Dark matter deficient galaxies in the Illustris flat-$\Lambda$CDM model structure formation simulation

Surveying dark matter deficient galaxies (those with dark matter mass to stellar mass ratio $M_{\rm dm}/M_{\rm star}<1$) in the Illustris simulation of structure formation in the flat-$\Lambda$CDM cosmogony, we find $M_{\rm star} \approx 2 \times 10^8\, M_\sun$ galaxies that have properties similar to those ascribed by \citet{vanDokkumetal2018a} to the ultra-diffuse galaxy NGC1052-DF2. The Illustris simulation also contains more luminous dark matter deficient galaxies. Illustris galaxy subhalo 476171 is a particularly interesting outlier, a massive and very compact galaxy with $M_{\rm star} \approx 9 \times 10^{10}\, M_\sun$ and $M_{\rm dm}/M_{\rm star} \approx 0.1$ and a half-stellar-mass radius of $\approx 2$ kpc. If the Illustris simulation and the $\Lambda$CDM model are accurate, there are a significant number of dark matter deficient galaxies, including massive luminous compact ones. It will be interesting to observationally discover these galaxies, and to also more clearly understand how they formed, as they are likely to provide new insight into and constraints on models of structure formation and the nature of dark matter.