THE GALACTIC STRUCTURE TOWARDS THE GALACTIC-CENTER .1. A STUDY OF THE PALOMAR GRONINGEN FIELD 3

In this paper we address the analysis of the structure of the Galaxy towards the Centre. For this purpose we developed the so-called HRD-GST, the Hertzsprung-Russell Diagram Galactic Software Telescope, which simulates the stellar distribution in the line of sight. With the aid of the HRD-GST we analyze the stellar content in field #3 of the Palomar-Groningen Survey (PG3), located at the periphery of the Galactic Bulge. The observations are well described by a two component model with a disc and spheroid. There is no need to include a population from the so-called thick disc in the direction of field #3. The majority of the stars (B(J) < 18m) in this field are located in the disc. The main stellar population is well described with the following structural parameters for the scale height and scale length: (z0 = 250 pc; h0 = 4.5 kpc) or (z0 = pc; h0 = 3.5 kpc). For the metallicity and age we obtained: Z = 0.008-0.015 and t = 7.0-4.5 Gyr for an exponentially decreasing star formation rate with a characteristic time scale equal to 2.5 Gyr. At brighter magnitudes there is a significant contribution from a younger disc population. This population is described with the following structural and evolutionary parameters: (z0 = 100 pc; h0 = 4.5 kpc), Z = 0.015-0.020, and t = 5.0-2.0 Gyr for a constant star formation rate. A peak at B(J) = 16m.4 and B(J)-R(F) congruent-to 1m.5 in the Colour-Magnitude Diagram is identified with stars on the Horizontal Branch from a stellar population within the spheroid. Two stellar populations are the main contributors to the spheroidal component. The analysis is restricted to a maximum metallicity Z = 0.020. For the older and metal poorer halo population we use a R1/4 - law with flattening q = 0.60, Z = 0.0004-0.0050, and t = 16-10 Gyr for an exponentially increasing star formation rate with a characteristic time scale equal to 3.0 Gyr. For the younger and metal richer bulge population we used a power-law with index n = 4.0 and flattening q = 0.8, Z = 0.005-0.020, and t = 10-5.0 Gyr for an exponentially decreasing star formation rate with a characteristic time scale equal to 2.5 Gyr.