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The spatial distribution of mono-abundance populations (MAPs, selected in[Fe/H] and [Mg/Fe]) reflect the chemical and structural evolution in a galaxyand impose strong constraints on galaxy formation models. In this paper, we useAPOGEE data to derive the intrinsic density distribution of MAPs in the MilkyWay, after carefully considering the survey selection function. We find that asingle exponential profile is not a sufficient description of the Milky Way'sdisc. Both the individual MAPs and the integrated disc exhibit a broken radialdensity distribution; densities are relatively constant with radius in theinner Galaxy and rapidly decrease beyond the break radius. We fit the intrinsicdensity distribution as a function of radius and vertical height with a 2Ddensity model that considers both a broken radial profile and radial variationof scale height (i.e., flaring). There is a large variety of structuralparameters between different MAPs, indicative of strong structure evolution ofthe Milky Way. One surprising result is that high-$\alpha$ MAPs show thestrongest flaring. The young, solar-abundance MAPs present the shortest scaleheight and least flaring, suggesting recent and ongoing star formation confinedto the disc plane. Finally we derive the intrinsic density distribution andcorresponding structural parameters of the chemically defined thin and thickdiscs. The chemical thick and thin discs have local surface mass densities of5.62$\pm$0.08 and 15.69$\pm$0.32 ${\rm M_{\odot} pc^{-2}}$, respectively,suggesting a massive thick disc with a local surface mass density ratio betweenthick to thin disc of 36%.

The Milky Way tomography with APOGEE: intrinsic density distribution and structure of mono-abundance populations