Direct Observations with the VLT of the formation of spiral galaxies
Contact Details
Francois HAMMER
Observatoire de Paris
+33 1 45 07 74 08
Francois.Hammer@obspm.fr
For several years, the astronomers have been intrigued by the compact and luminous galaxies observed in great number in the remote Universe. They are dwarf by their sizes, and however radiate ten to hundred times more energy than the dwarf galaxies observed in the local Universe. 7 billion years ago, these galaxies were the dominant population, whereas they are practically absent today. Recent observations cast a new light on this enigma.
A team of astronomers lead by FranÁois Hammer at Observatoire de Paris-Meudon (Nicolas Gruel, Observatoire de Paris and Hector Flores, CEA-Saclay in France, Trinh Xuan Thuan, University of
Virginia, USA and Leopoldo Infante, Catholic University in Chile) observed these galaxies with spectrograph FORS1 mounted on the VLT (European Southern Observatory) at Paranal (Chile). They could obtain spectra of an unequalled quality, detecting for the first time in remote galaxies, significant lines like those of iron and other elements.
These spectra enable to identify the stellar and gas contents of the luminous compact galaxies. They contain on one hand, old stars, rich in iron and other elements, and on the other hand, regions of intense stellar formation. The analysis of the spectra also shows that the luminous compact galaxies contain a large amount of gas and dust, and that a significant fraction of their energy is re-emitted in the infra-red, which is confirmed by the detection of several of them by the ISO satellite (European Space Agency). These already evolved galaxies are intensely transforming enormous masses of gas and dust into new generations of stars. The star formation is caused by interactions between a galaxy and its neighbors and even by the merger between two galaxies which are revealed by the remarkable quality of image of the Space Telescope.
These observations support in a spectacular way the scenario known as ‘ hierarchical ‘ formation of galaxies, in which two galaxies merge to form more massive objects, the phenomenon triggering an intense stellar formation. The compact galaxies show a considerable diversity of morphologies, including complete merger between
galaxies, and less direct interactions, building a very luminous bulge and a hardly developed disc. The team of astronomers, at the origin of the discovery presented here, suggests that the compact galaxies are the progenitors of bulges of massive galaxies similar to the massive bulge of our Galaxy, which later developped a disc with spiral arms. We would be thus witnessing an essential phase of the formation of the galaxies showing that the stars constituting the bulge are formed before those, which like our Sun, form the disc (and its spiral arms). These intense star formations occurred only in a secondary phase, the formation of a significant part of the massive galaxies observed today would be thus relatively recent.
An illustration of the morphologies of these galactic systems, two sections of representative spectra, and references are found at the URL address : http://www.obspm.fr
