Galaxy mergers produce spectacular bursts of star formation. These can be so bright that astronomers can detect them very far in the past, even from the earliest moments of the Universe. These celestial firworks are associated with the birth of huge stellar clusters, in which millions of stars are enclosed in a volume hardly larger than several tens of light years. A comparable volume around the Sun contains just a few stars. About twenty years ago, using the Hubble space telescope, astronomers had found these super clusters in the galaxies of the Anntenna group, but the physical processes responsible for their formation are still poorly understood. Now, by combining the first data acuired by ALMA with data obtained earlier with the VLT, the scientists are in a position to elucidate, for the first time, how the merging of galaxies can lead to the formation of stellar super clusters. This type of observation is well within the research objectives of ALMA.
This work forms the basis of Cinthya Herrera’s thesis ; she is preparing a doctorate at the Institut d’Astrophysique Spatiale (IAS) . She has a research scholarship within the framework of an agreement between the CNRS and the Chilean CONICYT.
1 The inaugural image obtained by ALMA was in fact of the galaxies in the Antenna group (cf. ALMAopens a new window on the Universe).
Rather than looking at stellar clusters which are already formed, her idea is to study the energy lost by the gas in which the clusters are born. The point is that stars are born in regions where the gas is very dense and cold. Now, the process of galaxy merging induces considerable turbulence in the gas, which must therefore lose this energy in order to condense, cool and collapse to form clusters of new stars.
Theoretical analysis shows that this energy loss is considerable. It can be traced today thanks to the combined observations of these two telescopes. As the gas settles down and loses its turbulent energy radiatively, it becomes visible in the infra-red, and so is observable by the VLT . The observations made by ALMA have highlighted the extremely turbulent nature of the gas in these gigantic clouds where stellar clusters are born. The turbulence is due to the gravitational energy released by the interaction of these two galaxies.
Observations have shown that in just one of these clouds, a considerable fraction of the turbulent energy contained in gas concentrations is radiated. The amount of gas in
this region is enough to make a super cluster. And where ALMA sees just one cloud surrounded by others, the VLT sees the most luminous object of the entire interaction region comprising the Antenna group. Theoretical analysis has shown that in just a few million years - the blink of an eye-lid on the scale of the Universe - this gas will have lost its turbulent energy and a new cluster will have been born. This first result heralds future discoveries which astronomers like Cinthya Herrera will be making with ALMA, which will soon be fully operational.
L’image en haut à gauche montre un zoom sur la région centrale, là où les deux galaxies s’interpénètrent, obtenue par le télescope spatial Hubble (NASA). Les images de droite, préparées par Cinthya Herrera, illustrent les observations faites par ALMA et le VLT. L’image ALMA, plus à gauche, montre les nuages où sont présents les amas stellaires les plus jeunes et de plus grande masse (symbolisés par des étoiles). L’image VLT à droite montre, elle, l’énergie rayonnée par les nuages. La source compacte brillante dans cette image (le pic de couleur rouge) est présente là où deux masses de gaz à des vitesses très différentes (couleurs rouge et bleue dans le cadre en bas à droite) se rencontrent. Les observations montrent que c’est là que la dissipation d’énergie est la plus forte et que les conditions sont réunies pour former un nouvel amas en quelques millions d’années.
- The turbulent birth of stellar super clusters in merging galaxies
Bibliography
ALMA CO and VLT/SINFONI H2 observations of the Antennae overlap region : mass and energy dissipation. Herrera, C. N., Boulanger F., Nesvadba, N. P. H., Falgarone, E. 2012, A&A, volume 538, L9