BY DR EMILY BALDWIN
Posted: 12 March, 2009
Using observations from the Hubble Space Telescope and ESO’s Very Large Telescope, astronomers have obtained unique three-dimensional views of distant galaxies.
The galaxies under scrutiny are being seen when the Universe was just half its current age, and will help astronomers learn about how galaxies formed in the distant past.
“This unique combination of Hubble and the VLT allows us to model distant galaxies almost as nicely as we can close ones,” says team leader Francois Hammer. The VLT’s FLAMES/ GIRAFFE spectrograph is used to obtain spectra of distant objects, and to resolve the motion of gas in galaxies. “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.”
By combining the powerful instruments on the Hubble Space Telescope with the ground-based VLT, astronomers have obtained unique three-dimensional views of distant galaxies. The diagram shows a sketch of a remote galaxy (in the box), how Hubble sees it (middle panel) and the gas motion measured with the VLT (left panel). Image: ESO.
By performing these observations, the team are piecing together the history of around one hundred remote galaxies, some of which are throwing up a few surprises. In one example, GIRAFFE revealed a region full of ionized gas - hot gas composed of atoms that have been stripped of one or several electrons - that is normally associated with the presence of very hot, young stars. But curiously, 11 days worth of Hubble observations did not show up any stars.
“Clearly this unusual galaxy has some hidden secrets,” says Mathieu Puech, lead author of one of the papers reporting this study. The science team compared the galaxy with computer simulations to suggest that two extremely gas-rich spiral galaxies collided, ionizing the gas and making it too hot for stars to form.
The inventory of galaxies goes from one extreme to the other, with another galaxy showing the opposite effect: a blue central region enshrouded in a reddish disc, almost totally obscured by dust. “The models indicate that gas and stars could be spiraling inwards rapidly,” says Hammer. The team speculate that this might be the first example of a disc rebuilt after a major merger event.
In a third example, another blue region was identified, but this time in an elongated bar and composed of young, massive stars rarely observed in nearby galaxies. Again, comparisons with computer simulations demonstrated that a collision of two galaxies of unequal mass would produce the observed properties.
Three of the galaxies studied so far with the Hubble Space Telescope (top panel) and ESO’s VLT (bottom panel). Parts which are red are moving away from us, while those that are blue are moving towards us. Image: ESO/Hammer et al.
“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.
Sebastien Periani is lead author on another paper reporting the results and says, “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.”
The astronomers will continue their analysis for the whole sample of galaxies observed by both Hubble and the VLT. Once complete, they will be able to compare the results with closer galaxies in order to create a storyboard of galaxy evolution over the past six to eight billion years.
Three papers describe the results so far and feature in various current and forthcoming editions of the journal Astronomy and Astrophysics.