## 5. Results and discussionResults of the preceding analyses are summarized in Tables 3 and 4, for both the individual fields and the combined survey of seven fields. In the following subsections, the optimized structural parameters found for the density models of all four Galactic components and the metallicity structures of the thin disk and halo are discussed in turn, before we finally present our preliminary conclusions on the metallicity structure of the thick disk component.
## 5.1. Optimized values and constraints: primary parametersThe determination of optimized parameter values and constraints is
based on the analysis of the -selection from a
large range of models, as described in Sect. 4.2. Due to the
systematic coverage of high latitudes, we expect the survey data to be
particularly suitable for measuring the scale heights of the old thin
and thick disks, and for providing reliable extrapolations of the
thick-disk and halo densities to the local volume near the sun. For
all the individual fields as well as for the combined survey of seven
fields, the -curves for these parameters do
indeed show the most pronounced minima, with growth rates increasing
to several times , fully vindicating the above
expectations. Thus, the most important conclusion to be drawn from the
results given in the bottom lines of Table 3 is that, apart from
its thin disk with canonical scale height ,
On the other hand, the present survey data are rather less sensitive to the two remaining primary parameters, the scale length of the old thin disk, , and the scale height of the young thin-disk dwarfs, . Over the full variation ranges explored for these parameters, the -curves have amplitudes which are of order or less only above their , and the corresponding frequency distributions for the good models are rather flat. Therefore, although optimized mean values could be derived with surprisingly little scatter from field to field, the actual constraints imposed on these values from the combined survey data are not very strong. ## 5.2. Consistency with external data: secondary parametersA gratifying result of the present analysis is that, upon
determination of optimized values and constraints for the On the other hand, due to the relatively limited survey range of the thin disk and the steep decline of the luminosity function toward bright stars, the present survey data are rather insensitive to the density parameters of the brighter dwarfs of the young thin disk and the giants of the old thin disk. Therefore, the mean values for these parameters (e.g., ) are not, in general, strongly constrained by the -curves and their associated frequency distributions. However, since the systematic coverage of many Galactic directions also provides adequate coverage of the different sensitivity ranges for all such low-sensitivity parameters - which also include the scale length of the thick disk, , and the halo parameters and -, the mean values derived from the data aggregate in seven fields are evidently derived consistent with independent investigations (Majewski 1993). Similarly, even though the present survey was
Thus, the "mean" metallicity suggested by the minimum of the global
-curve in Fig. 20 is fully consistent with the
On the other hand, because the present survey does not penetrate to
very faint magnitudes, the field
## 5.3. Preliminary constraints: metallicity structure of thick diskTable 4 summarizes the metallicity structure of the thick disk component that we find from the present (preliminary) analysis. As anticipated in 4.2.6, these results are derived assuming two different basic models: (1) for the chemically homogeneous model, we calculate a mean metallicity dex with dispersion dex, where dex is the dispersion intrinsic to the adopted color-magnitude and color-color relations and dex is the dispersion derived from the -curves and frequency distributions of the good models; (2) for the model including chemical gradients, the mean metallicities in each field are calculated from the best-fitting gradient solution and averaging all the individual stellar metallicities summed over the successive volumes. Interestingly, this model gives a marginally higher "mean" metallicity, dex, but with a somewhat larger dispersion dex which is slightly skewed toward lower metallicities. Clearly, the stronger derived gradients in fields 1, 3, and 6 of Table 4 predict lower metallicities for larger distances and, hence, for the implied larger count contributions, leading to lower "mean" metallicities for these fields as well. Even though the results of Table 4 do not yet provide a
definitive answer to the question of whether or not (a) gradient(s)
exist(s) in the thick disk - because the numerical data are still
consistent with either of the above model assumptions on the two-sigma
level -, what they seem to indicate however is that chemical
© European Southern Observatory (ESO) 1998 Online publication: March 3, 1998 |