If they weren’t gravitationally bound, they would have left the Milky Way billions of years ago. Also, “in the halo” doesn’t necessarily mean “far from the cen

If they weren’t gravitationally bound, they would have left the Milky Way billions of years ago. Also, “in the halo” doesn’t necessarily mean “far from the center of the galaxy”; it means “not orbiting in the disk”.[1] Some of these stars are closer to the galactic center than the Sun. [1] And further away from the center of the galaxy than the bulge — but the bulge is much smaller than the orbit of the Sun.