Star Formation History of a Young Super-Star Cluster in NGC 4038/39:
  Direct Detection of Low Mass Pre-Main Sequence Stars
  We present an analysis of the near-infrared spectrum of a young massive star
cluster in the overlap region of the interacting galaxies NGC 4038/39 using
population synthesis models. Our goal is to model the cluster population as
well as provide rough constraints on its initial mass function (IMF). The
cluster shows signs of youth such as thermal radio emission and strong hydrogen
emission lines in the near-infrared. Late-type absorption lines are also
present which are indicative of late-type stars in the cluster. The strength
and ratio of these absorption lines cannot be reproduced through either
late-type pre-main sequence (PMS) stars or red supergiants alone. Thus we
interpret the spectrum as a superposition of two star clusters of different
ages, which is feasible since the 1" spectrum encompasses a physical region of
~90 pc and radii of super-star clusters are generally measured to be a few
parsecs. One cluster is young (<= 3 Myr) and is responsible for part of the
late-type absorption features, which are due to PMS stars in the cluster, and
the hydrogen emission lines. The second cluster is older (6 Myr - 18 Myr) and
is needed to reproduce the overall depth of the late-type absorption features
in the spectrum. Both are required to accurately reproduce the near-infrared
spectrum of the object. Thus we have directly detected PMS objects in an
unresolved super-star cluster for the first time using a combination of
population synthesis models and pre-main sequence tracks. This analysis serves
as a testbed of our technique to constrain the low-mass IMF in young super-star
clusters as well as an exploration of the star formation history of young UC
HII regions.
