Extreme Tidal Stripping May Explain the Overmassive Black Hole in Leo I: a Proof of Concept

Leo I, at a distance of 255 kpc, is the most distant dwarf spheroidal galaxy of the Milky Way. A recent study found dynamical evidence of a supermassive black hole of $\sim 3 \times 10^{6} \, \rm M_\odot$ at its center. This black hole, comparable in mass to the Milky Way's Sgr A*, places the system >2 orders of magnitude above the standard $M_\bullet-M_{\star}$ relation. We investigate the possibility that Leo I's stellar system was originally much more massive, thus closer to the relation. Extreme tidal disruption from one or two close passages within the Milky Way's virial radius could have removed most of its stellar mass. A simple analytical model suggests that the progenitor of Leo I could have experienced a mass loss of $\sim 57\%$ from a single pericenter passage. This mass loss percentage increases to $\sim 78\%$ if the pericenter occurs at the lower limit current orbital reconstructions allow. Detailed N-body simulations show that the mass loss could reach $\sim 90\%$ with up to two pericenter passages. Despite very significant uncertainties in the properties of Leo I, we reproduce its current position and velocity dispersion, as well as the final stellar mass enclosed in 1 kpc ($\sim 5\times 10^6 \, \rm M_\odot$) within a factor <2. The most recent tidal stream produced is directed along our line of sight toward Leo I, making it challenging to detect. Evidence from this extreme tidal disruption event could be present in current Gaia data in the form of extended tidal streams.