  We explore the significance of bars in triggering central star formation (SF)
and AGN activity for spiral galaxy evolution using a volume-limited sample with
$0.020<z<0.055$, $M_{\rm r}<-19.5$, and $\sigma>70\rm km s^{-1}$ selected from
SDSS DR7. On a central SF rate-$\sigma$ plane, we measure the fraction of
galaxies with strong bars in our sample and also the AGN fractions for barred
and non-barred galaxies, respectively. The comparison between the bar and AGN
fractions reveals a causal connection between the two phenomena of SF quenching
and AGN activity. A massive BH and abundant gas fuels are sufficient conditions
to trigger AGNs. We infer that the AGNs triggered by satisfying the two
conditions drive the strong AGN feedback, suddenly suppressing the central SF
and leaving the SF sequence. We find that in galaxies where either of the two
conditions is not sufficient, bars are a great help for the AGN triggering,
accelerating the entire process of evolution, which is particularly evident in
pseudo-bulge galaxies. All of our findings are obtained only when plotted in
terms of their central velocity dispersion and central SFR (not galactic scale
SFR), indicating that the AGN-driven SF quenching is confined in the central
kpc region.
