New light on faint stars – VI. Structure and evolution of the Galactic spheroid

Recent observations of the structure of the Galactic spheroid are analysed and confirm the existence of a very flattened component of the spheroid. This component contains ∼ 2 per cent of the stars in the solar neighbourhood, but is not that identified in previous studies of high-velocity subdwarfs. It is distributed either in a flattened spheroid, axis ratio ∼ 1: 4, or as an exponential perpendicular to the plane, with scale height ∼ 1.5 kpc. It is photometrically distinct from the Galactic old disc (scale height ∼ 300 pc), in that the spatial density distribution changes abruptly near z = 1.5 kpc. This population is therefore a spheroidal population, not a part of the old disc. Its kinematic properties are, however, distinct from those of high-velocity subdwarfs, which show little or no net rotation and large anisotropic velocity dispersions. The inner spheroid in external spiral galaxies, and by implication our Galaxy, shows a smooth transition between disc and spheroid in kinematic properties with increasing distance from the plane. This model is consistent with all well-calibrated published star count data. We interpret these observations in a model in which the stars which dominate the outer spheroid formed before or early in the Galactic collapse. Debris from this period collapsed into a pressure supported disc with temperature ∼ 104 K, corresponding to a z velocity dispersion ∼ 60 km s−1. Those stars which formed in this disc make up the inner, rotating, flattened spheroid, while the remnant of this ‘thick disc’ and metal-rich ejecta from the other spheroidal stars dissipationally cooled into the Galactic thin disc.