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A Massive Clump of Stars Detected at its Birth in the Distant Universe

​Through the observation programme conducted using the Hubble space telescope, a team consisting of the CEA, the CNRS and the University of Paris-Diderot has discovered the birth of a massive clump of stars in a very distant galaxy. This is the first time that such a region of young star formation has been observed in the distant Universe, making it possible to study the physical mechanisms and formation conditions for stars that were created three billion years after the Big Bang. These results will be published on 7 May in Nature magazine.

Published on 6 May 2015

​This has enabled them to observe a galaxy 11 billion light years away, in which they have identified the signature of a gigantic cluster consisting of very young stars. This type of structure is already known. It consists of dense and very active regions of star formation within a galaxy. Yet the clump of stars discovered by the team was only formed ten million years ago and contains a large quantity of gas. In star terms, it forms the equivalent of 30 times the Sun’s mass annually, with a efficacy ten times greater than the average values observed during this era of cosmic history.

What very high resolution digital simulations reveal

To complete the interpretation of these amazing results, researchers also developed a set of very high resolution hydrodynamic simulations using the supercomputer at the CEA GENCI centre to reproduce the formation of these clumps. Simulations show that in gas-rich galaxies, the gases fragment and form numerous new stars in the same region during the first millions of years, attaining values that are consistent with the clump data observed.

After some fifteen million years, the effect of the stellar winds originating from massive young stars and the explosion of the earliest supernovas became sufficiently great to counterbalance the gravitational collapse of the gas. This caused star formation to decrease gradually.

A possible mechanism to explain
the growth of bulbs of galaxies

The discovery of a star formation clump as young as this in a distant galaxy has serious implications for understanding the formation of galaxies on a cosmological scale. Indeed, the clumps detected in other distant galaxies represent star clusters that are much more developed and older. The rarity of the phenomenon discovered here – a clump observed in its early stages and presenting with a very high rate of star formation – implies that the life span of clumps observed in the distant universe could be at least 500 million years.

This new limitation excludes certain theoretical scenarios that predict the swift destruction of young star clumps through the action of winds created by newly formed massive stars, and corroborates the notion that they can live for long enough to develop within the galactic disc within which they were formed. They can then migrate to the heart of the galaxy and play a major role in the growth of the bulb and the gigantic central black hole.

To accurately define the role of these massive clumps in the evolution of galaxies requires an even more precise determination of their physical properties, such as their size and dynamic mass. This is the aim at the heart of future work by the team of researchers, one that will require, in particular, the means of observation of the ALMA set of antennas (the Atacama Large Millimeter Array, in northern Chile), as well as the future James Webb Space Telescope (JWST) which is due to launch in late 2018.

Star formation in distant galaxies

When the Universe was only three billion years old, galaxies had very different characteristics from those with which it presents today. Their shapes were more irregular and their discs richer in gas, enabling them to form their stars much more quickly. The star were born within huge regions of stellar formation, ones much more massive and brighter than those typically observed in galaxies closer to our own. Yet the manner in which these massive clumps of stars assembled is not yet understood and their development on the scale of cosmic history is hotly debated.

Their apparently low luminosity requires the use of powerful means of observation.

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