The photometric structure of the inner Galaxy

The light distribution in the inner few kiloparsecs of the Milky Way isrecovered non-parametrically from a dust-corrected near-infrared COBE/DIRBEsurface brightness map of the inner Galaxy. The best fits to the photometry areobtained when the Sun is assumed to lie $\sim14\pm4\pc$ below the plane. Therecovered density distributions clearly show an elongated three-dimensionalbulge set in a highly non-axisymmetric disk. In the favoured models, the bulgehas axis ratios $1{:}0.6{:}0.4$ and semi-major axis length $\sim2\kpc$. Itsnearer long axis lies in the first quadrant. The bulge is surrounded by anelliptical disk that extends to $\sim2\kpc$ on the minor axis and $\sim3.5\kpc$on the major axis. In all models there is a local density minimum $\sim2.2\kpc$down the minor axis. The subsequent maximum $\sim3\kpc$ down the minor axis(corresponding to $l\simeq-22\deg$ and $l\simeq 17\deg$) may be associated withthe Lagrange point L$_4$. From this identification and the length of thebulge-bar, we infer a pattern speed $\Omega_b\simeq 60-70\kms\kpc^{-1}$ for thebar. Experiments in which pseudo-data derived from models with spiral structurewere deprojected under the assumption that the Galaxy is either eight-fold orfour-fold symmetric, indicate that the highly non-axisymmetric disks recoveredfrom the COBE data could reflect spiral structure within the Milky Way if thatstructure involves density contrasts greater than $\gta 3$ at NIR wavelengths.These experiments indicate that the angle $\phi_0$ between the Sun--centre lineand a major axis of the bulge lies near $20\deg$.