First galaxies older than expected

Abell 383 and a distorted background galaxy

The gravity of this giant cluster of galaxies, including the huge one in the middle, acts as a sort of magnifying glass, distorting and concentrating the light of a distant background galaxy. Visible as two tiny dots (labelled), the galaxy is seen as it was less than a billion years after the Big Bang.

ASTRONOMERS HAVE DISCOVERED a distant galaxy whose stars were born unexpectedly early in cosmic history.

“We have discovered a distant galaxy that began forming stars just 200 million years after the Big Bang,” says Johan Richard, the lead author of a new study. “This challenges theories of how soon galaxies formed and evolved in the first years of the Universe.”

Richard’s team spotted the galaxy in recent observations from the NASA/ESA Hubble Space Telescope, verified it with observations from the NASA Spitzer Space Telescope and measured its distance using W.M. Keck Observatory in Hawaii.

The distant galaxy is far beyond a cluster of galaxies called Abell 383, whose powerful gravity bends passing rays of light almost like a magnifying glass.

The chance alignment of the galaxy, the cluster and the Earth amplifies the light reaching us from the distant galaxy, enabling the astronomers to make detailed observations.

Without this gravitational lens, the galaxy would have been too faint to be seen even with today’s largest telescopes.

A young galaxy of old stars

After spotting the galaxy in Hubble and Spitzer images, the team carried out spectroscopic observations with the Keck-II telescope in Hawaii. Spectroscopy is the technique of breaking up light into its component colours.

By analysing the spectra, the team was able to make detailed measurements of the galaxy’s redshift and infer information about the properties of its component stars.

The redshift is 6.027, which means we’re seeing the galaxy as it was when the Universe was around 950 million years old.

Diagram explaining gravitational lensing

The gravity of huge galaxy clusters acts as a magnifying glass, amplifying the light of galaxies in the distant background and making them easier to see.

This doesn’t make it the most remote galaxy ever detected—several have been confirmed at redshifts of more than 8, and one has an estimated redshift of around 10, placing it 400 million years earlier.

However the newly discovered galaxy is very different to other distant ones, which generally shine brightly with only young stars.

“When we looked at the spectra, two things were clear,” explains co-author Eiichi Egami. “The redshift placed it very early in cosmic history, as we expected. But the Spitzer infrared detection also indicated that the galaxy was made up of surprisingly old and relatively faint stars.”

“This told us that the galaxy was made up of stars already nearly 750 million years old—pushing back the epoch of its formation to about 200 million years after the Big Bang, much further than we had expected,” adds Egami.

“This suggests that the first galaxies have been around for a lot longer than previously thought,” says Dan Stark, another co-author of the study.

Artist's impression of the James Webb Space Telescope

When operational, the James Webb Space Telescope will be able to see even further back in time.

Unseen army may solve the mystery

The discovery has implications beyond the question of when galaxies first formed. It might also help explain how the Universe became transparent to ultraviolet light in the first billion years after the Big Bang.

In the early years of the cosmos, a diffuse fog of neutral hydrogen gas blocked ultraviolet light in the Universe. Some source of radiation must have progressively ionised the diffuse gas, clearing the fog and making it transparent to ultraviolet rays as it is today—a process known as reionisation.

Astronomers believe that the radiation that powered this reionisation must have come from galaxies. But, so far, nowhere near enough of them have been found to provide the necessary radiation.

The new discovery may help solve this enigma.

“It seems probable that there are in fact far more galaxies out there in the early Universe than we previously estimated—it’s just that many galaxies are older and fainter, like the one we have just discovered,” says co-author Jean-Paul Kneib.

“If this unseen army of faint, elderly galaxies is indeed out there, they could provide the missing radiation that made the Universe transparent to ultraviolet light.”

As of today, astronomers can discover these galaxies only by seeing them through gravitational lenses. But the NASA/ESA/CSA James Webb Space Telescope, scheduled for launch later this decade, will be able to make high-resolution observations of the distant, highly redshifted bodies.

Astronomers hope then to be in a position to solve this mystery once and for all.

Adapted from information issued by the ESA Hubble Information Centre. Abell 383 image courtesy of NASA, ESA, J. Richard (CRAL) and J.-P. Kneib (LAM) (acknowledgement: Marc Postman (STScI)). Gravitational lensing diagram courtesy NASA, ESA & L. Calçada.

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