A universal minimal mass scale for present-day central black holes

Abstract: Intermediate-mass black holes (IMBHs) of mass $M_{\bullet} \approx 10^{2} - 10^{5}$ solar masses, $M_{\odot}$, are the long-sought missing link between stellar black holes, born of supernovae, and massive black holes, tied to galaxy evolution by the empirical $M_{\bullet}/\sigma_{\star}$ correlation. We show that low-mass black hole seeds that accrete stars from locally dense environments in galaxies following a universal $M_{\bullet}/\sigma_{\star}$ relation grow over the age of the Universe to be above ${\mathcal{M}}{0}\approx3\times10^{5}M{\odot}$ ($5\%$ lower limit), independent of the unknown seed masses and formation processes. The mass ${\mathcal{M}}{0}$ depends weakly on the uncertain formation redshift, and sets a universal minimal mass scale for present-day black holes. This can explain why no IMBHs have yet been found, and it implies that present-day galaxies with ${\sigma{\star}<{\mathcal{S}}_{0}\approx40\,\mathrm{km\,s}^{-1}}$ lack a central black hole, or formed it only recently. A dearth of IMBHs at low redshifts has observable implications for tidal disruptions and gravitational wave mergers.