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Register a new userChemical Cartography with APOGEE: Large-scale Mean Metallicity Maps of the Milky Way
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We present Galactic mean metallicity maps derived from the first year of the SDSS-III APOGEE experiment. Mean abundances in different zones of Galactocentric radius (0 < R < 15 kpc) at a range of heights above the plane (0 < |z| < 3 kpc), are derived from a sample of nearly 20,000 stars with unprecedented coverage, including stars in the Galactic mid-plane at large distances. We also split the sample into subsamples of stars with low and high-[{alpha}/M] abundance ratios. We assess possible biases in deriving the mean abundances, and find they are likely to be small except in the inner regions of the Galaxy. A negative radial gradient exists over much of the Galaxy; however, the gradient appears to flatten for R < 6 kpc, in particular near the Galactic mid-plane and for low-[{alpha}/M] stars. At R > 6 kpc, the gradient flattens as one moves off of the plane, and is flatter at all heights for high-[{alpha}/M] stars than for low-[{alpha}/M] stars. Alternatively, these gradients can be described as vertical gradients that flatten at larger Galactocentric radius; these vertical gradients are similar for both low and high-[{alpha}/M] populations. Stars with higher [{alpha}/M] appear to have a flatter radial gradient than stars with lower [{alpha}/M]. This could suggest that the metallicity gradient has grown steeper with time or, alternatively, that gradients are washed out over time by migration of stars.
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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.
We map the trends of elemental abundance ratios across the Galactic disk, spanning R = 3-15 kpc and midplane distance |Z|= 0-2 kpc, for 15 elements in a sample of 20,485 stars measured by the SDSS/APOGEE survey (O, Na, Mg, Al, Si, P, S, K, Ca, V, Cr, Mn, Fe, Co, Ni). Adopting Mg rather than Fe as our reference element, and separating stars into two populations based on [Fe/Mg], we find that the median trends of [X/Mg] vs. [Mg/H] in each population are nearly independent of location in the Galaxy. The full multi-element cartography can be summarized by combining these nearly universal median sequences with our measured metallicity distribution functions and the relative proportions of the low-[Fe/Mg] (high-alpha) and high-[Fe/Mg] (low-alpha) populations, which depend strongly on R and |Z|. We interpret the median sequences with a semi-empirical 2-process model that describes both the ratio of core collapse and Type Ia supernova contributions to each element and the metallicity dependence of the supernova yields. These observationally inferred trends can provide strong tests of supernova nucleosynthesis calculations. Our results lead to a relatively simple picture of abundance ratio variations in the Milky Way, in which the trends at any location can be described as the sum of two components with relative contributions that change systematically and smoothly across the Galaxy. Deviations from this picture and future extensions to other elements can provide further insights into the physics of stellar nucleosynthesis and unusual events in the Galaxys history.
The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high-resolution spectra for thousands of red giant stars distributed among the massive satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf (Sgr), Fornax (Fnx), and the now fully disrupted emph{Gaia} Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C, N, O, Mg, Al, Si, Ca, Fe, Ni, and Ce), as well as by fitting chemical evolution models to the [$alpha$/Fe]-[Fe/H] abundance plane for each galaxy. Results show that the chemical signatures of the starburst in the MCs observed by Nidever et al. in the $alpha$-element abundances extend to C+N, Al, and Ni, with the major burst in the SMC occurring some 3-4 Gyr before the burst in the LMC. We find that Sgr and Fnx also exhibit chemical abundance patterns suggestive of secondary star formation epochs, but these events were weaker and earlier ($sim$~5-7 Gyr ago) than those observed in the MCs. There is no chemical evidence of a second starburst in GSE, but this galaxy shows the strongest initial star formation as compared to the other four galaxies. All dwarf galaxies had greater relative contributions of AGB stars to their enrichment than the MW. Comparing and contrasting these chemical patterns highlight the importance of galaxy environment on its chemical evolution.
Galactic interstellar extinction maps are powerful and necessary tools for Milky Way structure and stellar population analyses, particularly toward the heavily-reddened bulge and in the midplane. However, due to the difficulty of obtaining reliable extinction measures and distances for a large number of stars that are independent of these maps, tests of their accuracy and systematics have been limited. Our goal is to assess a variety of photometric stellar extinction estimates, including both 2D and 3D extinction maps, using independent extinction measures based on a large spectroscopic sample of stars towards the Milky Way bulge. We employ stellar atmospheric parameters derived from high-resolution $H$-band APOGEE spectra, combined with theoretical stellar isochrones, to calculate line-of-sight extinction and distances for a sample of more than 2400 giants towards the Milky Way bulge. We compare these extinction values to those predicted by individual near-IR and near+mid-IR stellar colors, 2D bulge extinction maps and 3D extinction maps. The long baseline, near+mid-IR stellar colors are, on average, the most accurate predictors of the APOGEE extinction estimates, and the 2D and 3D extinction maps derived from different stellar populations along different sightlines show varying degrees of reliability. We present the results of all of the comparisons and discuss reasons for the observed discrepancies. We also demonstrate how the particular stellar atmospheric models adopted can have a strong impact on this type of analysis, and discuss related caveats.
We use data of $sim$13,000 stars from the SDSS/APOGEE survey to study the shape of the bulge MDF within the region $|ell|leq11^circ$ and $|b|leq13^circ$, and spatially constrained to ${rm R_{GC}leq3.5}$ kpc. We apply Gaussian Mixture Modeling and Non-negative Matrix Factorization decomposition techniques to identify the optimal number and the properties of MDF components. We find the shape and spatial variations of the MDF (at ${rm [Fe/H]geq-1}$ dex) are well represented as a smoothly varying contribution of three overlapping components located at [Fe/H]=+$0.32$, $-0.17$ and $-0.66$ dex. The bimodal MDF found in previous studies is in agreement with our trimodal assessment once the limitations in sample size and individual measurement errors are taken into account. The shape of the MDF and its correlations with kinematics reveal different spatial distributions and kinematical structure for the three components co-existing in the bulge region. We confirm the consensus physical interpretation of metal-rich stars as associated with the secularly evolved disk into a boxy/peanut X-shape bar. On the other hand, metal-intermediate stars could be the product of in-situ formation at high redshift in a gas-rich environment characterized by violent and fast star formation. This interpretation would help to link a present-day structure with those observed in formation in the center of high redshift galaxies. Finally, metal-poor stars may correspond to the metal-rich tail of the population sampled at lower metallicity from the study of RR Lyrae stars. Conversely, they could be associated with the metal-poor tail of the early thick disc.
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