Density gradients in the galactic halo (SA 82 and SA 107).
Abstract
We give the results of an RG U three-colour photometric investigation in two further fields of the Basle halo programme: SA 82 and SA 107; with galactic latitudes of +66 and +41 , respectively, and with a galactic longitude of about 6 , they are the two first fields of this programme with a direction component towards the galactic centre. 988 and 1693 stars, on areas of 1.20 and 0.95 square degrees and down to limiting magnitudes in G of and 18w75, have been measured for SA 82 and SA 107, respectively. The mean errors of the catalogue magnitudes are given in table 1. There is no interstellar reddening in these fields. In the usual manner, the two-colour diagrams have been used to separate the populations. 210 and 311 disk stars, as well as 588 and 1071 halo stars have been used for the determination of the density and luminosity functions of SA 82 and SA 107, respectively. Reduced to the brightest common limiting magnitude in G of 18W75, the fraction of the halo stars increases continuously with decreasing latitudes approaching the galactic centre, starting in the direction to the galactic North Pole (SA 57) with 67%, and reaching 73% and 77% in the directions to SA 82 and SA 107, respectively. The colour-magnitude diagrams (figures 5 to 8), as well as the two-colour diagrams (figures 2 and 3) for both populations show principally the same behaviour as in the other fields of the programme. The two-colour diagrams are needed for the determination of the absolute magnitudes, also necessary for the derivation of the density functions for consecutive luminosity intervals, one magnitude wide. The halo densities (figures 9 and 10) are nearly constant for the absolutely brighter stars from 3 to 9 kpc, and steeply decreasing for the absolutely fainter stars down to a distance of 2 to 3 kpc from the Sun, the density values for SA 107, in general, being larger than for SA 82. The density gradients for the disk (figure 11) are negative for both fields. Here too, the densities in SA 107 exceed the corresponding values in SA 82. At a distance of 1.5 kpc, the halo densities begin to exceed the disk densities. Figures 12 and 13 show the luminosity functions of the luminosity interval 4 <M(G) <:8 for the distance intervals 01 kpc and 1-2 kpc, respectively. Compared with the luminosity function of the solar neighbourhood, the sharp decay of the functions for the halo with increasing luminosities is most conspicuous. Key word: galactic structure - halo
- Publication:
-
Astronomy and Astrophysics Supplement Series
- Pub Date:
- June 1977
- Bibcode:
- 1977A&AS...28..377B