Chemical Cartography With Apogee: Large-Scale Mean Metallicity Maps of the Milky Way Disk

Abstract

We present Galactic mean metallicity maps derived from the first year of the SDSS-III APOGEE experiment. Mean abundances in different zones of projected 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 giant 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-[α/M] abundance ratios. We assess possible biases in deriving the mean abundances, and find that they are likely to be small except in the inner regions of the Galaxy. A negative radial metallicity 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-[α/M] stars. At R > 6 kpc, the gradient flattens as one moves off the plane, and is flatter at all heights for high-[α/M] stars than for low-[α/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-[α/M] populations. Stars with higher [α/M] appear to have a flatter radial gradient than stars with lower [α/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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“…Again the sample of Cepheids used in measuring the radial abundance gradients is concentrated close to the Galactic plane within | z| < 0.5 kpc (Luck & Lambert 2011). At R gc > 12 kpc and away from the midplane, the mean metallicity of about −0.3 dex and the flat radial gradient observed for the present sample of OCs is consistent with very similar values reported for field dwarfs (Cheng et al 2012) and field giants (Hayden et al 2014). These results suggest that the OCs and field stars yield consistent radial gradients if the comparison samples are drawn from the similar vertical slices.…”

“…As a result, the initial and the present-day gradient derived for the younger populations are very similar out to 12 kpc. This fairly explains the similarity in the radial gradients observed for the younger tracers such as OCs of age less than 2 Gyr ( Figure 4 and figure 5), field stars (Cheng et al 2012;Hayden et al 2014) and Cepheids (Luck & Lambert 2011;Genovali et al 2014) for Rgc < 12 kpc. The steep gradient of slope −0.058 dex kpc −1 for stars younger than 2 Gyr in the radial range 6−11 kpc and a shallow gradient of old stellar populations predicted after taking the radial migration into account is in fair agreement with our values of gradient measured for respective age groups.…”

“…Therefore, attempts to measure the radial gradients using the OCs distributed in the outer disc (R gc > 11 kpc) will be severely influenced by the vertical gradients whose affect is minimal for the sample in the vertical slice | z| < 0.5 kpc and thus unimportant for the radial gradient inferred for OCs in the radial range 6 to 11 kpc (Cheng et al 2012;Hayden et al 2014).…”

“…Both Cheng et al (2012) and Hayden et al (2014) have measured a steep radial gradients of about −0.06 dex kpc −1 (0.15 < | z| < 0.5 kpc) and −0.087±0.002 dex kpc −1 (0.0 < | z| < 0.25 kpc), respectively, close to the plane and have confirmed that the radial gradients become flatter as one moves away from the Galactic midplane in the vertical range 0.5 < | z| < 2 kpc. Again the sample of Cepheids used in measuring the radial abundance gradients is concentrated close to the Galactic plane within | z| < 0.5 kpc (Luck & Lambert 2011).…”

“…It is especially puzzling that the flattening of the metallicity distribution of OCs at large radii is not shown by the Galactic field stars (Cheng et al 2012;Hayden et al 2014) including Cepheids (Luck & Lambert 2011;Genovali et al 2014) where field stars suggest a constant steep decline of metallicity out to Rgc of 18 kpc. Another fascinating result, in common for both the field stars and OCs at large radii, is the evidence for enhanced [α/Fe] ratios in the outer Galactic disc (Yong et al 2005;Bensby et al 2011;Luck & Lambert 2011).…”