Chemical Cartography with APOGEE: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk (1503.02110v1)

Abstract: Using a sample of 69,919 red giants from the SDSS-III/APOGEE Data Release 12, we measure the distribution of stars in the [$\alpha$/Fe] vs. [Fe/H] plane and the metallicity distribution functions (MDF) across an unprecedented volume of the Milky Way disk, with radius $3<R\<15$ kpc and height $|z|\<2$ kpc. Stars in the inner disk ($R\<5$ kpc) lie along a single track in [$\alpha$/Fe] vs. [Fe/H], starting with $\alpha$-enhanced, metal-poor stars and ending at [$\alpha$/Fe]$\sim0$ and [Fe/H]$\sim+0.4$. At larger radii we find two distinct sequences in [$\alpha$/Fe] vs. [Fe/H] space, with a roughly solar-$\alpha$ sequence that spans a decade in metallicity and a high-$\alpha$ sequence that merges with the low-$\alpha$ sequence at super-solar [Fe/H]. The location of the high-$\alpha$ sequence is nearly constant across the disk, however there are very few high-$\alpha$ stars at $R\>11$ kpc. The peak of the midplane MDF shifts to lower metallicity at larger $R$, reflecting the Galactic metallicity gradient. Most strikingly, the shape of the midplane MDF changes systematically with radius, with a negatively skewed distribution at $3<R\<7$ kpc, to a roughly Gaussian distribution at the solar annulus, to a positively skewed shape in the outer Galaxy. For stars with $|z|\>1$ kpc or [$\alpha$/Fe]$>0.18$, the MDF shows little dependence on $R$. The positive skewness of the outer disk MDF may be a signature of radial migration; we show that blurring of stellar populations by orbital eccentricities is not enough to explain the reversal of MDF shape but a simple model of radial migration can do so.

• The paper demonstrates that the Milky Way disk exhibits distinct chemical evolution tracks between inner and outer regions.
• It analyzes metallicity distribution functions using APOGEE observations of nearly 70,000 red giants across 3–15 kpc and ±2 kpc vertically.
• The study finds that radial migration significantly shapes the metallicity profiles, challenging traditional closed-box models.

Chemical Cartography with APOGEE: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk

The paper "Chemical Cartography with APOGEE: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk" presents an extensive analysis of the metallicity distribution functions (MDF) and chemical structure of the Milky Way disk, leveraging data from 69,919 red giants from the SDSS-III/APOGEE Data Release 12. This paper provides detailed insights into the metallicity and chemical composition across a significant range of the Milky Way, with radial coverage from 3 to 15 kpc and vertical coverage up to 2 kpc above and below the galactic plane.

The authors focus on the distribution of stars in the [$\alpha$/Fe] versus [Fe/H] plane, revealing critical differences in the chemical composition of the inner and outer disk regions. In the inner disk, stars predominantly lie along a single evolutionary track, transitioning from $\alpha$-enhanced, metal-poor stars to solar $\alpha$ ratios and higher metallicity ([Fe/H]~+0.4). Conversely, at larger Galactocentric radii, the paper identifies two distinct sequences in the [$\alpha$/Fe] versus [Fe/H] space—a solar-$\alpha$ sequence spanning a broad metallicity range and a high-$\alpha$ sequence that merges with the lower-alpha sequence at super-solar metallicities (observed mainly at R<11 kpc).

A significant highlight of the paper is the systematic change in the MDF's shape with radius. The inner disk exhibits a negatively skewed distribution, which becomes more Gaussian at the solar radius and positively skewed in the outer regions of the Galaxy. This shift in the MDF's shape suggests the influence of processes such as radial migration, where the blurring of stellar populations due to orbital eccentricities fails to account for this skewness shift, but radial migration models can adequately explain it.

The implications of these findings are substantial for understanding the Milky Way's formation and evolution. The uniform high-alpha sequence suggests consistent star formation efficiency and outflow mass loading during the formation of these populations. The paper also suggests that radial migration has played a pivotal role in sculpting the observed MDFs, demanding a comprehensive understanding of the disk's evolutionary processes beyond simple closed-box models.

The authors stress the necessity for more sophisticated chemo-dynamic models incorporating gas dynamics and stellar migration to thoroughly match the observed stellar distributions and MDF shapes. Such models must consider both gas and stellar radial movements as integral to explaining the changing shapes and gradients observed in the Galaxy's MDFs.

In conclusion, this paper provides robust observational constraints on the Milky Way's chemical composition and offers a refined framework for developing models of Galaxy evolution that consider radial migration's intricate effects. Future work should aim at integrating these findings into detailed cosmological simulations to achieve deeper insights into Galactic formation and evolution.