Milky Way's outer disk isn't the smooth curve we thought

A team of Chinese astronomers has discovered wavy ripples stretching across the outer gas disk of the Milky Way – a finding that shows our galaxy's disk is far more complicated than scientists once thought.

The work, published online Friday in the journal Nature Astronomy, was led by researchers at the Purple Mountain Observatory, Chinese Academy of Sciences.

For decades, astronomers knew the Milky Way's disk bends and twists at its edges – a shape known as a "warp." But smaller ripples on top of that larger warp were too subtle to spot. The outer disk has long been a blank spot on the map.

To fill it in, the team used data from China's 13.7-meter millimeter-wave telescope. They tracked more than 30,000 clouds of cold carbon monoxide gas, a common marker of the "stuff" that shapes stars.

By building a careful model of the curve, the astronomers saw that the background structure was full of clear vertical ripples, or "folds." And these folds were everywhere across the outer disk.

That means the Milky Way's outer disk is not the smooth, gently bent shape earlier models suggested. Instead, it looks like a restless, textured sheet with ripples at different scales.

The most likely reason, the researchers say, is "bending waves" set off by gravitational shoves from outside. That's a clue about the Milky Way's past: it hints at a richer set of encounters with other bodies than previously thought, leaving lasting imprints in its cool gas.