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ABSTRACT We determine radial- and age-abundance gradients of the Small Magellanic Cloud (SMC) using spectra of 2062 red giant branch field stars observed by SDSS-IV / APOGEE-2S. With coverage out to $$\sim$$9 kpc in the SMC, these data taken with the high resolution ($$R \sim 22\,500$$) APOGEE H-band spectrograph afford the opportunity to measure extensive radial gradients for as many as 24 abundance ratios. The SMC has an overall metallicity ([Fe/H]) gradient of –0.0546 $$\pm$$ 0.0043 dex kpc−1. Ages are calculated for every star to explore the evolution of the different abundance gradients. As a function of age, many gradients flatten with decreasing age but begin to steepen for ages 3.7–5.6 Gyr, particularly in the $$\alpha$$-element [X/H] gradients. We previously detected similar evolutionary patterns $$\sim 1.7-$$3.6 Gyr earlier in the Large Magellanic Cloud (LMC) which are attributed to a recent interaction between the Magellanic Clouds. It is inferred that the feature in the SMC gradients was caused by the same interaction. The age–[X/Fe] trends, which track average [X/Fe] over time, are flat, demonstrating a slow enhancement with respect to Fe for the SMC. When comparing the SMC gradients to the LMC and MW, normalized to disc scale length ($$R_\text{d}$$), the [X/Fe] and [X/Mg] gradients are similar, but there is a dichotomy between the dwarfs and the Milky Way (MW) for the [X/H] gradients. The median MW [X/H] gradient around –0.119 dex/$$R_\text{d}$$ whilst the Clouds have gradients of about –0.082 dex/$$R_\text{d}$$ and –0.061 dex/$$R_\text{d}$$ for the SMC and LMC, respectively.
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Revealing the chemical structure of the Magellanic Clouds with APOGEE. I. Calculating individual stellar ages of RGB stars in the Large Magellanic Cloud
ABSTRACT Stellar ages are critical for understanding the temporal evolution of a galaxy. We calculate the ages of over 6000 red giant branch stars in the Large Magellanic Cloud (LMC) observed with SDSS-IV / APOGEE-S. Ages are derived using multiband photometry, spectroscopic parameters ($$\rm T_{eff}$$, $$\log {g}$$, [Fe/H], and [$$\alpha$$/Fe]) and stellar isochrones and the assumption that the stars lie in a thin inclined plane to get accurate distances. The isochrone age and extinction are varied until a best match is found for the observed photometry. We perform validation using the APOKASC sample, which has asteroseismic masses and accurate ages, and find that our uncertainties are $$\sim$$20 per cent and range from $$\sim$$1–3 Gyr for the calculated ages (most reliable below 10 Gyr). Here we present the LMC age map as well as the age–radius relation and an accurate age–metallicity relation (AMR). The age map and age–radius relation reveal that recent star formation in the galaxy was more centrally located and that there is a slight dichotomy between the north and south with the northern fields being slightly younger. The northern fields that cover a known spiral arm have median ages of $$\gtrsim$$2 Gyr, which is the time when an interaction with the Small Magellanic Cloud (SMC) is suggested to have happened. The AMR is mostly flat especially for older ages although recently (about 2.0–2.5 Gyr ago) there is an increase in the median [Fe/H]. Based on the time frame, this might also be attributed to the close interaction between the LMC and SMC.
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- Award ID(s):
- 1908331
- PAR ID:
- 10566333
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 533
- Issue:
- 3
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 3685 to 3707
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1093/mnras/stae1949
