What determines the local metallicity of galaxies: global stellar mass,
  local stellar mass surface density or star formation rate?
  The metallicity and its relationship with other galactic properties is a
fundamental probe of the evolution of galaxies. In this work, we select about
750,000 star-forming spatial pixels from 1122 blue galaxies in the MaNGA survey
to investigate the global stellar mass - local stellar mass surface density -
gas-phase metallicity ($M_*$ - $\Sigma_*$ - $Z$ ) relation. At a fixed $M_*$,
the metallicity increases steeply with increasing $\Sigma_*$. Similarly, at a
fixed $\Sigma_*$, the metallicity increases strongly with increasing $M_*$ at
low mass end, while this trend becomes less obvious at high mass end. We find
the metallicity to be more strongly correlated to $\Sigma_*$ than to $M_*$.
Furthermore, we construct a tight (0.07 dex scatter) $M_*$ - $\Sigma_*$ - $Z$
relation, which reduces the scatter in the $\Sigma_*$ - $Z$ relation by about
30$\%$ for galaxies with $7.8 < {\rm log}(M_*/M_\odot) < 11.0$, while the
reduction of scatter is much weaker for high-mass galaxies. This result
suggests that, especially for low-mass galaxies, the $M_*$ - $\Sigma_*$ - $Z$
relation is largely more fundamental than the $M_*$ - $Z$ and $\Sigma_*$ - $Z$
relations, meaning that both $M_*$ and $\Sigma_*$ play important roles in
shaping the local metallicity. We also find that the local metallicity is
probably independent on the local star formation rate surface density at a
fixed $M_*$ and $\Sigma_*$. Our results are consistent with the scenario that
the local metallicities in galaxies are shaped by the combination of the local
stars formed in the history and the metal loss caused by galactic winds.
