Searching for Milky Way twins: Radial abundance distribution as a strict criterion

Abstract

We search for Milky Way-like galaxies among a sample of approximately 500 galaxies. The characteristics we considered of the candidate galaxies are the following: stellar mass M⋆, optical radius R25, rotation velocity Vrot, central oxygen abundance (O/H)0, and abundance at the optical radius (O/H)R25. If the values of R25 and M⋆ of the galaxy were close to that of the Milky Way, then the galaxy was referred to as a structural Milky Way analogue (sMWA). The oxygen abundance at a given radius of a galaxy is defined by the evolution of that region (astration level, that is, the fraction of gas converted into stars, as well as gas exchange with the surroundings), and we then assumed that the similarity of (O/H)0 and (O/H)R25 in two galaxies suggests a similarity in their (chemical) evolution. If the values of (O/H)0 and (O/H)R25 in the galaxy were close to that of the Milky Way, then the galaxy was referred to as an evolutionary Milky Way analogue (eMWA). If the galaxy was simultaneously an eMWA and sMWA, then the galaxy was considered a Milky Way twin. We find that the position of the Milky Way on the (O/H)0–(O/H)R25 diagram shows a large deviation from the general trend in the sense that the (O/H)R25 in the Milky Way is appreciably lower than in other galaxies of similar (O/H)0. This feature of the Milky Way evidences that its (chemical) evolution is not typical. We identify four galaxies (NGC 3521, NGC 4651, NGC 2903, and MaNGA galaxy M-8341-09101) that are simultaneously sMWA and eMWA and can therefore be considered as Milky Way twins. In previous studies, Milky Way-like galaxies were selected using structural and morphological characteristics, that is, sMWAs were selected. We find that the abundances at the centre and at the optical radius (evolutionary characteristics) provide a stricter criterion for selecting real Milky Way twins.