The Cosmic Commute of Gas Clouds Towards Star and Planet Formation

The researchers found that cold molecular gas motions appear to fluctuate in velocity, reminiscent in appearance of waves on the surface of the ocean. These fluctuations represent gas motion. “The fluctuations themselves weren’t particularly surprising, we know that the gas is moving,” says Henshaw. Steve Longmore, co-author of the paper, based at Liverpool John Moores University, adds, “What surprised us was how similar the velocity structure of these different regions appeared. It didn’t matter if we were looking at an entire galaxy or an individual cloud within our own galaxy, the structure is more or less the same.”

Image of the molecular gas (carbon monoxide) distribution in the southern spiral arm of the galaxy NGC 4321 spanning roughly 15,000 light years across. The bright spots indicate giant molecular clouds that are semi-regularly spaced inside the ridge of more dilute gas inside the spiral arm. The cyan circles depict the locations of star forming complexes.

© J. Henshaw/MPIA

To better understand the nature of the gas flows, the team selected several regions for close examination, using advanced statistical techniques to look for differences between the fluctuations. By combining a variety of different measurements, the researchers were able to determine how the velocity fluctuations depend on the spatial scale.

“A neat feature of our analysis techniques is that they are sensitive to periodicity,” explains Henshaw. “If there are repeating patterns in your data, such as equally spaced giant molecular clouds along a spiral arm, we can directly identify the scale on which the pattern repeats.” The team identified three filamentary gas lanes, which, despite tracing vastly different scales, all seemed to show structure that was roughly equidistantly spaced along their crests, like beads on a string, whether it was giant molecular clouds along a spiral arm or tiny “cores” forming stars along a filament.

The team discovered that the velocity fluctuations associated with equidistantly spaced structure all showed a distinctive pattern. “The fluctuations look like waves oscillating along the crests of the filaments, they have a well-defined amplitude and wavelength,” says Henshaw adding, “The periodic spacing of the giant molecular clouds on large-scales or individual star-forming cores on small-scales is probably the result of their parent filaments becoming gravitationally unstable. We believe that these oscillatory flows are the signature of gas streaming along spiral arms or converging towards the density peaks, supplying new fuel for star formation.”

In contrast, the team found that the velocity fluctuations measured throughout giant molecular clouds, on scales intermediate between entire clouds and the tiny cores within them, show no obvious characteristic scale. Diederik Kruijssen, co-author of the paper based at Heidelberg University explains: “The density and velocity structures that we see in giant molecular clouds are ‘scale-free’, because the turbulent gas flows generating these structures form a chaotic cascade, revealing ever smaller fluctuations as you zoom in – much like a Romanesco broccoli, or a snowflake. This scale-free behaviour takes place between two well-defined extremes: the large scale of the entire cloud, and the small scale of the cores forming individual stars. We now find that these extremes have well-defined characteristic sizes, but in between them chaos rules.”

“Picture the giant molecular clouds as equally-spaced mega-cities connected by highways,” says Henshaw. “From a birds eye view, the structure of these cities, and the cars and people moving through them, appears chaotic and disordered. However, when we zoom in on individual roads, we see people who have travelled from far and wide entering their individual office buildings in an orderly fashion. The office buildings represent the dense and cold gas cores from which stars and planets are born.”