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.
Highlights
The environment is expected to play a major role in the galaxy mass assembly processes, star formation quenching, and morphological transformation of galaxies
We report the serendipitous discovery of a new large ionised gas structure observed in an over-dense region of a galaxy group at redshift z ∼ 0.72 with the Multi Unit Spectroscopic Explorer (MUSE; Bacon et al 2015)
Using the various measurements enabled by our MUSE data, we can constrain the physical properties of the ionised gas such as
Summary
The environment is expected to play a major role in the galaxy mass assembly processes, star formation quenching, and morphological transformation of galaxies. We report the serendipitous discovery of a new large ionised gas structure observed in an over-dense region of a galaxy group at redshift z ∼ 0.72 with the Multi Unit Spectroscopic Explorer (MUSE; Bacon et al 2015). This group is located in a larger scale structure that is identified as the COSMOS-Wall (Iovino et al 2016). The sensitivity, field of view, spectral range, and resolution of the MUSE integral field spectrograph allows the mapping of both emission lines fluxes and kinematics We use this capability here to infer the mass, origin, and sources of ionisation of the extended gas. Part of these ancillary datasets are used for the analysis and interpretation of the extended gas region
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