The photometric structure of the inner Galaxy

The light distribution in the inner few kiloparsecs of the Milky Way is recovered non-parametrically from a dust-corrected near-infrared COBE/DIRBE surface brightness map of the inner Galaxy. The best fits to the photometry are obtained when the Sun is assumed to lie ∼14 ± 4 pc above the plane. The recovered density distributions clearly show an elongated three-dimensional bulge set in a highly non-axisymmetric disc. In the favoured models, the bulge has axis ratios 1:0.6:0.4 and semi-major axis length ∼2 kpc. Its nearer long axis lies in the first quadrant. The bulge is surrounded by an elliptical disc that extends to ∼2 kpc on the minor axis and ∼3.5 kpc on the major axis. In all models there is a local density minimum ∼2.2 kpc down the minor axis. The subsequent maximum ∼3 kpc down the minor axis (corresponding to l = -22° and l ≃ 17°) may be associated with the Lagrange point L₄. From this identification and the length of the bulge-bar, we infer a pattern speed Ω_b ≃ 60-70 km s^-1 kpc^-1 for the bar. Experiments in which pseudo-data derived from models with spiral structure were deprojected under the assumption that the Galaxy is either eight-fold or four-fold symmetric indicate that the highly non-axisymmetric discs recovered from the COBE data could reflect spiral structure within the Milky Way if that structure involves density contrasts greater than ≳3 at near-infrared wavelengths. These experiments indicate that the angle φ₀ between the Sun-centre line and a major axis of the bulge lies near 20°.