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Abridged: We estimate the distances to ~48 million stars detected by theSloan Digital Sky Survey and map their 3D number density distribution in 100 <D < 20 kpc range over 6,500 deg^2 of sky. The data show strong evidence for aGalaxy consisting of an oblate halo, a disk component, and a number oflocalized overdensities with exponential disk parameters (bias-corrected for anassumed 35% binary fraction) H_1 = 300 pc, L_1 = 2600 pc, H_2 = 900 pc, L_2 =3600 pc, and local density normalization of 12%. We find the halo to be oblate,with best-fit axis ratio c/a = 0.64, r^{-2.8} profile, and the localhalo-to-thin disk normalization of 0.5%. We estimate the errors of derivedmodel parameters to be no larger than ~20% (disk scales) and ~10% (thick disknormalization). While generally consistent with the above model, the densitydistribution shows a number of statistically significant localized deviations.We detect two overdensities in the thick disk region at (R, Z) ~ (6.5, 1.5)kpcand (R, Z) ~ (9.5, 0.8) kpc, and a remarkable density enhancement in the halocovering >1000deg^2 of sky towards the constellation of Virgo, at distances of~6-20 kpc. Compared to a region symmetric with respect to the l=0 line, theVirgo overdensity is responsible for a factor of 2 number density excess andmay be a nearby tidal stream or a low-surface brightness dwarf galaxy mergingwith the Milky Way. After removal of the resolved overdensities, the remainingdata are consistent with a smooth density distribution; we detect no evidenceof further unresolved clumpy substructure at scales ranging from ~50pc in thedisk, to ~1 - 2 kpc in the halo.Comment: 60 pages, 46 figures (reduced resolution; see the ApJ for hi-res  version

The Milky Way Tomography with SDSS. I. Stellar Number Density Distribution