Observational signatures of massive black hole progenitor pathways: could Leo I be a smoking gun?

Abstract

ABSTRACT 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 that 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 an 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-centred MBHs and a central MBH would represent strong evidence of a heavy seeding pathway.