SDSS-IV MaNGA: the indispensable role of bars in enhancing the central
  star formation of low-$z$ galaxies
  We analyse two-dimensional maps and radial profiles of EW(H$\alpha$),
EW(H$\delta_A$), and D$_n$(4000) of low-redshift galaxies using integral field
spectroscopy from the MaNGA survey. Out of $\approx1400$ nearly face-on
late-type galaxies with a redshift $z<0.05$, we identify 121 "turnover"
galaxies that each have a central upturn in EW(H$\alpha$), EW(H$\delta_A$)
and/or a central drop in D$_n$(4000), indicative of ongoing/recent star
formation. The turnover features are found mostly in galaxies with a stellar
mass above $\sim$10$^{10}$ M$_{\odot}$ and NUV-$r$ colour less than $\approx5$.
The majority of the turnover galaxies are barred, with a bar fraction of
89$\pm$3\%. Furthermore, for barred galaxies the radius of the central turnover
region is found to tightly correlate with one third of the bar length.
Comparing the observed and the inward extrapolated star formation rate surface
density, we estimate that the central SFR have been enhanced by an order of
magnitude. Conversely, only half of the barred galaxies in our sample have a
central turnover feature, implying that the presence of a bar is not sufficient
to lead to a central SF enhancement. We further examined the SF enhancement in
paired galaxies, as well as the local environment, finding no relation. This
implies that environment is not a driving factor for central SF enhancement in
our sample. Our results reinforce both previous findings and theoretical
expectation that galactic bars play a crucial role in the secular evolution of
galaxies by driving gas inflow and enhancing the star formation and bulge
growth in the center.
