Description
The last decade has seen a rise in the number of known Milky Way (MW) satellites, primarily thanks to the discovery of ultrafaint systems at close distances. These findings suggest a higher abundance of satellites within $\sim30$kpc than predicted by cosmological simulations of MW-like halos in the CDM framework. If taken at face value, this discrepancy implies that halos as small as V$_{\rm peak}\sim10$ km/s should host galaxies, challenging CDM and conventional galaxy formation models based on atomic hydrogen cooling.
However, recent studies have highlighted that numerical simulations suffer from artificial subhalo disruption, leading to the premature loss of galaxies due to tidal stripping (so-called “orphan” galaxies).
We use the Aquarius MW-halo simulations combined with the GALFORM semi-analytic galaxy formation model to track orphan galaxies and estimate the true abundance and radial distribution of MW satellites expected in LCDM.
We estimate present-day luminosities and sizes by applying a stellar stripping model based on CDM “tidal tracks”. Strikingly, orphans make up half of all satellites in our highest-resolution run (m$_{p}\sim10^3$M$_\odot$), primarily occupying the central regions of the MW halo. We predict that dozens of satellites should be observable within ~30 kpc of the MW, awaiting discovery through deep-imaging surveys like LSST.
Our unprecedented results resolve any tension with current observations, and evidence the crucial need for "orphan" modelling in studies of the MW's ultrafaint regime. The use of semi-analytic galaxy formation models will be indispensable for the next generation of ultra-high-resolution MW simulations, where hydrodynamical counterparts will remain unfeasible.
