The spheroids of galaxies before and after disc formation

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Abstract
Disc galaxies probably started out as undressed spheroids, the discs forming gradually over several billion years. We show that most spheroid stars are probably on non-resonant orbits, and that the actions characterizing these orbits are reasonably invariant when a massive disc is slowly inserted into the spheroid. We argue that if the proto-disc material initially had angular momentum that was not parallel to the minor axis of the spheroid, the spheroid will have slewed as the disc formed, very much as if it were a rigid body. In so far as the actions Ji are conserved during disc formation, the spheroid may be characterized by the same distribution function f(J) before and after disc formation. An investigation of systems with well-defined f(J) that are confined by flattened Stäckel potentials illuminates the present large-scale structure of the classic metal-poor populations of the solar neighbourhood, and enables us to infer the structure of these populations before the disc formed. We find that the velocity ellipsoids of these populations near the Sun imply that they must now form highly flattened (≈E7) bodies. Before the disc formed, they would have resembled an E3 galaxy that is flattened by velocity anisotropy. Test-particle integrations in more realistic potentials confirm these conclusions.

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