G337.342-0.119 (the "Pebble"): A Cold, Dense, High-Mass Molecular Cloud
  with Unusually Large Linewidths and a Candidate High-Mass Star Cluster
  Progenitor
  Exactly how high-mass star clusters form, especially the young massive
clusters (YMCs: age $<100$ Myr; mass $>10^4$ solar masses), remains an open
problem, largely because they are so rare that examples of their cold, dense,
molecuar progenitors remain elusive. The molecular cloud G337.342$-$0.119, the
`Pebble,' is a candidate for such a cold progenitor. Although G337.342$-$0.119
was originally identified as four separate ATLASGAL clumps, the similarity in
their molecular line velocities and linewidths in the MALT90 dataset
demonstrate that these four clumps are in fact one single, coherent cloud. This
cloud is unique in the MALT90 survey for its combination of both cold
temperatures ($T_{dust} \sim 14$ K) and large linewidths $(\Delta V \sim 10$ km
s$^{-1}$). The near/far kinematic distance ambiguity is difficult to resolve
for G337.342$-$0.119. At the near kinematic distance (4.7 kpc), the mass is
5,000 solar masses and the size is $7\times2$ pc. At the far kinematic distance
(11 kpc), the mass is 27,000 solar masses and the size is $15 \times 4$ pc. The
unusually large linewidths of G337.342$-$0.119 are difficult to reconcile with
a gravitationally bound system in equilibrium. If our current understanding of
the Galaxy's Long Bar is approximately correct, G337.342$-$0.119 cannot be
located at its end. Rather, it is associated with a large star-forming complex
that contains multiple clumps with large linewidths. If G337.342$-$0.119 is a
prototypical cold progenitor for a high-mass cluster, its properties may
indicate that the onset of high-mass star cluster formation is dominated by
extreme turbulence.
