A Recent Supermassive Black Hole Binary in the Galactic Center Unveiled by the Hypervelocity Stars

Abstract

When a binary of early-type stars from the young stellar populations in the Galactic center (GC) region is scattered to the vicinity of the supermassive black hole (SMBH) Sgr~$\rm{A}^{*}$, one of the components would be tidally ejected as an early-type hypervelocity star (HVS) and the counterpart would be captured on a tight orbit around Sgr~$\rm{A}^{*}$. Dozens of B-type HVSs moving faster than the Galactic escape speed have been discovered in the Galactic halo and are produced most likely by the SMBH Sgr~$\rm{A}^{*}$. However, the velocity distribution and in particular the deficit of the HVSs above $700\, \rm{km\, s^{-1}}$ is seriously inconsistent with the expectations of the present models. Here we show that the high-velocity deficit is due to the deficiency in close interactions of stars with the SMBH Sgr~$\rm{A}^{*}$, because an orbiting intermediate-mass black hole (IMBH) of about 15,000 Solar mass kicked away slowly approaching stars 50--250 million years ago. The SMBH-IMBH binary formed probably after the merger of the Milky Way with the Gaia-Sausage-Enceladus dwarf galaxy, and coalesced about 10 million years ago, leading to a gravitational recoil of Sgr~$\rm{A}^{*}$ at a velocity of 0.3--0.5$\, \rm{km\, s^{-1}}$ and to a change of the HVS ejection scenarios. The SMBH-IMBH binary scenario predicts the formation of the S-star cluster at the GC with the distribution of the orbital size and stellar ages that are well consistent with the observations.