- 18 km s(-1), and 148(-16)(+14) km s(-1) relative to M87, and line broadenings sigma(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 H alpha, CIV lambda 1549, and He II lambda 1640, 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] lambda 158 mu m flux observed by Herschel-PACS from this region implies the existence of a massive cold (T similar to 10(3) K) component in the filament which contains significantly more mass (M similar to 8000 M-circle dot within our r approximate to 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] lambda 1354 in emission at 4-5 sigma. This line is emitted from 1 keV (T approximate to 10(7) K) plasma, and we use it to measure the bulk radial velocity (upsilon(r) = -92(-22)(+34) km s(-1)) and velocity dispersion (sigma(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 CII lambda 1335 and Ly alpha.