  Observational evidence is mounting regarding the population demographics of
Massive Black Holes (MBHs), from the most massive cluster galaxies down to the
dwarf galaxy regime. However, the progenitor pathways from which these central
MBHs formed remain unclear. Here we report a potentially powerful observational
signature of MBH formation in dwarf galaxies. We argue that a continuum in the
mass spectrum of MBHs in (fossil) dwarf galaxies would be a unique signature of
a heavy seed formation pathway. The continuum in this case would consist of the
usual population of stellar mass black holes, formed through stellar evolution,
plus a smaller population of heavy seed MBHs which have not yet sunk to the
centre of the galaxy. Under the robust assumption of initial fragmentation of
the parent gas cloud resulting in a burst of heavy seed production, a
significant fraction of these seeds will survive to the present day as
off-nuclear MBHs with masses less than that of the central object. Motivated by
the recent discovery of a MBH in the relatively low central density Leo I
galaxy, we show that such a continuum in MBH seed masses should persist from
the lightest black hole masses up to the mass of the central MBH in contrast to
the light seeding scenario where no such continuum should exist. The detection
of off-centered MBHs and a central MBH would represent strong evidence of a
heavy seeding pathway.
