We have performed the first detailed dissection of the stellar populations of the Milky Way disc in age, [Fe/H] and $\mathrm{[ \alpha \mathrm{/Fe]}}$ space, bridging the gap between the detailed observational understanding of MAPs (e.g. Bovy et al. 2012b, 2016b) and the plethora of studies of co-eval stellar populations in simulated galaxies (e.g Bird et al. 2013; Stinson et al. 2013; Martig et al. 2014a). We have placed novel constraints on models for the formation of the Milky Way disc by combining detailed density models fit to the mono-age, mono-[Fe/H] populations of the low and high $\mathrm{[ \alpha \mathrm{/Fe]}}$ disc, with surface mass density contributions calculated on the basis of these density fits and stellar evolution models. We summarize our key results as follows:
Radial and vertical profiles: The mono-age, mono-[Fe/H] populations of the $\mathrm{[ \alpha \mathrm{/Fe]}}$ poor disc are well fitted by a radially broken exponential, with a peak radius, Rpeak, that varies as a function of age and [Fe/H]. We find that the distance between Rpeak's of the low and high [Fe/H] populations increases with age, which we interpret as evidence for a decreasing [Fe/H] gradient with time (e.g. Anders et al. 2017). The radial variation of the stellar surface density of the high $\mathrm{[ \alpha \mathrm{/Fe]}}$ mono-age populations is found to have insignificant breaks, and they are better fit by a single exponential in this disc region. As these populations are the oldest, this may be a sign of the disc evolution washing out the density peak over time, or may point to a different formation scenario for high $\mathrm{[ \alpha \mathrm{/Fe]}}$ stars, where no density peak ever existed. These findings are in good agreement with earlier studies of MAPs (Bovy et al. 2016b). We measure an average high $\mathrm{[ \alpha \mathrm{/Fe]}}$ population scalelength of hR, in = 1.9 ± 0.1 kpc, and find scaleheights between 600 and 1000 pc, in good agreement with current measures of the $\mathrm{[ \alpha \mathrm{/Fe]}}$ rich disc scalelength and scaleheight (e.g. those outlined in Bland-Hawthorn & Gerhard 2016).Profile broadening: We show that the radial surface density profile of the low $\mathrm{[ \alpha \mathrm{/Fe]}}$ populations broadens with age in a given [Fe/H] bin, which we interpret as evidence of the gradual dispersal of mono-[Fe/H] populations, presumably due to radial migration and radial heating. The variation in shape of the broken exponential profile changes differently depending on the population [Fe/H], with low [Fe/H] populations inner profiles flattening faster, whereas the high [Fe/H] outer profiles flatten faster. We interpret this effect as tentative evidence for [Fe/H] dependent radial migration arising from pre-existing [Fe/H] gradients in the star-forming disc. We showed that our results qualitatively reproduce those of Hayden et al. (2015), finding a skewed MDF that varies as a function of R.Flaring: We find that flaring seems to be present in almost all mono-age populations, at differing levels. We have shown that the inverse flaring scalelength Rflare− 1 increases with age, meaning that the youngest populations flare most strongly. This finding appears inconsistent with that above, under the assumption that flaring is the result of radial migration. However, these results may be reconciled by invoking a more active accretion history in the early life of the disc, which could have suppressed flaring (e.g. Minchev et al. 2014b).The surface-mass density at R0: We have measured the surface mass density at the solar radius for each mono-age, mono-[Fe/H] population, finding a total surface mass density of $\Sigma _{R_0, {\rm tot}} = 20.0_{-2.9}^{+2.4}\mathrm{(stat.)}_{-2.4}^{+5.0}\mathrm{(syst.)}\ \mathrm{M_{{\odot }} \ pc^{-2}}$. Before allowing for systematics, this value is less than current estimates (e.g. Flynn et al. 2006; Bovy et al. 2012a; McKee et al. 2015), however, the systematic uncertainties are large, mainly due to a mismatch between the log g scales in APOGEE and the PARSEC models, and as such, we find our value to be consistent within the uncertainties. The relative contribution of high to low $\mathrm{[ \alpha \mathrm{/Fe]}}$ populations, $f_\Sigma$, is 18 per cent ± 5 per cent, which is consistent with existing measurements (e.g. Bland-Hawthorn & Gerhard 2016).The hZ distribution at R0: The shape of the mass-weighted hZ distribution found by this study is in good agreement with that of Bovy et al. (2012a), calling into question the existence of a vertical structural discontinuity in the Milky Way disc. The reconciliation of this finding with the discontinuity in chemical space (e.g. the bimodality in $\mathrm{[ \alpha \mathrm{/Fe]}}$ at fixed [Fe/H]: Nidever et al. 2014; Hayden et al. 2015) may shed new light on our understanding of the formation of the Galactic disc.The surface-mass density profile of the Milky Way: We have found the combined (from mono-age, mono-[Fe/H] populations at low and high $\mathrm{[ \alpha \mathrm{/Fe]}}$) surface-mass density-weighted profiles of the Milky Way disc as a function of $\mathrm{[ \alpha \mathrm{/Fe]}}$, age and [Fe/H], and found that the total surface density is also described by a broken exponential. We find that our results fail to determine the sign of the inner exponential to high significance out to ∼10 kpc, but detect a turnover to a declining exponential, at high significance, thereafter. We find evidence of a radial mean age and [Fe/H] gradient driven by the changing dominant population as a function of radius. A detailed comparison of these findings with numerical simulations is necessary for a proper interpretation. Our finding of a decline in stellar density may be consistent with that found in other studies (e.g. Reylé et al. 2009; Sale et al. 2010), albeit at shorter radii.
Citation: Bovy et al.; 2016b
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Citation: Bovy et al. 2016b
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