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

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 measure 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 +14 −16 km s −1 relative to M87, and line broadenings σ r = 171 ± 2 km s −1 , 189 +12 −11 km s −1 , and 128 +23 −17 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] λ158µm flux observed by Herschel-PACS from this region implies the existence of a massive cold (T ∼ 10 3 K) component in the filament which contains significantly more mass (M ∼ 8000M 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 ≈ 10 7 K) plasma, and we use it to measure the bulk radial velocity (v r = −92 +34 −22 km s −1 ) and velocity dispersion (σ r = 69 +79 −27 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.

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE)

“…Similar filamentary structures have also been observed in narrow-band Hα imaging (Arp 1967;Ford & Butcher 1979;Baum et al 1988;Heckman et al 1989;Sparks et al 1993Sparks et al , 2004Gavazzi et al 2000;Werner et al 2010), in different UV lines with HST (Sparks et al 2009, Anderson & Sunayaev 2018, and in the [CII]λ158 µm line with Herschel (Werner et al 2013), indicating the multi-temperature nature of the gas. In particular, the narrow-band Hα imaging data revealed a very complex filament extending to the southeast up to 8 kpc from the nucleus, with a few patchy regions, whose nature and origin are still not fully understood.…”

Ram Pressure Stripping in High-Density Environments

“…It is worth recalling that the coexistence of different gas phases, from the cold molecular gas, through the ionised phase, up to the hot plasma, has been also observed in other characteristic regions on the edges of galaxies and their surrounding ICM such as in cooling flows in NGC 1275 in Perseus (Conselice et al 2001;Salomé et al 2006) and M87 in Virgo (e.g. Forman et al 2007Sparks et al 2009Sparks et al , 2012Churazov et al 2008;Werner et al 2013;Simionescu et al 2017;Anderson & Sunyaev 2018;Boselli et al 2019b). These observations confirm that different gas phases at different temperature and density can cohabit the hostile intracluster environment.…”

Constraining Gas Motions in the Intra-Cluster Medium

“…More recently, these structures have been found to be multi-phase (e.g. Edge 2001; Salomé et al 2006;Sparks et al 2009;Salomé et al 2011;Mittal et al 2011Mittal et al , 2012Tremblay et al 2015;Anderson and Sunyaev 2018), consisting of gas from <100 K 10 4 K as detected through molecular CO, ionised C and O, and Ly-α emission lines, much of which has been observed to be extended and filamentary. In particular, the high spatial and spectral resolution of ALMA has been instrumental in linking the warm ionized and cold molecular gas phases and uncovering the total mass (most of which is in the cold phase) within the filamentary systems (e.g.…”