The dependence of the gradients of oxygen and nitrogen-to-oxygen on
  stellar age in MaNGA galaxies
  We derive the oxygen abundance (O/H), the nitrogen-to-oxygen (N/O) abundance
ratio, and their corresponding radial gradients for a sample of 1431 galaxies
from MaNGA DR15 survey using two different realizations of the strong line
method: empirical R calibration and the Bayesian model-based {\sc
HII-CHI-mistry} ({\sc HCm}) code. We find that both abundance calculation
methods reveal a correlation between the O/H gradient and the stellar mass of a
galaxy. This relation is non-linear, with the steepest average gradients in the
intermediate mass range and flatter average gradients for high- and low-mass
galaxies. The relation between the N/O gradient and the stellar mass is, on
average, non-linear with the steepest gradients in the intermediate mass range
($\log(M/M_\sun) \sim 10$), flatter gradients for high-mass galaxies, and the
flattest gradients for low-mass galaxies. However, the general trend of
steepening N/O gradient for higher masses, reported in previous studies,
remains evident. We find a dependence between the O/H and N/O gradients and the
galaxy mean stellar age traced by the $D$(4000) index. For galaxies of lower
masses, both gradients are, generally, steeper for intermediate values of
$D$(4000) and flatter for low and high values of $D$(4000). Only the most
massive galaxies do not show this correlation. We interpret this behaviour as
an evolution of the metallicity gradients with the age of stellar population.
Though the galaxies with a positive slope of the $D$(4000) radial gradient tend
to have a flatter O/H and N/O gradients, as compared to those with a negative
$D$(4000) gradient.
