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Wide-area deep imaging surveys have discovered large numbers of extremely lowsurface brightness dwarf galaxies, which challenge galaxy formation theory and,potentially, offer new constraints on the nature of dark matter. Here wediscuss one as-yet unexplored formation mechanism that may account for afraction of low surface brightness dwarfs. We call this the `ghost galaxy'scenario. In this scenario, inefficient radiative cooling prevents starformation in the `main branch' of the merger tree of a low mass dark matterhalo, such that almost all its stellar mass is acquired through mergers withless massive (but nevertheless star-forming) progenitors. Present-day systemsformed in this way would be `ghostly' isolated stellar halos with no centralgalaxy. We use merger trees based on the Extended Press-Schechter formalism andthe COCO cosmological N-body simulation to demonstrate that mass assemblyhistories of this kind can occur for low-mass halos in Lambda-CDM, but they arerare. They are most probable in isolated halos of present-day mass ~4x10^9M_sun, occurring for ~5 per cent of all halos of that mass under standardassumptions about the timing and effect of cosmic reionization. The stellarmasses of star-forming progenitors in these systems are highly uncertain;abundance-matching arguments imply a bimodal present-day mass function having abrighter population (median M_star ~3x10^6 M_sun) consistent with the tail ofthe observed luminosity function of ultra-diffuse galaxies. This suggestsobservable analogues of these systems may await discovery. We find that astronger ionizing background (globally or locally) produces brighter and moreextended ghost galaxies.

Ghostly galaxies: accretion-dominated stellar systems in low-mass dark matter halos