POPULATION STUDIES .11. THE EXTENDED DISK, HALO CONFIGURATION

A comparison has been made between the predictions of various putative disk, halo configurations in the solar neighborhood, and the body of data from newly available kinematically and nonkinematically selected samples. It is argued that the continuous extended disk configuration [having a considerable tail to lower abundance (down to [Fe/H] approximately -1.5) and hotter kinematics (sigma-W approximately 50-60 km s-1) at this abundance] together with a kinematically decoupled halo, is better able to represent the available observational material than is a three discrete component model containing a thin disk, a thick disk, and a halo having parameters similar to those suggested in the literature. Specifically, while both types of model are capable of producing the observed overall abundance distributions and the dependence of kinematics on abundance for kinematically selected samples, the extended disk-halo configuration is superior to the preferred thin disk-thick disk, halo configuration (which has thick disk sigma-W = 40 km s-1, and a normalization 0.06 relative to the thin disk) in two important respects. First, the thin disk-thick disk, halo configuration predicts too much material in the range [Fe/H] < -0.5 in comparison which that observed for unbiased samples in the solar neighborhood (Gaussian abundance distributions having been assumed for thin and thick disks). Second, and perhaps more decisively, only the extended disk, halo configuration has the potential of explaining the apparently discrepant results of Ratnatunga and Freeman, on the one hand, who report a rotational lag of 30 km s-1 for material having distances from the Galactic plane z = 2-5 kpc and [Fe/H] > -0.7, and of Wyse and Gilmore, on the other, who for the same distance regime but covering all abundances, present data indicating a considerably higher rotational lag of 80 km s-1. This phenomenon is reconcilable in terms of a model in which the kinematics of the disk are a continuously varying function of abundance such as the advocated extended disk, halo configuration, but not with the discrete thin disk-thick disk ones suggested in the literature, which contain no dependence of kinematics on abundance for the thick disk. The favored configuration is suggestive of a Galactic cosmogony in which the extended disk emerged from a central contraction similar to that advocated by Eggen, Lynden-Bell, and Sandage and modeled by Larson, whereas the formation of the halo occurred within the discrete fragment framework of Searle and Zinn. The thicker component of the disk has properties similar to those produced by the type of gaseous infall postulated to explain the G-dwarf problem and which are inherent in the type of model presented by Larson. It is not clear that later stochastic heating processes or merger events in the disk are required to explain the wealth of observational material concerning the Galaxy's thicker disk material in the vicinity of the Sun.