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) project. The region considered covers 17.4 deg2 and lies within ⊢?⊢<5°, ⊢b⊢<1.°5. The adopted methods for deriving photometry and astrometry in these crowded images, together with an analysis of the deficiencies nevertheless remaining, are presented. The numbers of objects extracted in I, J and K are 748 000, 851 000 and 659 000 respectively, to magnitude limits of 17, 15 and 13. Eighty per cent completeness levels typically fall at magnitudes 16, 13 and 10 respectively, fainter by about 2 mag than the usual DENIS limits as a result of the 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 for the dust plane, from these data in limited directions, gives a scalelength 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 value is likely to be affected by localized dust asymmetries. We convolve a detailed model of the systematic and random errors in the photometry with a simple model of the Galactic disc and dust distribution to simulate expected colour–magnitude diagrams. These are in good agreement with the observed diagrams, allowing us to isolate those stars from the inner disc and bulge. After correcting for local dust-induced asymmetries, we find evidence for longitude-dependent asymmetries in the distant J and K sources, consistent with the general predictions of some Galactic bar models. We consider complementary L-band observations in the companion paper.