Extremely Broad Lyα Line Emission from the Molecular Intragroup Medium in Stephan’s Quintet: Evidence for a Turbulent Cascade in a Highly Clumpy Multiphase Medium?

Abstract

We present Hubble Space Telescope Cosmic Origin Spectrograph (COS) UV line spectroscopy and integral-field unit (IFU) observations of the intragroup medium in Stephan’s Quintet (SQ). SQ hosts a 30 kpc long shocked ridge triggered by a galaxy collision at a relative velocity of 1000 km s−1, where large amounts of molecular gas coexist with a hot, X-ray-emitting, plasma. COS spectroscopy at five positions sampling the diverse environments of the SQ intragroup medium reveals very broad (≈2000 km s−1) Lyα line emission with complex line shapes. The Lyα line profiles are similar to or much broader than those of Hβ, [C ii]157.7 μm, and CO (1–0) emission. The extreme breadth of the Lyα emission, compared with Hβ, implies resonance scattering within the observed structure. Scattering indicates that the neutral gas of the intragroup medium is clumpy, with a significant surface covering factor. We observe significant variations in the Lyα/Hβ flux ratio between positions and velocity components. From the mean line ratio averaged over positions and velocities, we estimate the effective escape fraction of Lyα photons to be ≈10%–30%. Remarkably, over more than four orders of magnitude in temperature, the powers radiated by X-rays, Lyα, H2, and [C ii] are comparable within a factor of a few, assuming that the ratio of the Lyα to H2 fluxes over the whole shocked intragroup medium stay in line with those observed at those five positions. Both shocks and mixing layers could contribute to the energy dissipation associated with a turbulent energy cascade. Our results may be relevant for the cooling of gas at high redshifts, where the metal content is lower than in this local system, and a high amplitude of turbulence is more common.

The CO Emission in the Taffy Galaxies (UGC 12914/15) at 60 pc Resolution. I. The Battle for Star Formation in the Turbulent Taffy Bridge

“…They may be evidence for shocks associated with cloud collisions. Similar examples of very faint star formation embedded in shocked intergalactic emission-line gas exhibiting multiple line profiles is observed in Stephan's Quintet (see, for example, Xu et al 2003;Konstantopoulos et al 2014;Guillard et al 2022).…”

“…The other samples are the same as those described in Figure 9. (Guillard et al 2022) combined with previous observations of molecular hydrogen and X-rays (Appleton et al 2017;O'Sullivan et al 2009) show strong evidence that the dissipation of kinetic energy involves all gas phases. These are likely connected through a turbulent cascade involving atomic and molecular magnetohydrodynamic shocks (Lesaffre et al 2020;Durrive et al 2021), and turbulent mixing layers (Slavin et al 1993).…”

“…Additionally, there is evidence for shock-excited atomic emission-line gas in the bridge (Joshi et al 2019). Thus the CO-emitting clouds are likely immersed in a highly turbulent medium, similar to the intergalactic shocked filament in Stephan's Quintet (Appleton et al 2017;Guillard et al 2022). Vollmer et al (2021) discussed this in the context of their dynamical model of the Taffy system, which was tuned to provide a good match with the lower-resolution PdBI CO observations of the Taffy.…”

A 0.6 Mpc H i structure associated with Stephan’s Quintet

“…Stephan's Quintet (SQ, co-moving radial distance = 85 ± 6 Mpc, taken from the NASA/ IPAC Extragalactic Database (NED) 1 ) is unique among compact groups of galaxies [2][3][4][5][6][7][8][9][10][11][12] . Observations have previously shown that interactions between multiple members, including a high-speed intruder galaxy currently colliding into the intragroup medium, have probably generated tidal debris in the form of multiple gaseous and stellar filaments 6,8,13 , the formation of tidal dwarfs 7,14,15 and intragroup-medium starbursts 16 , as well as widespread intergalactic shocked gas 5,10,11,17 . The details and timing of the interactions and collisions remain poorly understood because of their multiple nature 18,19 .…”

Discovery of a 0.6 Mpc Diffuse HI Structure Associated with Stephan's Quintet

“…It is believed that in SQ, galaxy-galaxy and galaxy-IGM interactions have led to the complete removal of HI gas from its member galaxies 6,20 , a large-scale shocked lament ∼ 40 kpc in length 8 , an IGM starburst 9 , and long tidal tails (> 100 kpc) with tidal dwarf candidates 10 . It is a unique laboratory for studies of various processes associated with high-speed galaxy-galaxy/IGM collisions, such as the heating and cooling of warm H 2 gas powered by large scale shocks 11,12 , SFR quenching by turbulence in post-shock molecular gas 13 , and the possible transition of galaxies through the Green Valley (the evolutionary stage between star-forming galaxies and quiescent galaxies) during the evolution of compact groups 14,15 .A comprehensive understanding of the properties of SQ depends crucially on how accurately we know its interaction history, which is very complex due to the relatively large number (> 5) of participants.Theoretical simulations have yielded inconclusive and controversial results. The ducial model in Ref.…”