Understanding strong molecular hydrogen emission in astronomical
  environments
  Here I describe recent studies of objects with molecular hydrogen emission
that is strong relative to other spectral lines. Large telescopes and fast
spectrometers have made the 2 {\mu}m window accessible even for relatively
faint objects. I summarize several environments where strong molecular hydrogen
2.121 micron emission is observed. The line is hard to excite due to its large
excitation potential, and is most emissive in regions that have temperatures
that are nearly high enough to dissociate molecular hydrogen. I outline several
case studies. In the Helix planetary nebula strong emission is produced by
rapidly flowing molecular gas that is exposed to an intense ionizing radiation
field. This advective production of molecular hydrogen is a fundamentally
non-equilibrium process. In the filaments surrounding brightest cluster
galaxies in cool core clusters ionizing particles penetrate into magnetically
confined molecular cores and excite the gas. Finally, I outline ongoing work on
the Crab Nebula, where the first complete maps of molecular emission have only
recently been completed. Both ionizing particles and high-energy photons may be
important. Finally I speculate on the origin of the correlation between H2 / H
I intensity ratios and other properties found in Active Galaxies. This is
suggestive of a hardening of the radiation field along the Eigenvector 1
sequence. In all of this work I take the approach of understanding H2 emission
along with emission from low and moderate ionization species, a necessary step
if we are to really understand the context in which molecular hydrogen emission
forms.
