Structure in the first quadrant of the Galaxy: an analysis of TMGS star counts using the SKY model

We analyse the stellar content of almost 300 deg2 of the sky close to the Galactic plane by directly comparing the predictions of the SKY model (Cohen and Wainscoat et al.) with star counts taken from the Two Micron Galactic plane Survey (TMGS: Garzón et al.). Through these comparisons we can examine discrepancies between counts and model and thereby elicit an understanding of Galactic structure. Over the vast majority of areas in which we have compared the TMGS data with the SKY predictions we find very good accord; so good that we are able to remove the disc source counts to highlight structure in the plane. The exponential disc is usually dominant, but by relying on the predicted disc counts of SKY we have been able to probe the molecular ring, spiral arms, and parts of the bulge. The latter is clearly triaxial. We recognize a number of off-plane dust clouds not readily included in models. However, we find that, whilst the simple exponential extinction function works well in the outer Galaxy, closer than about 4 kpc to the Galactic Centre the extinction drops dramatically. We also examine the shape of the luminosity function of the bulge and argue that the cores of all spiral arms we have observed contain a significant population of supergiants that provides an excess of bright source counts over those of a simple model of the arms. Analysis of one relatively isolated cut through an arm near longitude 65 degrees categorically precludes any possibility of a sech2z stellar density function for the disc.