Ionised gas structure of 100 kpc in an over-dense region of the galaxy group COSMOS-Gr30 at z ~ 0.7^⋆

¹ Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse, CNRS, UPS, CNES, 13013 Toulouse, France
e-mail: benoit.epinat@lam.fr
² Aix-Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, 13013 Marseille, France
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁵ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Univ. Lyon, Univ. Lyon 1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR 5574, 69230 Saint-Genis-Laval, France
⁷ Institute for Astronomy, Department of Physics, ETH Zürich, Wolfgang-Pauli-Strasse, 27, 8093 Zürich, Switzerland
⁸ Institut fur Astrophysik, Universitat Gottingen, Friedrich-Hund Platz 1, 37077 Göttingen, Germany
⁹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

Received: 1 September 2017
Accepted: 27 October 2017

Abstract

We report the discovery of a 10⁴ kpc² gaseous structure detected in [O ii]λλ3727, 3729 in an over-dense region of the COSMOS-Gr30 galaxy group at z ~ 0.725 with deep MUSE Guaranteed Time Observations. We estimate the total amount of diffuse ionised gas to be of the order of (~5 ± 3) × 10¹⁰ M_⊙ and explore its physical properties to understand its origin and the source(s) of the ionisation. The MUSE data allow the identification of a dozen group members that are embedded in this structure through emission and absorption lines. We extracted spectra from small apertures defined for both the diffuse ionised gas and the galaxies. We investigated the kinematics and ionisation properties of the various galaxies and extended gas regions through line diagnostics (R23, O32, and [O iii]/Hβ) that are available within the MUSE wavelength range. We compared these diagnostics to photo-ionisation models and shock models. The structure is divided into two kinematically distinct sub-structures. The most extended sub-structure of ionised gas is likely rotating around a massive galaxy and displays filamentary patterns that link some galaxies. The second sub-structure links another massive galaxy that hosts an active galactic nucleus (AGN) to a low-mass galaxy, but it also extends orthogonally to the AGN host disc over ~ 35 kpc. This extent is likely ionised by the AGN itself. The location of small diffuse regions in the R23 vs. O32 diagram is compatible with photo-ionisation. However, the location of three of these regions in this diagram (low O32, high R23) can also be explained by shocks, which is supported by their high velocity dispersions. One edge-on galaxy shares the same properties and may be a source of shocks. Regardless of the hypothesis, the extended gas seems to be non-primordial. We favour a scenario where the gas has been extracted from galaxies by tidal forces and AGN triggered by interactions between at least the two sub-structures.