The largest galaxies in the Universe may grow up in cosmic oceans of cold gas, scientists have discovered. This finding is published in the journal Science today by an international team of astronomers, lead by Bjorn Emonts of the Centro de Astrobiología (CSIC-INTA) in Madrid.
Galaxies often huddle together in hundreds or thousands, forming a cluster. At the heart of these clusters lie the largest galaxies in the Universe, which are enormous spheres of stars. “We expected that these super-galaxies in the distant, early Universe would form from small galaxies falling together under their own gravity and merging, just like we see in the nearby Universe”, said Bjorn Emonts, “but the story is much more complicated than that”, he added.
The astronomers turned radio telescopes in Australia and the US on an embryonic cluster, 10 billion light-years away. This cluster has a super-galaxy nacknamed the “Spiderweb” forming at its center. There the team found something surprising: the merging galaxies wallowing in a cosmic cloud of very cold gas.
The enormous gas cloud contains roughly 100 billion times the mass of our Sun. This cold gas consists mostly of hydrogen molecules, the basic material from which stars and galaxies form.
Rather than seeing the hydrogen directly, the astronomers located it by detecting a tracer gas, carbon monoxide, which is easier to detect. “We expected the cold gas to be found within the merging galaxies”, said co-author Helmut Dannerbauer from the Instituto de Astrofísica de Canarias (IAC), who in 2014 revealed that the Spiderweb is surrounded by many galaxies hidden in a thick layers of dust.
But observations with the Australia Telescope Compact Array and the Very Large Array telescope in the US revealed that most of the cold gas did not show there, but instead occupies the vast space between the galaxies. The astronomers now think that this super-galaxy is condensing directly out of this cosmic ocean of cold gas.
Co-author Montserrat Villar-Martín from the Centro de Astrobiología said: “We know now how and where to look for the giant reservoirs of cold gas from which the largest galaxies in the Universe grow. From now on, the most advanced astronomical technology can race in their quest”.
Where the cold gas came from is still a puzzle. “The carbon monoxide that we detected is a by-product of previous stars, a form of cosmic recycling, but we cannot say for sure where the gas came from or how it accumulated in the cluster core”, Bjorn Emonts said. “To find out we’d have to look even deeper into the Universe’s history”.
Figure: Artist’s impression of the cosmic “ocean” of very cold gas discovered in the heart of an embryonic cluster of galaxies, about 10 billion light-years away. Lumpy and misshapen, the central region was dubbed “Spiderweb Galaxy” (formally known as MRC 1138-262), because it appears to be made of small galaxies hurled together by gravity, just as flies are captured by a spider’s web. The cold gas stretches across a quarter of a million light-years, and is the raw material from which new stars are formed. A single super-galaxy is expected to condense out of this cosmic gas cloud. Modified from ESO Science Release 1431. Credit: ESO/M. Kornmesser. This figure is licensed under CC BY 4.0 International License (https://creativecommons.org/licenses/by/4.0/)