The survey of planetary nebulae in Andromeda (M31). IV. Radial oxygen and argon abundance gradients of the thin and thicker disc

Souradeep Bhattacharya, Magda Arnaboldi, Nelson Caldwell, Ortwin Gerhard, Chiaki Kobayashi, Johanna Hartke, Kenneth C. Freeman, Alan W. McConnachie, Puragra Guhathakurta

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We obtain a magnitude-limited sample of Andromeda (M 31) disc PNe with chemical abundance estimated through the direct detection of the [O III] 4363 Å line. This leads to 205 and 200 PNe with oxygen and argon abundances, respectively. We find that high- and low-extinction M 31 disc PNe have statistically distinct argon and oxygen abundance distributions. In the radial range of 2−30 kpc, the older low-extinction disc PNe are metal-poorer on average with a slightly positive radial oxygen abundance gradient (0.006 ± 0.003 dex kpc−1) and slightly negative for argon (−0.005 ± 0.003 dex kpc−1), while the younger high-extinction disc PNe are metal-richer on average with steeper radial abundance gradients for both oxygen (−0.013 ± 0.006 dex kpc−1) and argon (−0.018 ± 0.006 dex kpc−1), similar to the gradients computed for the M 31 H II regions. The M 31 disc abundance gradients are consistent with values computed from major merger simulations, with the majority of the low-extinction PNe being the older pre-merger disc stars in the thicker disc, and the majority of the high-extinction PNe being younger stars in the thin disc, formed during and after the merger event. The chemical abundance of the M 31 thicker disc has been radially homogenized because of the major merger. Accounting for disc scale lengths, the positive radial oxygen abundance gradient of the M 31 thicker disc is in sharp contrast to the negative one of the MW thick disc. However, the thin discs of the MW and M 31 have remarkably similar negative oxygen abundance gradients.
Original languageEnglish
Pages (from-to)2343-2359
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date24 Sept 2022
Publication statusPublished - 1 Dec 2022


  • astro-ph.GA
  • astro-ph.SR
  • Galaxies: evolution
  • Galaxies: individual M 31
  • planetary nebulae: general
  • Galaxies: abundances
  • Galaxies: structure


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