We present the scientific case for a high resolution spectro-imager
in the mid-IR that would
be dedicated to the mapping of H2 emission
in its four lowest rotational transitions,
at 28.2, 17.0, 12.3 and 9.7 micron with a spectral
resolution ~104 sufficient to provide kinematical distances in
galaxies. The proposed instrument on a 2 m-class telescope
will be most sensitive
to H2 line emission
simultaneously extended and structured at small scale, in gas at
temperatures higher than 80 K. Colder H2,
which may contribute most of the baryonic dark matter in
galaxies, will be traced by the emission of glitters of warm H2
heated, throughout the medium, by the dissipation of omnipresent turbulence.
The main scientific objectives are to (i) directly measure the
mass and temperature distribution of the warm H2, far from star forming
regions, in particular in low metallicity environments where the
traditional tracers of H2 (CO, dust emission) fail, (ii)
trace the dissipation of turbulence in the perspective of building
a global view of the star formation process in galaxies and (iii)
trace baryonic dark matter in the form of cold H2
in a large sample of galaxies along the Hubble sequence.