Astronomers have known for decades that turbulent motion mixes and heats the interstellar medium (ISM), the dust and gases between the stars. But until now no one had been able to actually photograph this motion. An article published yesterday in Nature by a team of astronomers not only reveals this turbulent gas, but also that it moves at a low but supersonic velocity.

The images were taken by a group of astronomers led by Bryan Gaensler, and used the Australia Telescope Compact Array. It was focused on a region in the southern galactic plane about 10,000 light-years away, in the constellation Norma.

Turbulent, supersonic gas is characterized mainly by the Reynolds number (the ratio of momentum to viscous forces, which determines when a flow becomes turbulent) and the Mach number (the flow speed divided by the local speed of sound). These quantities have been difficult to measure in the past, since astronomers haven’t been able to get an adequate picture of the gas, but they do know that structures formed by the turbulent motion range from 1,000 km to 100 parsecs (over 1015 km).

Now, describing the ISM as "gas" is a bit of an overstatement, if you’re picturing gas as in our atmosphere. The ISM is extremely low density, ranging from 10-4-106 molecules per cm3 depending on the temperature (for some perspective, the density of air at ocean level on Earth is about 1019 molecules per cm3).

We can’t see the ISM using visible light, so astronomers observe radio waves emitted from the Milky Way and altered by passing through this turbulent region. The gases are ionized, so their swirling, mixing turbulent motion forms electrical and magnetic fields (known as magnetohydrodynamic flow), which in turn affects the polarization of radio waves.

These changes in polarization can be detected by radio telescopes (like the one the team used) and indirectly reveal the direction of flowing gases. The researchers used these measurements to make an image of the region (see above), which shows a complex field of tangled, twisting tendrils (say that three times fast) of gas. These structures are formed by turbulent vortices and intersecting shock waves.

In order to figure out if the gas was moving at subsonic or supersonic speeds, they also performed three-dimensional computer simulations of the ISM, comparing subsonic (Mach number below one), transonic (Mach number around 1), and supersonic (Mach number well above one) models with the real images. The team found that the subsonic and transonic results better matched the network of tendrils seen in the images. Based on this, and prior studies, they concluded that the gas flows at a low Mach number (less than two). Sound moves pretty quickly there, however, so this corresponds to about 70,000 km per hour (even though the density is much lower than in our atmosphere, the temperature and pressure are higher than you'd expect, and the speed of sound depends on the ratio of pressure to density).

Using the same approach, the team plans on studying other properties of this region, and hope to calculate the properties of the ISM more accurately. A better picture of the ISM is important because it will help us better understand the overall motion of galaxies and formation of stars, which occurs in the (relatively) dense regions.

Nature, 2011. DOI: 10.1038/nature10446 (About DOIs)