To Circularize or Not to Circularize? -- Orbital Evolution of Satellite Galaxies

Abstract

We investigated the orbital evolution of satellite galaxies using numerical simulations. It has been long believed that the orbit suffers circularization due to the dynamical friction from the galactic halo during orbital decay. This circularization was confirmed by numerous simulations where dynamical friction is added as external force. However, some of the resent N-body simulations demonstrated that circularization is much slower than expected from approximate calculations. We found that the dominant reason for this discrepancy is the assumption that Coulomb logarithm log Lambda is constant, which has been used in practically all recent calculations. Since the size of the satellite is relatively large, accurate determination of the outer cutoff radius is crucial to obtain good estimate for the dynamical friction. An excellent agreement between $N$-body simulations and approximate calculations was observed when the outer cutoff radius is taken to be the distance of the satellite to the center of the galaxy. When satellite is at the perigalacticon, the distance to the center is smaller and therefore log Lambda becomes smaller. As a result, the dynamical friction becomes less effective. We apply our result to the Large Magellanic Cloud. We found that the expected lifetime of the LMC is twice as long as that would be predicted with previous calculations. Previous study predicts that the LMC will merge into the Milky Way after 7 G years, while we found that the merging will take place after 14 G years from now. Our result suggests that generally satellites formed around a galaxy have longer lifetime than previous estimates.