Tiny galaxies and dark substructures: exploring the "dark" subhaloes in TNG50

TL;DR

This study uses the TNG50 simulation to explore the existence and distribution of dark matter subhaloes without luminous counterparts, finding they are common in galaxy clusters and may be detectable through gravitational lensing.

Contribution

It provides the first detailed analysis of dark subhaloes in the TNG50 simulation, highlighting their prevalence and potential observability in galaxy clusters.

Findings

Dark subhaloes are common in galaxy groups and clusters.

More massive dark subhaloes tend to be located farther from cluster centers.

Subhaloes with masses between 4.5×10^7 and 2.1×10^8 M_sun are promising targets for strong lensing searches.

Abstract

Dark matter haloes and subhaloes that host no luminous counterpart are predicted within our current understanding of galaxy formation within a $\Lambda$CDM 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 $\rm 10^{9.7} \ M_{\odot}$ that are entirely dark. However, under realistic effective surface brightness cuts of $\rm \leq 29 \ mag \ arcsec^{-2}$, the inference of the most massive dark subhalo in galaxy groups and clusters becomes $\rm M_{DM} \gtrsim 2 \times 10^{10} \ M_{\odot}$. 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. We find that subhaloes in the mass range of $\rm 4.5 \times 10^{7} \leq M_{DM}/M_{\odot} \leq 2.1\times 10^{8}$ 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.