Tiny galaxies and dark substructures: exploring the ‘dark’ subhaloes in TNG50

Dark matter haloes and subhaloes that host no luminous counterpart are predicted within our current understanding of galaxy formation within a ΛCDM paradigm. Observational tests, such as gravitational lensing, have made potential detections of such objects around individual galaxies as well as in galaxy groups and clusters. The question of whether or not a dim counterpart might reside in these objects remains an open question. We investigate this point using the TNG50-1 simulation of the IllustrisTNG project. Under the assumption of TNG50’s galaxy formation model, we do not find haloes or subhaloes above a total mass of 109.7 \ M that are entirely dark. However, under realistic effective surface brightness cuts of ≤ 29 \ mag \ arcsec⁻², the inference of the most massive dark subhalo in galaxy groups and clusters becomes MDM 2 × 10¹⁰ \ M. Concentrating on galaxy groups and clusters, we find that dark subhaloes are ubiquitous, with more massive dark subhaloes tending to preferentially reside further from the centers of clusters. Subhaloes in the mass range of 4.5 × 10⁷ ≤ MDM/ M ≤ 2.1× 10⁸ tend to be the most likely to reside in the strong lensing regions of galaxy groups and clusters, and argue that future dark subhalo searches should investigate this mass range.