Why are quasi-periodic eruptions only found in low-mass galaxies?

Abstract

ABSTRACT I consider the current sample of galaxy nuclei producing quasi-periodic eruptions (QPEs). If the quasi-period results from the orbital motion of a star around the central black hole, the dearth of associated black hole masses ≳106 M⊙ places tight constraints on models. It disfavours those assuming wide orbits and small eccentricities, because there is ample volume within pericentre to allow significantly more massive holes in QPE systems than are currently observed. If instead the orbiting star is assumed to pass close to the black hole, the same lack of large black hole masses strongly suggests that the stellar orbits must be significantly eccentric, with 1 − e ≲ few × 10−2. This favours a tidal disruption near-miss picture where QPEs result from repeated accretion from an orbiting star (in practice a white dwarf) losing orbital angular momentum to gravitational radiation, even though this is not assumed in deriving the eccentricity constraint. Given the tight constraints resulting from the current small observed sample, attempts to find QPE systems in more massive galaxies are clearly important.