The history of the universe – in 3D!

In an international project, astronomers have obtained exceptional 3D images of distant galaxies, seen when the Universe was half its current age. And by looking at this unique “history book” of the universe, at an epoch when the Sun and the Earth did not even exist, scientists hope to solve the puzzle of how galaxies formed in the remote past.

The team – consisting of scientists from the US’ NASA, the European Space Agency and the European Southern Observatory – obtained the images by combining the Hubble Space Telescope’s acute eye with the capacity of the Very Large Telescope (VLT) to probe the motions of gas in tiny objects.

“Hubble and VLT are real ‘time machines’ for probing the universe’s history,” said Sébastien Peirani, lead author of one of the papers reporting on this study.

DOUBLING UP

For decades, distant galaxies that emitted their light six to eight billion years ago – over half the age of the universe – were no more than small specks of light on the sky.

With the launch of the Hubble Space Telescope in the early 1990s, astronomers were able to scrutinise the structure of these galaxies in some detail for the first time.

Now, researchers are using the Hubble’s capabilities in conjunction with those of the VLT – an array of four separate optical telescopes, situated at the Paranal Observatory in northern Chile. Its Fibre Large Array Multi Element Spectrograph (FLAMES) can observe up to 130 targets at a time, which enabled the team to measure the velocity of the gas in the distant galaxies.

“By seeing how the gas is moving, it provides us with a 3D view of galaxies halfway across the universe,” said François Hammer, who led the project.

The team is now reconstituting the history of about 100 remote galaxies. The project has already provided useful insights for three such galaxies.

UNRAVELLING GALACTIC SECRETS

In one galaxy, FLAMES revealed a region full of ionised gas – hot gas composed of atoms that have been stripped of one or several electrons. This is normally due to the presence of very hot, young stars.

However, even after staring at the region for more than 11 days, Hubble did not detect any stars! “Clearly, this unusual galaxy has some hidden secrets,” researcher Mathieu Puech said.

Computer simulations suggested that the explanation lies in the collision of two very gas-rich spiral galaxies. The heat produced by the collision would ionise the gas, making it too hot for stars to form.

Another galaxy that the scientists studied showed the opposite effect. There, they discovered a bluish central region enshrouded in a reddish disc, almost completely hidden by dust.

“The models indicate that gas and stars could be spiralling inwards rapidly,” said Hammer. “This might be the first example of a disc rebuilt after a major merger.”

Finally, in a third galaxy, the team saw a rare sight: an extremely blue, elongated structure – a bar, in fact – composed of young, massive stars. Comparisons with computer simulations showed that the properties of this object are well reproduced by a collision between two galaxies of unequal mass.

“The combination of Hubble and the VLT, along with modern computer simulations, allows us to model distant galaxies almost as nicely as the close ones,” Hammer said.

The team is now extending its analyses to the whole sample of galaxies observed.

“The next step will then be to compare this with closer galaxies, and so, piece together a picture of the evolution of galaxies over half the age of the universe,” he concluded.


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