The Swansong of the Galactic Center Source X7: An Extreme Example of Tidal Evolution near the Supermassive Black Hole

Abstract

We present two decades of new high-angular-resolution near-infrared data from the W. M. Keck Observatory that reveal extreme evolution in X7, an elongated dust and gas feature, presently located half an arcsecond from the Galactic Center supermassive black hole. With both spectro-imaging observations of Br-γ line emission and Lp (3.8 μm) imaging data, we provide the first estimate of its orbital parameters and quantitative characterization of the evolution of its morphology and mass. We find that the leading edge of X7 appears to be on a mildly eccentric (e ∼ 0.3), relatively short-period (170 yr) orbit and is headed toward periapse passage, estimated to occur in ∼2036. Furthermore, our kinematic measurements rule out the earlier suggestion that X7 is associated with the stellar source S0-73 or with any other point source that has overlapped with X7 during our monitoring period. Over the course of our observations, X7 has (1) become more elongated, with a current length-to-width ratio of 9, (2) maintained a very consistent long-axis orientation (position angle of 50°), (3) inverted its radial velocity differential from tip to tail from −50 to +80 km s−1, and (4) sustained its total brightness (12.8 Lp magnitudes at the leading edge) and color temperature (425 K), which suggest a constant mass of ∼50 M Earth. We present a simple model showing that these results are compatible with the expected effect of tidal forces exerted on it by the central black hole, and we propose that X7 is the gas and dust recently ejected from a grazing collision in a binary system.