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Stars and Galaxies


Fig. 3 shows the distribution of stars. The most striking feature is the rapid steepening of the density gradient towards the Milky Way region at the right and left hand edges of the total field, apparent from the dramatic change in colour in this false colour presentation. Noticable changes in star density across even short distances are indicated by colour changes within individual plates. The overall impression is that of a smooth stellar background with no trace of large-scale structures.

[Fig. 3]Fig. 3 [Fig. 4]Fig. 4


The galaxies, whose distribution must be known to provide an observational stepping stone for cosmology, are shown in Fig. 4. The magnitudes for all plates are calibrated to a common zero point (some patchiness due to very deep magnitude limits can still be seen).

It is rewarding to see that the distributions of stars and galaxies are quite different - as was expected. It gives us some confidence that the tricky process of star/galaxy separation works well. Visual checks of the J-data suggest that the automatic star/galaxy separation yields errors smaller than 10 %.

An automized comparison of the data from the R-plates with those from the J-plates has the advantage that the classification of all objects can be checked, something which is not possible visually. Although the quality of the procedures is not tested by this comparison, the reliability with which the chosen routines work on the same object at different brightness levels and on plates taken under different observing conditions will be given. Another test is the comparison between automatically determined morphological types of galaxies on R- and J-plates. Its outcome will be more difficult to interpret because physical effects will also play a role. It will be interesting to see whether a colour dependence of morphological classification can be quantified.

Next: The stage and Up: The ESO Red Sky Previous: Artefacts