Star Formation Efficiencies at Giant Molecular Cloud Scales in the
  Molecular Disk of the Elliptical Galaxy NGC 5128 (Centaurus A)
  We present ALMA CO(1-0) observations toward the dust lane of the nearest
elliptical and radio galaxy, NGC 5128 (Centaurus A), with high angular
resolution ($\sim$ 1 arcsec, or 18 pc), including information from large to
small spatial scales and total flux. We find a total molecular gas mass of
1.6$\times$10$^9$ $M_\odot$ and we reveal the presence of filamentary
components more extended than previously seen, up to a radius of 4 kpc. We find
that the global star formation rate is $\sim$1 \Msol yr$^{-1}$, which yields a
star formation efficiency (SFE) of 0.6 Gyr$^{-1}$ (depletion time $\tau =$1.5
Gyr), similar to those in disk galaxies. We show the most detailed view to date
(40\,pc resolution) of the relation between molecular gas and star formation
within the stellar component of an elliptical galaxy, from several kpc scale to
the circumnuclear region close to the powerful radio jet. Although on average
the SFEs are similar to those of spiral galaxies, the circumnuclear disk (CND)
presents SFEs of 0.3 Gyr$^{-1}$, lower by a factor of 4 than the outer disk.
The low SFE in the CND is in contrast to the high SFEs found in the literature
for the circumnuclear regions of some nearby disk galaxies with nuclear
activity, probably as a result of larger shear motions and longer AGN feedback.
The higher SFEs in the outer disk suggests that only central molecular gas or
filaments with sufficient density and strong shear motions will remain in
$\sim$1 Gyr, which will later result in the compact molecular distributions and
low SFEs usually seen in other giant ellipticals with cold gas.
