H2 energetics and gravitational fragmentation in the overlap region in the Antennae. Early stages of star formation

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There is compelling evidence that interaction between gas-rich galaxies can induce IR luminous bursts of star formation, but our understanding of how star formation is physically triggered is unclear. To explain the ongoing starburst in the overlap region of the Antennae, the galaxy interaction must trigger star formation where and as the galaxies collide, over time scales much shorter than the tidally induced inflow of the gas towards the nuclei. Our main goal is to test this dynamical view of star formation in mergers by looking for gas cooling and fragmentation. SINFONI near-IR spectro-imaging observations allow us to study how the gas, which has been heated by the collision, cools, fragments, and collapse to form stars. We focus on the H2 and HII gas spatial distribution, kinematics, excitation and energetics within a field of 600 pc in size in the overlap region of the Antennae. The H2 emission extends over the whole area and presents an exceptionally broad line width (mechanical energy released by the galaxies collision has not yet dissipated). We also find two compact sources, one of them detected in the H2 emission, while the other appears in the map of the Brγ line. We identify these sources as early steps in the formation and evolution of super star clusters. The H2 source is compact (60 pc diameter), turbulent (140 km/s H2 FWHM) and massive 3×107 Msun where star formation has not yet started or is just starting. The ionized compact source is a very young <3 Myr super star cluster that is still embedded in its parent cloud. We are observing rapid star formation where gravitational fragmentation and energy dissipation occur over the cloud crossing time. For star formation to occur these time scales must be smaller than the dynamical time scale of the gas compression by the large scale galaxies dynamics. Finally, there is a tentative discovery of early steps in the formation of Super Star Clusters by looking at a tracer of mechanical energy dissipation (H2 line emission). This interpretation needs to be confirmed by looking at mass tracers (dust and CO with ALMA). If we are right we are opening a new observational perspective on star formation.

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