  Spitzer space telescope spectroscopy reveal a new class of H2-luminous
galaxies with enhanced H2 line emission, but where star formation is strongly
suppressed. This is in sharp contrast with what is observed in standard star
forming galaxies. These sources are all in active phases of galaxy evolution
(galaxy interactions, AGN feedback, gas accretion in galaxy clusters, etc.).
Why is H2 present in violent phases of galaxy evolution? How is the H2 emission
powered? Why is the H2 gas inefficient at forming stars? What can we learn from
these "H2 galaxies" about galaxy formation? This thesis addresses these
questions, and discuss the theoretical and observational perspectives of this
work (in particular Herschel and JWST).
