2005A&A...433L..17H

The collision of gas-rich galaxies is believed to produce strong shocks between their gas clouds which cause the onset of the observed bursts of extended star formation. However, the so far observed shock signatures in colliding galaxies can be explained essentially by winds from already existing massive stars and supernovae and thus do not give any evidence for an outstanding pre-starburst phase. Either pre-starburst gas shocks are too short-lived to be detected or one has to modify our perception of colliding galaxies. A dedicated analysis of ISOCAM-CVF mid-infrared spectral maps led us to the discovery of exceptional H₂ v=0-0 S(3) λ=9.66µm line emission from the ``Antennae'' galaxy pair, which is at an early stage of galaxy collision. Its H₂ line luminosity, normalized by the far-infrared luminosity, exceeds that of all other known galaxies and the strongest H₂ emission is spatially displaced from the known starbursts regions. This implies that most of the excited H₂ gas in the Antennae must be shocked due to the collision of the two galaxies. These observations indicate that the outstanding phase of pre-starburst shocks exists, and that they might be a key to our understanding of the formation of the first proto-galaxies.