Two sequences of spiral galaxies with different shapes of the metallicity gradients

Abstract

We considered two sequences of spiral galaxies with different shapes of the radial gas-phase oxygen abundance distributions from the galaxies in the survey Mapping Nearby Galaxies at Apache Point Observatory (MaNGA): (1) Galaxies in which the gradient is well approximated by a single linear relation across the whole disc, that is, galaxies with an S (slope) gradients, (2) galaxies in which the metallicity in the inner region of the disc is at a nearly constant level and the gradient is negative at larger radii, that is, galaxies with level-slope (LS) gradients. We also selected galaxies with a nearly uniform oxygen abundance across the whole galaxy, that is, galaxies with level (L) gradients (or O/H uniform galaxies) with a high oxygen abundance that can be the final evolutionary stage of the two galaxy sequences described above. The radial nitrogen abundance distributions in galaxies with LS oxygen abundance distributions also show breaks at radii smaller than the O/H distribution breaks. The observed behaviour of the oxygen and nitrogen abundances with radius in these galaxies can be explained by the time delay between the nitrogen and oxygen enrichment together with the variation in the star formation history along the radius. These galaxies clearly show the effect of the inside-out disc evolution model, which predicts that the galactic centre evolves more rapidly than the regions at greater galactocentric distances. We find that the shape of the radial abundance distribution in a galaxy is not related to its macroscopic characteristics (rotation velocity, stellar mass, isophotal radius, and star formation rate) and is independent of its present-day environment. The correlations between the gradient slopes and macroscopic characteristics of galaxies are weak in the sense that the scatter of the points in each diagram is large. The galaxies with different abundance distributions (S, LS, or L) in our sample are located within the main sequence of the star-forming galaxies in the diagram of star formation rate–stellar mass. We also examined the properties of the Milky Way in the context of the considered galaxy samples.