The outer regions of the Galactic bulge - II. Analysis

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Abstract
Velocity, chemical abundance and spatial distribution data for some 1500 K and M giants in the Galactic bulge. We provide three levels of analysis: an almost model-independent determination of basic symmetry properties of the Galatic bulge; a more detailed analysis based on conservative assumptions, which investigates the basic kinematics and chemical abundance distributions in terms of a few parameters; and a more detailed investigation of possible functional forms for the density and kinematic distributions that are consistent with the data using maximum-likelihood fits. The bulge has a well-determined linear rotation curve over the range 700 pc ≤R≤3500 pc, amplitude ~ 25 km s−1 kpc−1. Several velocity dispersion models are found to fit the data. We test the oblate isotropic bulge model of Kent by integrating the kinematics predicated by that model into our Galaxy model, and comparing with our data. We find reasonable agreement except in the most distant of our fields from the Galatic Center. We do not find a significant requirement for asymmetry or a bar from our kinematics. Our data allow a quantitative comparison between the Milky Way bulge and the spiral galaxy bulges studied by Kormendy & Illingworth over a similar Galactocentric distance range; the Galactic bulge appears to be characteristic of the extragalatic bulge population. We determine the metallicity distribution of K giants in that subset of our fields where the photometric calibration is adequate. Our metallicity index measures the Mg‘b’ features, and is calibrated against local fields standards. Thus our abundance scale assumes a similar relationship between [Mg/Fe] and [Fe/H] in the bulge to that which is seen in the solar neighbourhood. Given that assumption, the mean metallicity in these fields is [Fe/H]≈ −0.3 and does not vary significantly over the regions investigated. This mean is very close to the mean abundance gradient in the Galactic bulge over the Galactocentric range 500 pc≤RGC≤3500 pc. The overall bulge abundance distribution is in good agreement with a model that combines standard solar neighbourhood determinations of the abundance distribution functions for the halo and thick disc, together with a closed-box ‘simple model’ of chemical evolution with yield ~0.7 Z⊙ to describe the bulge component. We derive the distribution momentum for the bulge from our data, and compare it with determinations for the halo, the thick disc and the thin disk from Wyse & Gilmore. We confirm that the bulge and the halo have angular momentum distributions that are indistinguishable, as do the thick disc and the thin disc. The bulge-halo distribution is however, very different from the thick disc-thin disc distribution. This is perhaps the strongest available clue to the evolutionary relationships between different Galactic structural components.
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