  Starburst galaxies have elevated star formation rates (SFRs) for their
stellar mass. In Ellison et al. (2018) we used integral field unit (IFU) maps
of star formation rate surface density (Sigma_SFR) and stellar mass surface
density (Sigma_*) to show that starburst galaxies in the local universe are
driven by SFRs that are preferentially boosted in their central regions. Here,
we present molecular gas maps obtained with the Atacama Large Millimeter Array
(ALMA) observatory for 12 central starburst galaxies at z~0 drawn from the
Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. The ALMA
and MaNGA data are well matched in spatial resolution, such that the ALMA maps
of molecular gas surface density (Sigma_H2) can be directly compared with MaNGA
maps at kpc-scale resolution. The combination of Sigma_H2, Sigma_* and
Sigma_SFR at the same resolution allow us to investigate whether central
starbursts are driven primarily by enhancements in star formation efficiency
(SFE) or by increased gas fractions. By computing offsets from the resolved
Kennicutt-Schmidt relation (Sigma_H2 vs. Sigma_SFR) and the molecular gas main
sequence (Sigma_* vs. Sigma_H2), we conclude that the primary driver of the
central starburst is an elevated SFE. We also show that the enhancement in
Sigma_SFR is accompanied by a dilution in O/H, consistent with a triggering
that is induced by metal poor gas inflow. These observational signatures are
found in both undisturbed (9/12 galaxies in our sample) and recently merged
galaxies, indicating that both interactions and secular mechanisms contribute
to central starbursts.
