Best Observational Evidence of First Generation Stars in the Universe

 

Astronomers using ESO’s Very Large Telescope have discovered by far the brightest galaxy yet found in the early Universe and found strong evidence that examples of the first generation of stars lurk within it. These massive, brilliant, and previously purely theoretical objects were the creators of the first heavy elements in history — the elements necessary to forge the stars around us today, the planets that orbit them, and life as we know it. The newly found galaxy, labelled CR7, is three times brighter than the brightest distant galaxy known up to now.

Source: ESO

Astronomers have long theorised the existence of a first generation of stars — known as Population III stars — that were born out of the primordial material from the Big Bang. The name Population III arose because astronomers had already classed the stars of the Milky Way as Population I (stars like the Sun, rich in heavier elements and forming the disc) and Population II (older stars, with a low heavy-element content, and found in the Milky Way bulge and halo, and globular star clusters).

The first stars must have formed out of the only elements to exist prior to stars: hydrogen, helium and trace amounts of lithium.

These Population III stars would have been enormous — several hundred or even a thousand times more massive than the Sun — blazing hot, and transient — exploding as supernovae after only about two million years. But until now the search for physical proof of their existence had been inconclusive. (Finding these stars is very difficult: they would have been extremely short-lived, and would have shone at a time when the Universe was largely opaque to their light.)

A team led by David Sobral has now used ESO’s Very Large Telescope (VLT) to peer back into the ancient Universe, to a period known as reionisation, approximately 800 million years after the Big Bang. Instead of conducting a narrow and deep study of a small area of the sky, they broadened their scope to produce the widest survey of very distant galaxies ever attempted.

The team discovered — and confirmed — a number of surprisingly bright very young galaxies. One of these, labelled CR7, was an exceptionally rare object, by far the brightest galaxy ever observed at this stage in the Universe. CR7’s nickname is an abbreviation of COSMOS Redshift 7, a measure of its place in terms of cosmic time. (The higher the redshift, the more distant the galaxy and the further back in the history of the Universe it is seen.)

CR7 is three times brighter in terms of ultraviolet light emission than the previous titleholder, Himiko, which was thought to be one of a kind at this very early time. The energy coming from CR7 is mostly ultraviolet/visible light.

The X-shooter and SINFONI instruments on the VLT found strong ionised helium emission in CR7 but — crucially and surprisingly — no sign of any heavier elements in a bright pocket in the galaxy. This meant the team had discovered the first good evidence for clusters of Population III stars that had ionised gas within a galaxy in the early Universe.

Within CR7, bluer and somewhat redder clusters of stars were found, indicating that the formation of Population III stars had occurred in waves — as had been predicted. What the team directly observed was the last wave of Population III stars, suggesting that such stars should be easier to find than previously thought: they reside amongst regular stars, in brighter galaxies, not just in the earliest, smallest, and dimmest galaxies, which are so faint as to be extremely difficult to study.

Further observations with the VLT, ALMA, and the NASA/ESA Hubble Space Telescope are planned to confirm beyond doubt that what has been observed are Population III stars, and to search for and identify further examples.