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

B. Epinat, T. Contini, H. Finley, L. A. Boogaard, A. Guerou, J. Brinchmann, D. Carton, L. Michel-Dansac, R. Bacon, S. Cantalupo, C. M. Carollo, S. Hamer, W. Kollatschny, D. Krajnović, R. A. Marino, J. Richard, G. Soucail, P. Weilbacher, L. Wisotzki

Abstract
We report the discovery of a 10(4) kpc(2) gaseous structure detected in [OII] lambda lambda 3727, 3729 in an over-dense region of the COSMOS-Gr30 galaxy group at z similar to 0.725 thanks to deep MUSE Guaranteed Time Observations. We estimate the total amount of diffuse ionised gas to be of the order of (similar to 5 +/- 3)x10(10) M-circle dot 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 of group members embedded in this structure from 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 thanks to line diagnostics (R23, O32 and [O III]/H beta) available within the MUSE wavelength range. We compared these diagnostics to photo-ionisation models and shock models. The structure is divided in two kinematically distinct sub-structures. The most extended sub-structure of ionised gas is likely rotating around a massive galaxy and displays filamentary patterns linking some galaxies. The second sub-structure links another massive galaxy hosting an Active Galactic Nucleus to a low mass galaxy but also extends orthogonally to the AGN host disk over similar to 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 large velocity dispersions. One edge-on galaxy shares the same properties and may be a source of shocks. Whatever 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.

Keywords
galaxies: evolution; galaxies: kinematics and dynamics; intergalactic medium; galaxies: interactions; galaxies: groups: general; galaxies: high-redshift; Astrophysics - Astrophysics of Galaxies

Astronomy and Astrophysics
Volume 609, Article Number A41
2018 January

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia