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