It's a bad week, research-wise. But I did chat with Bonaca (Harvard) this morning, and she showed that it is at least possible (not confirmed yet, but possible) that the dark substructure we infer from the GD-1 stream has kinematics consistent with it having fallen into the Milky Way along with the Sagittarius dwarf galaxy. This, if true, could lead to all sorts of new inferences and measurements.
Reminder: The idea is that there is a gap and spur in the stream, which we think was caused by a gravitational interaction with an unseen, compact mass. We took radial-velocity data which pin down the kinematics of that mass, and put joint constraints on mass, velocity, and timing. Although these constraints span a large space, it would still be very remarkable, statistically, if the constraints overlap the Sagittarius galaxy stream.
Philosophically, this connects to interesting ideas in inference: We can assume that the dark mass has nothing to do with Sag. This is conservative, and we get conservative constraints on its properties. Or we can assume that it is associated with Sag. This is not conservative, but if we make the assumption, it will improve enormously what we can measure or predict. It really gets at the conditionality or subjectivity of inference.
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