  The phenomenological study of evolving galaxy populations has shown that star
forming galaxies can be quenched by two distinct processes: mass quenching and
environment quenching (Peng et al. 2010). To explore the mass quenching process
in local galaxies, we study the massive central disk galaxies with stellar mass
above the Schechter characteristic mass. In Zhang et al. (2019), we showed that
during the quenching of the massive central disk galaxies as their star
formation rate (SFR) decreases, their molecular gas mass and star formation
efficiency drop rapidly, but their HI gas mass remains surprisingly constant.
To identify the underlying physical mechanisms, in this work we analyze the
change during quenching of various structure parameters, bar frequency, and
active galactic nucleus (AGN) activity. We find three closely related facts. On
average, as SFR decreases in these galaxies: (1) they become progressively more
compact, indicated by their significantly increasing concentration index,
bulge-to-total mass ratio, and central velocity dispersion, which are mainly
driven by the growth and compaction of their bulge component; (2) the frequency
of barred galaxies increases dramatically, and at a given concentration index
the barred galaxies have a significantly higher quiescent fraction than
unbarred galaxies, implying that the galactic bar may play an important role in
mass quenching; and (3) the "AGN" frequency increases dramatically from 10% on
the main sequence to almost 100% for the most quiescent galaxies, which is
mainly driven by the sharp increase of LINERs. These observational results lead
to a self-consistent picture of how mass quenching operates.
