Space and Physics

Two Early Galaxies Grew Old Before Their Time


Stephen Luntz

Freelance Writer

clockApr 30 2014, 14:00 UTC
802 Two Early Galaxies Grew Old Before Their Time
NASA/STScI; S. Allam and team; and the Master Lens Database (, L. A. Moustakas, K. Stewart, et al. (2014) The galaxy S0901 was behaving old when the universe was still young
Two galaxies have been observed that were rotating in a way expected to require billions of years to achieve, despite the fact that we are seeing them as they were not that long after the Big Bang. The findings could trigger a rethink of what it takes to stimulate star formation.
Galaxies did not appear until at least 200 million years after the Big Bang and when they did they at first looked different from those we see around us today. Gradually their gravity attracted gas clouds around them, which fell onto the galaxy proper triggering bursts of star formation that in turn account for the open clusters that dot the arms of spiral galaxies like our own. In the process they grew up, becoming not only larger but also more stable with consistent rotation rather than turbulent motion.
Professors James Rhoads and Sangeeta Malhotra of Arizona State University investigated two galaxies, The Clone and S0901. At 10 billion light years away both are being seen as they were around 4 billion years after the universe began, and therfore fairly early in their lives.
"The purpose of this project is to study the physical conditions of gas in those galaxies. We wanted to know: are they similar to the galaxies around us or is there some difference in their physical conditions," says Rhoads. Aside from names that sound like they belong to the light relief in a science fiction film The Clone and S0901 were chosen not because they themselves are unusual, but because they sit behind closer galaxies positioned to act as a gravitational lens. The bending of light that occurs when huge concentrations of mass act as gravitational lenses focuses light from more distant objects in a particular part of space. When the alignment is right for us we get treated to views of the early universe we would never be able to see with our current equipment.
With less time to cannibalize surrounding galaxies S0901 and The Clone are only 10-20% of the size of the Milky Way as we see them, although they have probably grown to a similar size over the intervening 10 billion years. However, despite at first seeming like pretty typical specimens of their era Rhoads and Malhotra reported in The Astrophysical Journal that S0901 had a rotation speed of 120km/s, while The Close is turning at 79km/s. These are speeds far more common in nearby highly evolved galaxies than those we see early in their lives. It seems the pair are well ahead of their peers.
The situation for The Clone was ambiguous, with Rhoads and Malhotra finding two vastly different models of the dispersion of gasses could explaining the measurements of ionized carbon from star forming regions. However, S0901 was a different matter with a clear indication of an orderly rotation such as we see in modern galaxies, rather than the turbulence that typifies young galaxies. 
"Usually, when astronomers examine galaxies at this early era, they find that turbulence plays a much greater role than it does in modern galaxies. But S0901 is a clear exception to that pattern, and the Clone could be another," says Rhoads. “This galaxy is the equivalent of a 10-year-old. I can tell you from watching my kids’ classes that 10-year-olds like to fidget! S0901 is unusual because it’s not fidgeting, and instead is very well behaved,” Rhoads told RedOrbit.
"Galaxies 10 billion years ago were making stars more actively than they do now," says Malhotra, "They usually also show more turbulence, likely because they are accumulating gas faster than a modern galaxy does. But here we have cases of early galaxies that combine the 'calm' rotation of a modern one with the active star formation of their early peers. This suggests first that these galaxies have finished accumulating their gas, at least for now. But it also seems that turbulence is not actually required to trigger that early, active star formation." 
Whether these two are curious exceptions or indicative of an unrecognized class of young but mature galaxies remains to be seen. "This is not the last word on this. We need a bigger sample to be sure of our conclusions," Malhotra says. Sadly this may take some time. Rhoads and Malhotra used the European Space Agency's Herschel Space Telescope. However, Herschel's liquid helium coolant ran out last year. Work has continued using the Atacama Large Millimeter/submillimeter Array seeking galaxies of similar age that can be examined in similar detail.

Space and Physics