THE INNER GALAXY RESOLVED AT IJK USING DENIS DATA

We present the analysis of three-colour optical/near-infrared images, in IJK, taken for the DEep Near Infrared Southern Sky Survey (DENIS) p roject. The region considered covers 17.4 deg(2) and lies within \l\ < 5 degrees, \b\ <1.5 degrees. The adopted methods for deriving photomet ry and astrometry in these crowded images, together with an analysis o f the deficiencies nevertheless remaining, are presented. The numbers of objects extracted in I, J and K are 748 000, 851 000 and 659 000 re spectively, to magnitude limits of 17, 15 and 13. Eighty per cent comp leteness levels typically fall at magnitudes 16, 13 and 10 respectivel y, fainter by about 2 mag than the usual DENIS limits as a result of t he crowded nature of these fields. A simple model to describe the disc contribution to the number counts is constructed, and parameters for the dust layer are derived. We find that a formal fit of parameters fo r the dust plane, from these data in limited directions, gives a scale length and scaleheight of 3.4 +/- 1.0 kpc and 40 +/- 5 pc respectively , and a solar position 14.0 +/- 2.5 pc below the plane. This latter va lue is likely to be affected by localized dust asymmetries. We convolv e a detailed model of the systematic and random errors in the photomet ry with a simple model of the Galactic disc and dust distribution to s imulate expected colour-magnitude diagrams. These are in good agreemen t with the observed diagrams, allowing us to isolate those stars from the inner disc and bulge. After correcting for local dust-induced asym metries, we find evidence for longitude-dependent asymmetries in the d istant J and K sources, consistent with the general predictions of som e Galactic bar models. We consider complementary L-band observations i n the companion paper.