By Daniel Cray
Hurricane Isaac fades into history and Hurricane Leslie stirs to life. As it happens, a stiff wind in space shouldn’t be overlooked either. So-called galactic winds, which are primarily photons shed from massive stars but sometimes also include thermal energy from exploding stars, are basically interstellar housekeepers. They careen outward from a galaxy’s core, applying pressure to dust molecules which in turn sweep gas and debris to the galaxy’s border — but most of the time no farther. The winds from massive stars are simply not powerful enough to push any harder, unless they have help from black holes, the suspected source of such prodigious energy.
Or that’s the way it always seemed. But astrophysicists have just discovered evidence that intense star formation in nearly two dozen small, dense galaxies is fueling galactic winds so fierce that they’re reaching escape velocity, driving free-floating dust and gas completely out into intergalactic space. The result of such a thorough housecleaning: the winds are stripping the galaxies of their fuel for producing new generations of stars.
“Usually gas gets blown out but then falls back into a galaxy, where it can form more stars,” says Aleksandar Diamond-Stanic, a fellow at the University of California, San Diego, and the lead author of a paper on the winds recently published in Astrophysical Journal Letters. “In these cases, the gas gets blown out and never comes back.”
That takes a lot of oomph. If you think Isaac’s 80-mph wind speeds were bad, try 1,500 miles — per second. That’s the velocity researchers clocked within four galaxies, each of them several billion light years from Earth. Overall, they found 14 galaxies with outflow velocities topping 621 miles (1,000 km) per second, fast enough to escape most galaxies’ gravitational pull.
To figure out what was going on, Diamond-Stanic and his colleagues had to become galactic storm chasers, attempting to answer the obvious question: what’s driving the high winds? They began by scrutinizing spectrographic data from two telescopes — the Multiple Mirror Telescope in Arizona and Hawaii’s Keck telescope. The researchers probed ultraviolet light from the gas located in front of 50 galaxies for precise changes in wavelength, which reveals the speed the gas was traveling. They also studied the galaxies’ infrared light, which serves as an interstellar thermometer since star-heated dust emits energy in that wavelength.
Finally, the astronomers sampled 29 of the 50 galaxies using the Hubble Space Telescope, then overlaid their information with data from one of the best maps available, compiled by NASA’s Wide-field Infrared Survey Explorer (WISE), which scanned the entire sky twice in infrared light. By combining that high-tech hodgepodge of data, the team was able to refine their wind velocity estimates and — significantly — calculate the physical size of the 29 galaxies.
That last measurement turned out the be the key: the 22 galaxies with high outflow velocities are only about 167 parsecs (500 light years) in diameter — tiny compared to, say, our Milky Way, which spans about 10,000 parsecs (or 30,000 light years). “Until then, we didn’t realize these galaxies are so small and so incredibly compact,” Diamond-Stanic says. The galaxies are packed with hot, newly formed stars, some 10,000 times the star density of the Milky Way, and all that star formation in such small regions concentrates and amplifies the galactic winds.
“Essentially you have many bombs going off very close to each other at about the same time, and each bomb can evacuate the area around it,” Diamond-Stanic says. “Then the next star pushes that gas even further, until eventually all the gas can be evacuated on the scale of that galaxy.”
No one knows whether such galactic tempests are unusual events or a routine phase of galaxy development — and we could hardly expect to have such answers yet. Terrestrial meteorology is already enough of a puzzle, and we live in the midst of it. Cosmic meteorology — studied at cosmic distances — will require a whole lot more study still.