Searching for the GD-1 Stream Progenitor in Gaia DR2 with Direct N-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 reveals the GD-1 stream extends over $\approx 100^\circ$, allowing us to determine the stream's leading and trailing ends. Our models suggest the length of the stream is consistent with a dynamical age of approximately 3400 Myr and the exact length and location of the GD-1 stream correspond to the stream's progenitor being located between $-30^\circ < \phi_{1,\mathrm{pro}} < -45^{\circ}$ in the standard GD-1 coordinate system. The model stream density profiles reveal that intact progenitors leave a strong over-density, 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's density profile yields two possible scenarios for its progenitor's history: a) the progenitor reached dissolution approximately 500 Myr ago during the cluster's last perigalactic pass and is both located at and responsible for the observed gap at $\phi_1=-40^{\circ}$ or b) the progenitor reached dissolution over 2.5 Gyr ago, the fully-dissolved remnant is at $-30^\circ < \phi_1 < -45^{\circ}$, and an observational signature of its location no longer exists. That the dissolved progenitor is in the range $-30^\circ < \phi_1 < -45^{\circ}$ implies that density fluctuations outside of this range, e.g., a deep gap at $\phi_1 \approx -20^\circ$, are likely produced by compact baryonic or dark-matter perturbers.