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The origin of supersonic infrared and radio recombination nebular lines oftendetected in young and massive superstar clusters are discussed. We suggest thatthese arise from a collection of repressurizing shocks (RSs), actingeffectively to re-establish pressure balance within the cluster volume and fromthe cluster wind which leads to an even broader although much weaker component.The supersonic lines are here shown to occur in clusters that undergo a bimodalhydrodynamic solution (Tenorio-Tagle et al. 2007), that is within clusters thatare above the threshold line in the mechanical luminosity or cluster mass vsthe size of the cluster (Silich et al. 2004). The plethora of repressurizingshocks is due to frequent and recurrent thermal instabilities that take placewithin the matter reinserted by stellar winds and supernovae. We show that themaximum speed of the RSs and of the cluster wind, are both functions of thetemperature reached at the stagnation radius. This temperature depends only onthe cluster heating efficiency ($\eta$). Based on our two dimensionalsimulations (Wunsch et al. 2008) we calculate the line profiles that resultfrom several models and confirm our analytical predictions. From a comparisonbetween the predicted and observed values of the half-width zero intensity ofthe two line components we conclude that the thermalization efficiency in SSC'sabove the threshold line must be lower than 20%.

Supersonic Line Broadening within Young and Massive Super Star Clusters