The Post-periapsis Evolution of Galactic Center Source G1: The Second Case of a Resolved Tidal Interaction with a Supermassive Black Hole

We present new adaptive optics (AO) imaging and spectroscopic measurements of Galactic center source G1 from W. M. Keck Observatory. Our goal is to understand its nature and relationship to G2, which is the first example of a spatially resolved object interacting with a supermassive black hole (SMBH). Both objects have been monitored with AO for the past decade (2003-2014) and are comparatively close to the black hole (a min ∼ 200-300 au) on very eccentric orbits (e G1 ∼ 0.99; e G2 ∼ 0.96). While G2 has been tracked before and during periapsis passage (T 0 ∼ 2014.2), G1 has been followed since soon after emerging from periapsis (T 0 ∼ 2001.3). Our observations of G1 double the previously reported observational time baseline, which improves its orbital parameter determinations. G1's orbital trajectory appears to be in the same plane as that of G2 but with a significantly different argument of periapsis (Δω = 21 ± 4°). This suggests that G1 is an independent object and not part of a gas stream containing G2, as has been proposed. Furthermore, we show for the first time that (1) G1 is extended in the epochs closest to periapsis along the direction of orbital motion, and (2) it becomes significantly smaller over time (450 au in 2004 to less than 170 au in 2009). Based on these observations, G1 appears to be the second example of an object tidally interacting with an SMBH. G1's existence 14 yr after periapsis, along with its compactness in epochs further from the time of periapsis, suggest that this source is stellar in nature. © 2017. The American Astronomical Society. All rights reserved.