THE TILTED OLD GALACTIC DISC, AND THE POSITION OF THE SUN

With the use of data from COBE, IRAS and the Two-micron Galactic Survey (TMGS), we examine the effect on star counts and surface brightness maps of the position of the Sun above the Galactic plane and of a possible tilt between the b = 0 degrees plane and the plane of the old disc (to a distance of 3 or 4 kpc from the Sun). We find that the Sun is 15.5 +/- 3 pc above the plane, and that b = 0 degrees is tilted by 0 degrees.40 +/- 0 degrees.03. These results lead to errors of up to 20 per cent when comparing the flux at the same absolute latitude above and below the plane, and should be taken into account when comparing models with measurements. We find that the plane of the local disc is flat to within the errors and that a large part of the effects previously attributed to the warp could be explained by the tilt alone. This result indicates that the plane of the local old disc is tilted with respect to the average H I or stellar disc of the inner Galaxy (R < 7 kpc), suggesting that the old disc itself is corrugated.