G337.342-0.119 (The "Pebble"): A cold, dense, high-mass molecular cloud with unusually large line widths and a candidate high-mass star cluster progenitor

How high-mass star clusters form, especially young massive clusters (YMCs: age <100 Myr; mass >10⁴ M_⊙), remains an open problem, largely because they are so rare that examples of their cold, dense, molecular progenitors continue to be elusive. The molecular cloud G337.342−0.119, the "Pebble," is a candidate cold progenitor. Although G337.342−0.119 was originally identified as four separate ATLASGAL clumps, the similarities in their molecular line velocities and line widths in the MALT90 data set 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 ~ 14 K) and large line widths (ΔV ~ 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 5000 M_⊙ and the size is 7 × 2 pc. At the far kinematic distance (11 kpc), the mass is 27,000 M_⊙ and the size is 15 × 4 pc. The unusually large line widths 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 line widths. 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.