Spiral galaxies with flat radial abundance gradients at large radii

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, Rb, outer. The outer break radius can be considered as a dividing radius between the galaxy and the circumgalactic medium (CGM). The values of the Rb, 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 Rb, inner. The metallicity gradient in the outer part of the galaxy (Rb, inner < R < Rb, outer) is steeper than in the inner part (R < Rb, 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.