Once upon a time, before our Sun and Earth existed, distant galaxies were being created. Because this happened so long ago, astronomers know very little about how these galaxies formed. But now, by combining the Hubble Space Telescope’s acute vision with the Very Large Telescope’s spectrograph, astronomers have obtained exceptional 3-D views of distant galaxies, seen when the Universe was half its current age. By looking at this unique “history book” of our Universe, scientists hope to solve the puzzle of how galaxies formed in the remote past.
Hubble allows fine details of galaxies to be seen, while the VLT’s FLAMES/GIRAFFE spectrograph can obtain simultaneous spectra from small areas of extended objects, and resolving the motions of the gas in these distant galaxies.
“This unique combination of Hubble and the VLT allows us to model distant galaxies almost as nicely as we can close ones,” said François Hammer, who led the team. “In effect, FLAMES/GIRAFFE now allows us to measure the velocity of the gas at various locations in these objects. This means that we can see how the gas is moving, which provides us with a three-dimensional view of galaxies halfway across the Universe.”
The team has been reconstructing the history of about one hundred remote galaxies that have been observed with both Hubble and GIRAFFE on the VLT. The first results are coming in and have already provided useful insights for three galaxies.
In one galaxy, GIRAFFE revealed a region full of ionized gas, that is, 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,” said Mathieu Puech, lead author of one of the papers reporting this study. Comparisons with computer simulations suggest 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.
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Another galaxy that the astronomers 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 astronomers identified a very unusual, extremely blue, elongated structure — a bar — composed of young, massive stars, rarely observed in nearby galaxies. Comparisons with computer simulations showed the astronomers that the properties of this object are well reproduced by a collision between two galaxies of unequal mass.
“The unique combination of Hubble and FLAMES/GIRAFFE at the VLT makes it possible to model distant galaxies in great detail, and reach a consensus on the crucial role of galaxy collisions for the formation of stars in a remote past,” says Puech. “It is because we can now see how the gas is moving that we can trace back the mass and the orbits of the ancestral galaxies relatively accurately. Hubble and the VLT are real ‘time machines’ for probing the Universe’s history,” added Sébastien Peirani, lead author of another paper reporting on this study.
The astronomers are now extending their analysis 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 the past six to eight billion years, that is, over half the age of the Universe,” said Hammer.