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We report the discovery of a 10^4 kpc^2 gaseous structure detected in [OII]in an over-dense region of the COSMOS-Gr30 galaxy group at z~0.725 thanks todeep MUSE Guaranteed Time Observations. We estimate the total amount of diffuseionised gas to be of the order of (~5+-3)x10^10 Msun and explore its physicalproperties to understand its origin and the source(s) of the ionisation. TheMUSE data allow the identification of a dozen of group members embedded in thisstructure from emission and absorption lines. We extracted spectra from smallapertures defined for both the diffuse ionised gas and the galaxies. Weinvestigated the kinematics and ionisation properties of the various galaxiesand extended gas regions thanks to line diagnostics (R23, O32 and[OIII]/H\beta) available within the MUSE wavelength range. We compared thesediagnostics to photo-ionisation models and shock models. The structure isdivided in two kinematically distinct sub-structures. The most extendedsub-structure of ionised gas is likely rotating around a massive galaxy anddisplays filamentary patterns linking some galaxies. The second sub-structurelinks another massive galaxy hosting an Active Galactic Nucleus to a low massgalaxy but also extends orthogonally to the AGN host disk over ~35 kpc. Thisextent is likely ionised by the AGN itself. The location of small diffuseregions in the R23 vs. O32 diagram is compatible with photo-ionisation.However, the location of three of these regions in this diagram (low O32, highR23) can also be explained by shocks, which is supported by their largevelocity dispersions. One edge-on galaxy shares the same properties and may bea source of shocks. Whatever the hypothesis, the extended gas seems to be nonprimordial. We favour a scenario where the gas has been extracted from galaxiesby tidal forces and AGN triggered by interactions between at least the twosub-structures.

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