Densities and mass assembly histories of the Milky Way satellites are not a challenge to $\Lambda$CDM

Abstract

We use the \texttt{GRUMPY} galaxy formation model based on a suite of zoom-in, high-resolution, dissipationless $\Lambda$ Cold Dark Matter ($\Lambda$CDM) simulations of the Milky Way (MW) sized haloes to examine total matter density within the half-mass radius of stellar distribution, $\rho_{\rm tot}(<r_{1/2})$, of satellite dwarf galaxies around the MW hosts and their mass assembly histories. We compare model results to $\rho_{\rm tot}(<r_{1/2})$ estimates for observed dwarf satellites of the Milky Way spanning their entire luminosity range. We show that observed MW dwarf satellites exhibit a trend of decreasing $\rho_{\rm tot}(<r_{1/2})$ with increasing stellar mass. This trend is in general agreement with the trend predicted by the model. None of the observed satellites are overly dense compared to the results of our $\Lambda$CDM-based model. We also show that although the halo mass of many satellite galaxies is comparable to the halo mass of the MW progenitor at $z> 10$, at these early epochs halos that survive as satellites to $z=0$ are located many virial radii away from the MW progenitors and thus do not have a chance to merge with it. Our results show that neither the densities estimated in observed Milky Way satellites nor their mass assembly histories pose a challenge to the $\Lambda$CDM model. In fact, the broad agreement between density trends with the stellar mass of the observed and model galaxies can be considered as yet another success of the model.