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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?

Guillard, P. and Appleton, P. N. and Boulanger, F. and Shull, J .M. and Lehnert, M. D. and Pineau des Forêts, G. and Falgarone, E. and Cluver, M. E. and Xu, C. K. and Gallagher, S. C. and Duc, P. A. (2022) 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? Astrophysical Journal, 925 (1). Art. No. 63. ISSN 0004-637X. doi:10.3847/1538-4357/ac313f.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Guillard, P.0000-0002-2421-1350
Appleton, P. N.0000-0002-7607-8766
Lehnert, M. D.0000-0003-1939-5885
Falgarone, E.0000-0003-0693-2477
Cluver, M. E.0000-0002-9871-6490
Xu, C. K.0000-0002-1588-6700
Gallagher, S. C.0000-0001-6217-8101
Duc, P. A.0000-0003-3343-6284
Alternate Title:Extremely broad Lyman-alpha line emission from the molecular intra-group medium in Stephan's Quintet: evidence for a turbulent cascade in a highly clumpy multi-phase medium?
Additional Information:© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 February 12; revised 2021 October 18; accepted 2021 October 18; published 2022 January 26. P.G. thank the Centre National d'Etudes Spatiales (CNES), the University Pierre and Marie Curie, and the "Programme National de Cosmologie and Galaxies" (PNCG) and the "Physique Chimie du Milieu Interstellaire" (PCMI) programs of CNRS/INSU for their financial supports. We thank Daniel Kunth and Brigitte Rocca for very useful physical and technical discussions about Lyα scattering. Support for Program number HST-GO-13321.001-A was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. PA thanks Guillermo Blanc (Carnegie Observatories) and Emily Freeland (formerly Texas A&M University) for providing assistance with observations, and software/data reduction associated with the Mitchell Spectrograph. This work used observations carried out under project number U020 (P.I. Guillard) with the IRAM NOEMA Interferometer, reduced and analyzed with the GILDAS software ( IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). We also thank the referee who has contributed to improve both the figures and content of the paper.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Centre National d’Études Spatiales (CNES)UNSPECIFIED
University Pierre and Marie CurieUNSPECIFIED
Programme National de Cosmologie and Galaxies (PNCG)UNSPECIFIED
Physique Chimie du Milieu Interstellaire (PCMI)UNSPECIFIED
Centre National de la Recherche Scientifique (CNRS)UNSPECIFIED
Institut national des sciences de l'Univers (INSU)UNSPECIFIED
Max Planck SocietyUNSPECIFIED
Instituto Geográfico Nacional (IGN)UNSPECIFIED
Subject Keywords:Galaxy groups; Hickson compact group; Galaxy mergers; Galaxy interactions; Interacting galaxies; Intergalactic medium phases; Intergalactic medium; Cool intergalactic medium; Galaxy evolution; Galaxy processes; Galaxy quenching; Interstellar dust
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Galaxy groups (597); Hickson compact group (729); Galaxy mergers (608); Galaxy interactions (600); Interacting galaxies (802); Intergalactic medium phases (814); Intergalactic medium (813); Cool intergalactic medium
Record Number:CaltechAUTHORS:20220203-317857000
Persistent URL:
Official Citation:P. Guillard et al 2022 ApJ 925 63
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:113253
Deposited By: George Porter
Deposited On:03 Feb 2022 22:35
Last Modified:03 Feb 2022 22:35

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