Exactly how high-mass star clusters form , especially the young massive clusters ( YMCs : age < 100 Myr ; mass > 10 ^ { 4 } M _ { \odot } ) , remains an open problem , largely because they are so rare that examples of their cold , dense , molecuar progenitors continue to be 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 ~ { } \mbox { 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 M _ { \odot } and the size is 7 \times 2 pc .
At the far kinematic distance ( 11 kpc ) , the mass is 27,000 M _ { \odot } 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 can not 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 .