One of the most remarkable discoveries from our MPE guaranteed-time
program is the existence of enormous amounts (M ~ 10^9 Msun) of warm
(T=100-200K) molecular hydrogen (H2) gases in bright IR galaxies.  By
observing a number of rovibrational H2 lines, we will determine
whether the H2 is excited by UV radiation field or shocks.  With this
information in hand, we will further investigate the mechanism which
is generating the large luminosities of these galaxies.