Intermediate-mass Black Hole and stellar orbits in the Galactic Center

Abstract

Many young stars reside within the central half-parsec from SgrA*, the supermassive black hole in the Galactic Center. The origin of these stars remains a puzzle. Recently, Hansen and Milosavljevic (2003, HM) have argued that an Intermediate-Mass Black Hole (IMBH) could have delivered the young stars to the immediate vicinity of SgrA*. Here we focus on the final stages of the HM scenario. Namely, we integrate numerically the orbits of stars which are initially bound to the IMBH, but are stripped from it by the tidal field of SgrA*. Our numerical algorithm is a symplectic integrator designed specifically for the problem at hand; however, we have checked our results with SYMBA, a version of the widely available SWIFT code. We find that the distribution of the post-inspiral orbital parameters is sensitive to the eccentricity of the inspiraling IMBH. If the IMBH is on a circular orbit, then the inclinations of numerically computed orbits relative to the inspiral plane are almost always smaller than 10 degrees, and therefore (a) the simulations are in good agreement with the observed motions of stars in a clockwise-moving stellar disc, (b) the simulations never reproduce the orbits of stars outside this disc, which include those in the second thick ring of stars and the randomly oriented unrelaxed orbits of some of the S-stars. If the IMBH's orbital eccentricity is e=0.6, then approximately half of the stars end up with orbital inclinations below 10 degrees, and another half have inclinations anywhere between 0 and 180 degrees; this is somewhat closer to what's observed. We also show that if IRS13 cluster is bound by an IMBH, as has been argued by Maillard et. al. 2004, then the same IMBH could not have delivered all of the young stars to their present location.