Spiral galaxies with flat radial abundance gradients at large radii

¹ Vilnius University, Faculty of Physics, Institute of Theoretical Physics and Astronomy Sauletekio av. 3 10257 Vilnius, Lithuania
² Main Astronomical Observatory, National Academy of Sciences of Ukraine 27 Akademika Zabolotnoho St 03680 Kiev, Ukraine

^★ Corresponding author.

Received: 27 November 2025
Accepted: 30 December 2025

Abstract

We consider the oxygen abundance distributions for a sample of massive (log(M_★/M_⊙) ≳ 10) spiral galaxies from the Mapping Nearby Galaxies at the Apache Point Observatory (MaNGA) survey in which the radial abundance gradient flattens to a constant value outside of the outer break radius, R_b, outer. The outer break radius can be considered as a dividing radius between the galaxy and the circumgalactic medium (CGM). The values of the R_b, outer range from ∼0.8 R₂₅ to ∼1.45 R₂₅, where R₂₅ is the optical (isophotal) radius of the galaxy. The oxygen abundances in the CGM range from 12+log(O/H) ∼8.0 to ∼8.5. The oxygen abundance distribution in each of our galaxies also shows the inner break in the radial abundance profile at the radius R_b, inner. The metallicity gradient in the outer part of the galaxy (R_b, inner < R < R_b, outer) is steeper than in the inner part (R < R_b, inner). The behaviour of the radial abundance distributions in these galaxies can be explained by assuming an interaction with (capture of the gas from) a small companion and adopting the model for the chemical evolution of galaxies with a radial gas flow. The interaction with a companion results in the mixing of gas and a flat metallicity gradient in the CGM. The capture of the gas from a companion increases the radial gas inflow rate and changes the slope of the radial abundance gradient in the outer part of the galaxy.