Investigating the origin of observed central dips in radial metallicity profiles

Bethan Easeman, Patricia Schady, Stijn Wuyts, Robert M Yates

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Radial metallicity trends provide a key indicator of physical processes such as star formation and radial gas migration within a galaxy. Large integral field unit surveys allow for detailed studies of these radial variations, with recent observations detecting central dips in the metallicity, which may trace the impact of various evolutionary processes. However, the origin of these dips has not been conclusively determined, with suggestions that they may be diagnostic dependent. In this paper, we use the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at Apache Point Observatory survey to investigate whether the observed dips represent genuine decreases in the central metallicity, or whether they could be an artefact of the diagnostic used. Using a sub-sample of 758 local star-forming galaxies at low inclinations, we investigate in detail the impact of using different strong line diagnostics on the shapes of the returned profiles, and the prevalence of dips. We find no clear evidence of the dips being caused by changing values of the ionization parameter within galaxies. To investigate physical causes, we explore both global and spatially resolved parameters, finding that galaxies exhibiting central dips in the O3N2 metallicity profile have on average lower H α equivalent width values out to R/Re∼1.5⁠, and higher values of DN(4000) in the central regions. We additionally find a higher prevalence of dips in galaxies with high stellar mass, and lower values of global specific star formation rate, suggesting a possible link to central quenching. Nevertheless, these results are dependent on the diagnostic used, suggesting caution should be taken when interpreting observed features in galaxy metallicity gradients.
Original languageEnglish
Pages (from-to)371-392
Number of pages22
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date12 Jan 2022
Publication statusPublished - 12 Jan 2022

Bibliographical note

Funding Information:
ACKNOWLEDGEMENTS We thank the anonymous referee for their insightful comments and helpful suggestions. We greatly appreciate the MPA-JHU group for making their catalogue public, and C. Avery for sharing their data on the pre v alence of outflo ws within the galaxies in our sample. This research made use of ASTROPY , 1 a community-developed core PYTHON package for astronomy (Astropy Collaboration 2013 ; Price- Whelan et al. 2018 ). This work makes use of data from SDSS-IV. Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the participating institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS website is . SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration.

Publisher Copyright:
© 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.


  • H ii regions
  • ISM: abundances
  • galaxies: abundances

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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