Galaxies are complex sources made by hundred of billions stars, gas and dust. Emmanuel Kant, German philosopher, was the first to hypothesise the existence of such island universe lying in the intergalactic space. Understanding of the galaxy nature has considerably progressed since their first identification as extra-galactic sources by Edwin Hubble in 1920. For instance both the Milky Way and the Andromeda galaxy have been identified as spiral galaxies, made of a central bulge surrounded by a disk with arms. In the same mass range, most of the galaxies of the local Universe also show spiral structures. However there is still no consensus on how these galaxies have been formed. The most accepted scenario assumes that they have been formed in the early Universe, some 12 billions years ago, through the gravitational collapse of large clouds of primordial gas. After this birth stage, they would have evolved in a quiescent way, without many interactions, similarly to the island universes of Kant. This scenario is indeed challenged by the new observations made by the Paris Observatory team.
They found that 6 billions years ago, the gaseous phases of distant galaxies were half less enriched in Oxygen than in present-day galaxies. Qualitatively such evolution is expected as massive stars are continuously producing heavy elements such as Oxygen that is expelled in the intergalactic medium. However this enrichment is found to be much more rapid than expectations from models in which galaxies are isolated. The observations indeed predict that a third of the stars in present-day galaxies have been formed from gas coming from external sources.
Thus no galaxy is an island. Their content has been fed and regulated by interaction with their environment, including galaxy interactions, gas infall or outflows. This result is in agreement with the hierarchical scenario of galaxy formation that predict that massive galaxies are formed by merger of smaller sub-units.
Reference
- IMAGES IV : strong evolution of the oxygen abundance in gaseous phases of intermediate mass galaxies from z 0.8, Rodrigues, M. ; Hammer, F. ; Flores, H. ; Puech, M. ; Liang, Y. C. ; Fuentes-Carrera, I. ; Nesvadba, N. ; Lehnert, M. ; Yang, Y. ; Amram, P. ; Balkowski, C. ; Cesarsky, C. ; Dannerbauer, H. ; Delgado, R. ; Guiderdoni, B. ; Kembhavi, A. ; Neichel, B. ; Östlin, G. ; Pozzetti, L. ; Ravikumar, C. D. ; Rawat, A. ; di Serego Alighieri, S. ; Vergani, D. ; Vernet, J. ; Wozniak, H. , Astronomy and Astrophysics, Volume 492, Issue 2, 2008, pp.371-388
Contact
- Myriam Rodrigues
Observatoire de Paris, GEPI, et Instituto Superior Tecnico, CENTRA, Portugal