The browser you are using is not supported by this website. All versions of Internet Explorer are no longer supported, either by us or Microsoft (read more here:

Please use a modern browser to fully experience our website, such as the newest versions of Edge, Chrome, Firefox or Safari etc.

Photo of Paul McMillan

Paul McMillan


Photo of Paul McMillan

Understanding inverse metallicity gradients in galactic discs as a consequence of inside-out formation


  • Ralph Schönrich
  • Paul J. McMillan

Summary, in English

The early stages of a galaxy's evolution leave an imprint on its metallicity distribution. We discuss the origins and evolution of radial metallicity gradients in discs of spiral galaxies using an analytical chemical evolution model. We explain how radial metallicity gradients in stellar populations are determined by three factors: the radial metallicity profile of the star-forming interstellar medium (ISM), radial changes in the star formation history (in particular, inside-out formation) and radial mixing of stars. Under reasonable assumptions, inside-out formation steepens the negative ISM metallicity gradient, but contributes positively to the stellar metallicity gradient, up to inverting the metallicity profile to a positive d[Fe/H]/dR. This reconciles steep negative d[Fe/H]/dR in some high-redshift galaxies to generally flatter gradients in local observations. We discuss the evidence for inverse radial metallicity gradients (positive d[X/H]/dR) at high redshifts and the inverse relationship between azimuthal velocity and the metallicity (positive dVφ/d[Fe/H]) of stars for the Milky Way's thick disc. The former can be achieved by high central gas-loss rates and re-distribution processes, e.g. re-accretion of enriched material in conjunction with the inside-out formation and near-disc galactic fountaining. For the Milky Way-thick disc, we show that the positive dVφ/d[Fe/H] correlation points to comparable time-scales for inside-out formation, initial metal enrichment and SNIa enrichment. We argue that the original ISM metallicity gradient could be inferred with better data from the high-metallicity tail of the alpha-enhanced population. Including inside-out formation in our models changes the local vertical metallicity gradient by about −0.2 dex kpc−1, in line with local measurements.


  • Lund Observatory

Publishing year







Monthly Notices of the Royal Astronomical Society





Document type

Journal article


Oxford University Press


  • Astronomy, Astrophysics and Cosmology


  • Galaxies: abundances
  • Galaxies: evolution
  • Galaxies: stellar content
  • Galaxy: disc
  • Galaxy: evolution
  • Galaxy: kinematics and dynamics




  • ISSN: 0035-8711