Heart of Darkness: dust obscuration of the central stellar component in
  globular clusters younger than ~100Myr in multiple stellar population models
  To explain the observed anomalies in stellar populations within globular
clusters, many globular cluster formation theories require two independent
episodes of star formation. A fundamental prediction of these models is that
the clusters must accumulate large gas reservoirs as the raw material to form
the second stellar generation. We show that young clusters containing the
required gas reservoir should exhibit the following observational signatures:
(i) a dip in the measured luminosity profile or an increase in measured
reddening towards the cluster centre, with Av >10mag within a radius of a few
pc; (ii) bright (sub)mm emission from dust grains; (iii) bright molecular line
emission once the gas is dense enough to begin forming stars. Unless the IMF is
anomalously skewed towards low-mass stars, the clusters should also show
obvious signs of star formation via optical emission lines (e.g. H_alpha) after
the stars have formed. These observational signatures should be readily
observable towards any compact clusters (radii of a few pc) in the nearby
Universe with masses > 10^6 Msun and ages <100Myr. This provides a
straightforward way to directly test globular cluster formation models which
predict large gas reservoirs are required to form the second stellar
generation. The fact that no such observational evidence exists calls into
question whether such a mechanism happens regularly for YMCs in galaxies within
a few tens of Mpc.
