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To explain the observed anomalies in stellar populations within globularclusters, many globular cluster formation theories require two independentepisodes of star formation. A fundamental prediction of these models is thatthe clusters must accumulate large gas reservoirs as the raw material to formthe second stellar generation. We show that young clusters containing therequired gas reservoir should exhibit the following observational signatures:(i) a dip in the measured luminosity profile or an increase in measuredreddening towards the cluster centre, with Av >10mag within a radius of a fewpc; (ii) bright (sub)mm emission from dust grains; (iii) bright molecular lineemission once the gas is dense enough to begin forming stars. Unless the IMF isanomalously skewed towards low-mass stars, the clusters should also showobvious signs of star formation via optical emission lines (e.g. H_alpha) afterthe stars have formed. These observational signatures should be readilyobservable towards any compact clusters (radii of a few pc) in the nearbyUniverse with masses > 10^6 Msun and ages <100Myr. This provides astraightforward way to directly test globular cluster formation models whichpredict large gas reservoirs are required to form the second stellargeneration. The fact that no such observational evidence exists calls intoquestion whether such a mechanism happens regularly for YMCs in galaxies withina few tens of Mpc.

Heart of Darkness: dust obscuration of the central stellar component in globular clusters younger than ~100Myr in multiple stellar population models