Evrim Yazgin
Cosmos science journalist
NASA’s James Webb Space Telescope (JWST) has yet again peered into the early universe giving scientists new insights into how the first stars formed galaxies.
Our universe is about 13.8 billion years old. The JWST has wowed astronomers by revealing ancient galaxies that challenge our understanding of how they form. Within the first 6 months of its scientific operations in 2022, JWST broke Hubble’s record for the furthest confirmed galaxy.
JWST’s galaxy, known as JADES-GS-z13-0 is believed to have formed “only” 325 million years after the Big Bang. This is more than 100 million years earlier than Hubble’s record which was held by the galaxy GN-z11, discovered in 2015.
Galaxies forming so close to the dawn of time have left astronomers scratching their heads. Remember, we’re talking about massive structures.
Our Milky Way galaxy is, for example, is about 100,000 light-years (~1 billion billion km) across and is home to an estimated 100–400 billion stars. At the centre of most galaxies is a supermassive black hole millions of times the mass of our Sun.
As if all that wasn’t enough, current theories suggest that galaxies form around dark matter halos which pull in stars and dust gravitationally.
It should take lots of time for such assemblages to come about.
Yet, in a relatively short period of time after the Big Bang our universe was beginning to form galaxies.
To understand this, astronomers are using the JWST’s instruments which make it the most powerful telescope ever. Two research teams have used JWST to peer back at the jewel in Hubble’s crown, GN-z11.
GN-z11 is 13.4 billion years old. It is also one of the youngest galaxies observed in the early universe. So it is a perfect candidate to help understand the evolution of galaxies so close to the birth of the cosmos.
In a preprint paper accepted for publication in Astronomy & Astrophysics, a team led by University of Cambridge professor Roberto Maiolino analysed stars in GN-z11 using JWST instruments. They found evidence that the stars are Population III stars – the first generation of, and hence oldest, stars in the universe.
Different populations of stars have different abundances of heavy elements.
Population I stars are the youngest, forming in the last 5 or 6 billion years like our Sun. These have the highest abundance of heavy elements – vestiges from the more ancient stars which died, out of which Population I stars were born.
Population II stars have fewer heavy elements. They can be found in the oldest regions of the Milky Way.
Population III stars have been theoretical until now.
Maiolino’s team found no elements heavier than helium in GN-z11. “The fact that we don’t see anything else beyond helium suggests that this clump must be fairly pristine,” Maiolino, first author on the paper, says. “This is something that was expected by theory and simulations in the vicinity of particularly massive galaxies from these epochs – that there should be pockets of pristine gas surviving in the halo, and these may collapse and form Population III star clusters.”
The same team found the first clear evidence that at GN-z11’s centre is a supermassive black hole. This analysis is published in Nature.
“Webb’s NIRCam (Near-Infrared Camera) has revealed an extended component, tracing the host galaxy, and a central, compact source whose colours are consistent with those of an accretion disk surrounding a black hole,” says co-author Dr Hannah Übler, also from Cambridge.
The supermassive black hole is about 2 million times heavier than the Sun and is in a very active phase of consuming matter, making it extremely luminous.
