The measurement of the structure of stellar populations in the Milky Way disk places fundamental constraints on models of galaxy formation and evolution .
Previously , the disk ’ s structure has been studied in terms of populations defined geometrically and/or chemically , but a decomposition based on stellar ages provides a more direct connection to the history of the disk , and stronger constraint on theory .
Here , we use positions , abundances and ages for 31,244 red giant branch stars from the SDSS-APOGEE survey , spanning 3 < R _ { \mathrm { gc } } < 15 kpc , to dissect the disk into mono-age and mono- \mathrm { [ Fe / H ] } populations at low and high \mathrm { [ } \alpha \mathrm { / Fe ] } .
For each population , with \Delta \mathrm { age } < 2 Gyr and \Delta \mathrm { [ Fe / H ] } < 0.1 dex , we measure the structure and surface-mass density contribution .
We find that low \mathrm { [ } \alpha \mathrm { / Fe ] } mono-age populations are fit well by a broken exponential , which increases to a peak radius and decreases thereafter .
We show that this profile becomes broader with age , interpreted here as a new signal of disk heating and radial migration .
High \mathrm { [ } \alpha \mathrm { / Fe ] } populations are well fit as single exponentials within the radial range considered , with an average scale length of 1.9 \pm 0.1 kpc .
We find that the relative contribution of high to low \mathrm { [ } \alpha \mathrm { / Fe ] } populations at R _ { 0 } is f _ { \Sigma } = 18 \% \pm 5 \% ; high \mathrm { [ } \alpha \mathrm { / Fe ] } contributes most of the mass at old ages , and low \mathrm { [ } \alpha \mathrm { / Fe ] } at young ages .
The low and high \mathrm { [ } \alpha \mathrm { / Fe ] } populations overlap in age at intermediate \mathrm { [ Fe / H ] } , although both contribute mass at R _ { 0 } across the full range of \mathrm { [ Fe / H ] } .
The mass weighted scale height h _ { Z } distribution is a smoothly declining exponential function .
High \mathrm { [ } \alpha \mathrm { / Fe ] } populations are thicker than low \mathrm { [ } \alpha \mathrm { / Fe ] } , and the average h _ { Z } increases steadily with age , between 200 and 600 pc .