This artist’s impression shows a galaxy in the distant Universe, just two billion years
after the Big Bang, in the process of pulling in cool gas (shown in orange) from its
surroundings.
Credit:

ESO/L. Calçada/ESA/AOES Medialab

Astronomers have recently caught a distant galaxy in the act of consuming a vast cloud of primeval gas, allowing it to kick off a new round of star formation and increase its own lifespan. It’s long been suspected that galaxies can do this, but new observations by the ESO’s Very Large Telescope (VLT) in Chile have provided the first direct proof. These results were published in the 5 July 2013 issue of the journal Science (also available on arXiv at arXiv:1306.0134v2 [astro-ph.CO]).
It’s taken so long to prove because gas in intergalactic space is very hard to see. It is extremely diffuse and, with no nearby stars to illuminate it, is completely dark. Fortunately for Nicolas Bouché of the Research Institute in Astrophysics and Planetology (IRAP) in Toulouse, France, lead author of the new paper, his team found a distant galaxy in almost perfect alignment with an even more distant quasar. With the bright quasar backlighting the galaxy, it became possible to study the surrounding gas and see it interact with the galaxy.
When galaxies first form, they are rich with gas and dust, but these resources are rapidly used up to create hundreds of billions of stars. If galaxies are to produce newer generations of stars, such as are typically seen in the outer regions of galaxies, then there must be a source of fresh gas. Astronomers had long suspected that the gravitational pull of galaxies must draw in material from the intergalactic medium. As the gas draws nearer, the predicted behaviour was that the principle of conservation of angular momentum would cause it to start spiralling inwards, rotating with the galaxy.
The astronomer’s observations showed both how the glaxy was rotating, and that the gas was moving according to theory. “The properties of this vast volume of surrounding gas were exactly what we would expect to find if the cold gas was being pulled in by the galaxy,” says co-author Michael Murphy (Swinburne University of Technology, Melbourne, Australia). “The gas is moving as expected, there is about the expected amount and it also has the right composition to fit the models perfectly. It’s like feeding time for lions at the zoo — this particular galaxy has a voracious appetite, and we’ve discovered how it feeds itself to grow so quickly.”