Searching for the GD-1 stream progenitor inGaiaDR2 with directN-body simulations

We perform a large suite of direct N-body simulations aimed at revealing the location of the progenitor, or its remnant, of the GD-1 stream. Data from Gaia DR2 reveal that the GD-1 stream extends over ≈100°, allowing us to determine the stream’s leading and trailing ends. Our models suggest that the length of the stream is consistent with a dynamical age between 2 and 3 Gyr and the exact length, width, and location of the GD-1 stream correspond to the stream’s progenitor being located between −30° < ϕ1, pro < −45° in the standard GD-1 coordinate system. The model stream density profiles reveal that intact progenitors leave a strong overdensity, recently dissolved progenitors appear as gaps in the stream as escaped stars continue to move away from the remnant progenitor’s location, and long-dissolved progenitors leave no observational signature on the remaining stream. Comparing our models to the GD-1 stream yields two possible scenarios for its progenitor’s history: (a) the progenitor reached dissolution approximately 500 Myr ago during the cluster’s previous perigalactic pass and is both located at and responsible for the observed gap at ϕ1 = −40° or (b) the progenitor reached dissolution over 2.5 Gyr ago, the fully dissolved remnant is at −30° < ϕ1 < −45°, and an observational signature of its location no longer exists. That the dissolved progenitor is in the range −30° < ϕ1 < −45° implies that density fluctuations outside of this range, e.g. a deep gap at ϕ1 ≈ −20°, are likely produced by compact baryonic or dark-matter perturbers.