Ultra-diffuse galaxies (UDGs) are the lowest-surface-brightness galaxies known, with typical stellar masses of dwarf galaxies but sizes similar to those of larger galaxies such as the Milky Way1. The reason for their extended sizes is debated, with suggested internal processes such as angular momentum2, feedback3,4 or mergers5 versus external mechanisms6,7,8,9 or a combination of both10. Observationally, we know that UDGs are red and quiescent in groups and clusters11,12 whereas their counterparts in the field are blue and star-forming13,14,15,16. This dichotomy suggests environmental effects as the main culprits. However, this scenario is challenged by recent observations of isolated quiescent UDGs in the field17,18,19. Here we use the ΛCDM (or Λ cold dark matter, where Λ is the cosmological constant) cosmological hydrodynamical simulation to show that isolated quenched UDGs are formed as backsplash galaxies that were once satellites of another galactic, group or cluster halo but are today a few Mpc away from them. These interactions, albeit brief, remove the gas and tidally strip the outskirts of the dark matter haloes of the now quenched and seemingly isolated UDGs, which are born as star-forming field UDGs occupying dwarf-mass dark matter haloes. Quiescent UDGs may therefore be found in non-negligible numbers in filaments and voids, bearing the mark of past interactions as stripped outer haloes devoid of dark matter and gas compared to dwarfs with similar stellar content.
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This letter is based on snapshots, subhalo catalogues and merger trees from the cosmological hydrodynamical TNG50 simulation25,26 of the IllustrisTNG project37,38,39,40,41,42,43. These data are publicly available at https://www.tng-project.org/. ASCII tables with the simulation data for our sample of UDGs in Figs. 1, 2 and 4 are available in the public repository https://github.com/josegit88/public_data_files/tree/main/ascii_files_isolated_UDGs_TNG50. Source data are provided with this paper.
Scripts used for reading of and access to the snapshot, merger trees and subhalo data are publically available at the TNG database. Visualizations were made using the publicly available Py-SPHViewer code61. Any correspondence and/or request for materials pertaining to this manuscript should be directed to J.A.B.
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J.A.B. and M.G.A. acknowledge financial support from CONICET through PIP grant 11220170100527CO. L.V.S. is grateful for support from NSF grant CAREER-1945310 and NASA grant ATP-80NSSC20K0566. A.P. acknowledges support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through project 138713538 - SFB 881 (‘The Milky Way System’, subproject C09). D.N. acknowledges funding from the DFG through an Emmy Noether Research Group grant (NE 2441/1-1). F.M. acknowledges support through the programme ‘Rita Levi Montalcini’ of the Italian MUR. M.C. is partially supported by NSF grants AST-1518257 and AST-1815475. P.T. acknowledges support from NSF grants AST-1909933 and AST-200849 and from NASA ATP grant 80NSSC20K0502. M.V. acknowledges support from NASA ATP grants 16-ATP16-0167, 19-ATP19-0019, 19-ATP19-0020 and 19-ATP19-0167 and from NSF grants AST-1814053, AST-1814259, AST-1909831 and AST-2007355.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Arianna Di Cintio and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Source Data Fig. 1
ASCII files with data per column: stellar masses (M⊙) and three-dimensional half-mass radius (kpc).
Source Data Fig. 2
ASCII files with data per column: stellar masses (M⊙), colour (g−r) and star formation rate (M⊙ yr−1).
Source Data Fig. 4
ASCII files with data per column: stellar masses (M⊙), stellar ages (Gyr), κrot and virial masses (M⊙).
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Benavides, J.A., Sales, L.V., Abadi, M.G. et al. Quiescent ultra-diffuse galaxies in the field originating from backsplash orbits. Nat Astron 5, 1255–1260 (2021). https://doi.org/10.1038/s41550-021-01458-1
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