ALMA observations of fragmentation, sub-structure, and protostars in
  high-mass starless clump candidates
  (Abridged) The initial physical conditions of high-mass stars and
protoclusters remain poorly characterized. To this end we present the first
targeted ALMA 1.3mm continuum and spectral line survey towards high-mass
starless clump candidates, selecting a sample of 12 of the most massive
candidates ($400-4000\, M_\odot$) within 5 kpc. The joint 12+7m array maps have
a high spatial resolution of $\sim 3000\, \mathrm{au}$ ($\sim
0.8^{\prime\prime}$) and have point source mass-completeness down to $\sim
0.3\, M_\odot$ at $6\sigma$ (or $1\sigma$ column density sensitivity of
$1.1\times10^{22}\, \mathrm{cm^{-2}}$). We discover previously undetected
signposts of low-luminosity star formation from CO (2-1) and SiO (5-4) bipolar
outflows and other signatures towards 11 out of 12 clumps, showing that current
MIR/FIR Galactic Plane surveys are incomplete to low- and intermediate-mass
protostars ($\lesssim 50\, L_\odot$). We compare a subset of the observed cores
with a suite of radiative transfer models of starless cores. We find a
high-mass starless core candidate with a model-derived mass consistent with
$29^{52}_{15}\, M_\odot$ when integrated over size scales of $2\times10^4\,
\mathrm{au}$. Unresolved cores are poorly fit by starless core models,
supporting the interpretation that they are protostellar even without detection
of outflows. Substantial fragmentation is observed towards 10 out of 12 clumps.
We extract sources from the maps using a dendrogram to study the characteristic
fragmentation length scale. Nearest neighbor separations when corrected for
projection are consistent with being equal to the clump average thermal Jeans
length. Our findings support a hierarchical fragmentation process, where the
highest density regions are not strongly supported against thermal
gravitational fragmentation by turbulence or magnetic fields.
