We report on a study of young star cluster complexes in the spiral galaxy M51. Recent studies have confirmed that star clusters do not form in isolation, but instead tend to form in larger groupings or complexes. We use {it HST} broad and narrow band images (from both {it WFPC2} and {it ACS}), along with {it BIMA}-CO observations to study the properties and investigate the origin of the e complexes. We find that the complexes are all young ($< 10$ Myr), have sizes between $sim$85 and $sim$240 pc, and have masses between 3-30 $times 10^{4} M_{odot}$. Unlike that found for isolated young star clusters, we find a strong correlation between the complex mass and radius, namely $Mpropto R^{2.33 pm 0.19}$. This is similar to that found for giant molecular clouds (GMCs). By comparing the mass-radius relation of GMCs in M51 to that of the complexes we can estimate the star formation efficiency within the complexes, although this value is heavily dependent on the assumed CO-to-H$_2$ conversion factor. The complexes studied here have the same surface density distribution as individual young star clusters and GMCs. If star formation within the complexes is proportional to the gas density at that point, then the shared mass-radius relation of GMCs and complexes is a natural consequence of their shared density profiles. We briefly discuss possibilities for the lack of a mass-radius relation for young star clusters. We note that many of the complexes show evidence of merging of star clusters in their centres, suggesting that larger star clusters can be produced through the build up of smaller clusters.
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