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Ghostly Stellar Haloes and their Relationship to Ultra-faint Dwarfs

Ghostly stellar haloes are extended haloes of stars composed solely of debris of pre-reionization fossil galaxies and should exist in dwarf galaxies with total masses $<10^{10}$ M$_\odot$. Fossil galaxies are even smaller mass dwarf galaxies that stopped forming stars after the epoch of reionization and have been identified in the Local Group as the ultra-faint dwarf satellites. Using cosmological N-body simulations we present an empirical model for the shapes and masses of ghostly stellar haloes. We compare the model to available observations of stellar haloes in six isolated dwarf galaxies in the Local Group (Leo T, Leo A, IC 10, WLM, IC 1613, NGC 6822) to infer the star formation efficiency in dwarf galaxies at the epoch of reionization. We find an efficiency of star formation in dark matter haloes with masses $10^6 - 10^8$ M$_\odot$ at $z\sim7$ in rough agreement with independent methods using data on the luminosity function of ultra-faint dwarf galaxies but systematically higher by a factor of 3-5. The systematic uncertainty of our results is still large, mainly because available observations of stellar halo profiles do not extend over a sufficiently large distance from the center of the host dwarf galaxy. Additional observations, easily within reach of current telescopes, can significantly improve the accuracy of this method and can also be used to constrain the present day dark matter masses of dwarf galaxies in the Local Group. Our method is based on a set of observations never used before, hence it is a new independent test of models of hierarchical galaxy formation.