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Astron. Astrophys. 348, 98-112 (1999)

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4. Discussion and conclusions

Since the important paper of Reid & Majewski (1993) on the analysis of deep star counts in the North Galactic Pole (NGP) field SA 57, a number of publications have appeared which are based on new analyses of the same and/or similar data from new Galactic survey work. Apart from the unidirectional survey of the NGP by Spagna et al. (1996), Robin et al. (1996) and Ng et al. (1997) also investigate several fields at intermediate latitudes along the Galactic meridian, including SA 54 and M5 (which are also part of the present survey). The first two of these recent studies make use of broad-band multicolor photometric and proper motion data, and all three of them provide analyses of the data in terms of appropriately consistent combinations of models including the kinematics, space density distributions, histories of star formation, and chemical evolution of the Galactic stellar populations.

Obviously, the combined aggregate of these survey data has large statistical weight which, along with the different methodologies chosen for the different analyses, should provide a substantial standard for assessing the reliability of the thick-disk picture that can currently be derived from such approaches. We shall thus present a brief comparison with the present work.

Determinations of relevant Galactic thick-disk properties are summarized in Table 5., where [FORMULA] is the local density normalization in percent of the local thin-disk density, d and h are the exponential scale length and scale height, respectively, and [FORMULA] is the mean metallicity. Generally speaking, the overall average parameter values resulting from the recent individual studies are still well described by the plain figures given in the review by Morrison (1996). In particular, the results of the present paper are in excellent agreement with this global picture of the thick disk - which is all the more remarkable as they were derived from photometric data only, i.e., without supplementary information from proper motions!


Table 5. Recent determinations of Galactic thick-disk properties.
1) Based on their Table 1B.
2) Same model also shown consistent with star count and proper motion data discussed in Soubiran (1993), Ojha et al. (1994a,b), Perrin et al. (1995), and Haywood et al. (1997a,b).
3) Study of North Galactic Pole (NGP) field exclusively.

The important next question then concerns the reliability of this global thick-disk model: how well do we now know the constraints to its parameters? Obviously in Table 5, there is (still) substantial dispersion among the different determinations of the scale height. In fact, the tabular data are (weakly) anti-correlated with the local density parameter in the sense that higher densities are associated with lower scale heights, or, for that matter, with lower Galactic latitudes. As anticipated in Sect. 3.2.2 above, this should however be expected if the star counts observed in a given field are fitted by a density model of the form [FORMULA], and is indeed also conspicuous in the resulting best-fitting data given for the seven individual fields of the present multi-directional survey in columns 6 ([FORMULA]) and 8 ([FORMULA]) of Table 4. On the other hand, as exposed in detail in Paper I and again in Sect. 3 above, calculation of the optimized parameter values for the combined survey explicitly accounts for this model dependency by including all model solutions that satisfy the constraint set by the (external) accuracy of the data for all seven fields simultaneously. Thus, the optimized mean parameter values are associated with relatively large dispersions, which however also appear to adequately accommodate the dispersions exhibited by the independent studies of Table 5.

In summary, the present data and analysis corroborate the larger-scale structural picture of the thick disk which has gradually emerged over the past years. We have demonstrated that, even though it is possible to describe this component by an average model specified by a unique set of parameter values, the actual constraints on these values, as derived from either the present data or from the current literature, do not appear to be very narrow. We cannot yet discard as insignificant the sizeable dispersions in local densities and, particularly, in scale heights and also in mean metallicities. If, indeed, their significance can be established by an appropriate [FORMULA]- test - which will be provided in Paper V based on the analysis of yet more comprehensive RGU data -, these larger dispersions may have to be taken as indications of nonuniformity in the physical and chemical makeup of the real thick disk - implying a likely nonmonotonic formation history of this component as well.

While such a conclusion would also be supported by the fact that we have found the thick disk to have a luminosity function which is distinctly different from those of the thin disk and halo, more definitive results on these issues can now be expected from the following next steps of this project.

  1. Analysis of the second catalog of new homogeneous RGU survey data in seven additional intermediate-latitude ([FORMULA]) fields will provide a more complete mapping and will allow us to obtain a more precise view of the "graininess" of the thick disk. In particular, two fields toward the center and anticenter directions should significantly enhance the reliability of the scale length determination, while from the distribution of the remaining fields we expect to gain a handle for breaking the "density-scale-height-degeneracy".

  2. Implementation of a complete synthetic photometry-based calibration and transformation model for the UBV and RGU systems will provide the capability for full exploitation of the metallicity sensitivity of the RGU data (i.e., primarily of the U-G color) essentially down to the limit of photographic noise. From the combined survey of homogeneous, three-dimensional [FORMULA] data in 14 fields, we then expect to detect the presence or confirm the absence of large-scale metallicity gradients reliably enough for a significant discrimination of formation scenarios.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999