An international team of astronomers using the William E. Gordon Telescope at the Arecibo Observatory in Puerto Rico has discovered a 2.6-million-light-year-long bridge of atomic hydrogen gas between galaxies in the NGC 7448 galaxy group, located about 500 million light years away.
The 2.6-million-light-year-long stream of atomic hydrogen gas, shown in green, stretches from the large galaxy at the bottom left to the group of galaxies at the top; a third nearby galaxy to the right also has a shorter stream of gas attached to it; the three insets show expanded views of the different galaxies and the green circle indicates the Arecibo telescope beam. Image credit: Rhys Taylor / Arecibo Galaxy Environment Survey / the Sloan Digital Sky Survey Collaboration, www.sdss.org.
“This was totally unexpected,” said Dr Rhys Taylor of the Czech Academy of Sciences, who is the lead author of the paper reporting the discovery in the Monthly Notices of the Royal Astronomical Society.
“We frequently see gas streams in galaxy clusters, where there are lots of galaxies close together, but to find something this long and not in a cluster is unprecedented.”
“It is not just the length of the stream that is surprising but also the amount of gas found in it,” added co-author Roberto Rodriguez of the University of Puerto Rico in Humacao.
“We normally find gas inside galaxies, but here half of the gas – 15 billion times the mass of the Sun – is in the bridge. That’s far more than in the Milky Way and Andromeda galaxies combined!”
The astronomers are still investigating the origin of the stream.
One notion surmises that the large galaxy at one end of the stream passed close to the group of smaller galaxies at the other end in the past, and that the gas bridge was drawn out as they moved apart, while a second notion presumes that the large galaxy plowed straight through the middle of the group, pushing gas out of it.
The scientists plan to find out which of these ideas can best match the shape of the bridge that is seen with the Arecibo Telescope.
_____
R. Taylor et al. 2014. The Arecibo Galaxy Environment Survey – VII. A dense filament with extremely long H i streams. MNRAS 443 (3): 2634-2649; doi: 10.1093/mnras/stu1305
