Blue horizontal branch field stars in the galactic halo - observations versus kinematic models

A kinematic analysis of a sample of 185 blue horizontal branch field (bhbf) stars situated in the galactic halo at galactocentric distances r < 40 kpc has been performed. The stars are locaied in four fields: the NGP/SGP, 22h, Fll7 and F867 fields |$[(l,b)=(0^\circ, \pm \enspace 90^\circ),(38^\circ,-51^\circ),(270^\circ,-45^\circ)\enspace\text {and}\enspace(352^\circ, 52^\circ)]$|⁠. The bhbf stars are found to constitute a well mixed system. The systemic rotation is found to be |$\upsilon_\text{rot}=10\pm24\enspace \text {km}\enspace \text s^{-1}$| relative to a galactic restframe, assuming |$\upsilon_\odot=220\enspace \text {km}\enspace \text s^{-1}$|⁠. The systemic rotation of the system of bhbf stars is thus dynamically insignificant. The observed line-of-sight velocity dispersions in the four fields have been compared with the predictions of dynamical halo models of White (W) and Sommer-Larsen (SL) and with the phenomenological model of Ratnatunga & Freeman (RF). The main result of the paper is that the SL model is found to provide a good fit to the kinematic data in all four fields, whereas neither the W model nor the RF model fits the data in all four fields. The NGP/SGP data turns out to be the most discrminatory with respect to the models. The SL model enjoys a further advantage over the other models in that it is spatially round, whereas the W model and (it is argued in the paper) the RF model correspond to spatially highly flattened systems inconsistent with the observational evidence that the halo is quite round. Possible formation scenarios for the galactic halo in relation to the SL model are discussed. A characteristic of the SL model is that the velocity distribution of halo stars is radially anisotropic in the inner halo |$(r\sim r_\odot)$|⁠, but tangemially anisotropic in the outer parts of the halo. It appears difficult to reconcile this feature of the SL model with a formation scenario in which the halo formed through satellite accretion and disruption. A formation scenario, which by including the effects of gas dynamical processes might be able to account for the above feature of the SL model, is described qualitatively. This scenario might also provide a natural explanation of the halo ‘second parameter’ problem. In ihe appendix some global kinematic properties of halo tracer systems, characterized by scale-free distribution functions, are derived.