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.
