  Galactic bars are often suspected to be a channel of gas inflow to the
galactic center and trigger central star formation and active galactic nuclei
(AGN) activity. However, the current status on this issue based on empirical
studies is unsettling, especially on AGN. We investigate this question based on
the Sloan Digital Sky Survey (SDSS) Data Release 7. From the nearby (0.01 < z <
0.05) bright (Mr < -19) database, we have constructed a sample of 6,658
relatively face-on late-type galaxies through visual inspection. We found 36%
of them to have a bar. Bars are found to be more common in galaxies with
earlier morphology. This makes sample selection critical. Parameter-based
selections would miss a large fraction of barred galaxies of early morphology.
Bar effects on star formation or AGN are difficult to understand properly
because multiple factors (bar frequency, stellar mass, black-hole mass, gas
contents, etc.) seem to contribute to them in intricate manners. In the hope of
breaking these degeneracies, we inspect bar effects for fixed galaxy
properties. Bar effects on central star formation seem higher in redder
galaxies. Bar effects on AGN on the other hand are higher in bluer and less
massive galaxies. These effects seem more pronounced with increasing bar
length. We discuss possible implications in terms of gas contents, bar
strength, bar evolution, fueling time-scale, and the dynamical role of
supermassive black hole.
