  We have used the Sloan Digital Sky Survey (SDSS) to undertake an
investigation of lopsidedness in a sample of ~25,000 nearby galaxies (z <
0.06). We use the m=1 azimuthal Fourier mode between the 50% and 90% light
radii as our measure of lopsidedness. The SDSS spectra are used to measure the
properties of the stars, gas, and black hole in the central-most few-kpc-scale
region. We show that there is a strong link between lopsidedness in the outer
parts of the galactic disk and the youth of the stellar population in the
central region. This link is independent of the other structural properties of
the galaxy. These results provide a robust statistical characterization of the
connections between accretion/interactions/mergers and the resulting star
formation. We also show that residuals in the galaxy mass-metallicity relation
correlate with lopsidedness (at fixed mass, the more metal-poor galaxies are
more lopsided). This suggests that the events causing lopsidedness and enhanced
star formation deliver lower metallicity gas into the galaxy's central region.
Finally, we find that there is a trend for the more powerful active galactic
nuclei to be hosted by more lopsided galaxies (at fixed galaxy mass, density,
or concentration). However if we compare samples matched to have both the same
structures and central stellar populations, we then find no difference in
lopsidedness between active and non-active galaxies. This leads to the
following picture. The presence of cold gas in the central region of a galaxy
(irrespective of its origin) is essential for both star-formation and black
hole growth. The delivery of cold gas is aided by the processes that produce
lopsidedness. Other processes on scales smaller than we can probe with our data
are required to transport the gas to the black hole.
