Probing the mysteries of the Cosmic Snake

In a study, the subject matter of which sounds like a topic for a New Age journal, astronomers using the Hubble Space Telescope (HST) are probing the anatomy of a galactic feature known as the “Cosmic Snake”, hoping to understand star-formation processes that occurred billions of years ago.

Too faint to have an official name, the Cosmic Snake is a galaxy that indeed looks like a giant worm. It appears to be winding its way through a cluster of galaxies known as MACS1206.2-0847. But instead of being part of the cluster, it is a more distant galaxy that happens to lie in the same line of sight.

That distant galaxy isn’t really snake-shaped. Instead, its image has been stretched, fragmented, and pieced back together again by an effect known as gravitational lensing, which occurs as its light passes through the cluster. 

“The mass of the cluster is able to warp space-time,” says Antonio Cava, an astrophysicist at the University of Geneva, Switzerland. “The light coming from the background galaxy can follow different paths, producing multiple deformed images.”

That which we see as a snake, he says, really consists of four separate images of the galaxy, twisted and lying end to end. A fifth image, which he calls the “counter-image,” lies off to the side. 

The Cosmic Snake would be interesting enough if it were simply an exotic optical effect. But in addition to distorting and multiplying images, gravitational lensing also magnifies them. 

In a study published this week in Nature Astronomy, an international team led by Cava used this magnification to study the background galaxy in unprecedented detail. In particular, they looked for bright regions known as clumps, in which massive numbers of stars are caught in the process of forming.

Prior studies of distant galaxies had shown these clumps to be enormous — as large as 3000 light years across. That makes them a thousand times larger than similar clumps in nearby galaxies, in which star formation is also occurring. 

This difference in scale has been a conundrum for astrophysicists, who could not explain why the more distant galaxies had such dramatically larger clumps. One partial explanation was that that the light from those more distant galaxies has been traveling for so long that by looking at them we are, in effect, peering billions of years back into the youth of the universe. 

Could star formation process back then have been somehow different than now? If so, astrophysicists couldn’t figure out why.

What Cava’s team learned from the Cosmic Snake’s giant “natural telescope” is that without the help of gravitational lensing, even our best telescopes are simply not powerful enough to see the clumps in sufficient detail.

To prove it, they took advantage of the fact that different parts of the Cosmic Snake are magnified by different amounts. 

The counter-image, for example, is only magnified by a factor of five by the lensing effect. It shows a small number of large clumps, similar to those been seen for other galaxies. 

But the snake itself is magnified by as much as a factor of 100, Cava says, allowing the HST to see details 100 times smaller than could otherwise be detected. 

That revealed that instead of having a relatively small number of giant clumps, the distant galaxy actually had numerous smaller ones, not all that different from those seen in nearby galaxies.

“We have reduced the differences between what we observe in the nearby universe and in distant galaxies from a factor of 1000 to a factor of 10,” says Daniel Schaerer, a professor at Geneva Observatory who was also part of the study team. 

Ryo Ando, an astronomer from the University of Tokyo whose own study of star-forming clumps in a nearby galaxy was published recently in The Astrophysical Journal, calls the new study ingenious. 

Astronomers studying nearby galaxies, he says, had previously realised that looking at them without sufficient resolution might produce an “averaged view” in which numerous smaller clumps merged to look bigger than they really are. The new study proves that this clump-merging effect is real, he says: “It demonstrates the importance of high-resolution observations. It is really intriguing.”

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