The researchers compared their new optical images with archival data: HI measurements from the THINGS survey and ultraviolet observations from the GALEX satellite. The faint stellar structure they found lines up with the outer contours of the hydrogen gas and with the previously known ultraviolet-extended disk.

"Now, our ultra-deep optical data reconcile the galaxy's apparent size with the observed HI and UV extents," the team wrote in its paper.

How galaxies grow

The result speaks to a broader question in astronomy: how do galaxies grow? Over time, galaxies accumulate gas from their surroundings. Because this incoming gas spins faster relative to the center, it cannot collapse all the way to the core. Instead, it settles into the outer disk, where it eventually sparks the formation of new stars. Astronomers have observed this process, but they still do not know whether it proceeds smoothly and gradually or in fast, episodic bursts. Mergers or interactions with satellite galaxies can also trigger bursts of star formation in the outskirts of galactic disks, making them grow bigger.

One challenge in identifying such growth is definitional. Astronomers still debate how to draw a galaxy's "optical boundary," which makes XUV disks difficult to classify in the first place.

A possible culprit: a passing satellite

To understand what lit up M74's outskirts, the researchers analyzed the colors of the newly found stars using a stellar population model—a tool that links a star population's color to its age and chemical makeup. The analysis suggests the stars formed roughly 640 million years ago. The estimate depends on metallicity, the abundance of elements heavier than hydrogen and helium; possible ages range from about 260 million years to just over 1 billion years. Even the upper end of that range is extremely recent on galactic timescales.

Based on that age estimate, the team favors a specific trigger: a close gravitational encounter with an irregular, blue satellite galaxy. "Given this and the visual inspection of the field, we propose a recent flyby of UGC 1176 at least 1 Gyr ago as the trigger for the new star formation in the outer regions," they conclude. A gigayear, abbreviated Gyr, is one billion years.

The researchers reached this scenario by comparing the estimated age of the new stars with how long such an encounter would take to trigger star formation in a galaxy's outskirts. It is worth keeping in mind that this is one interpretation of the evidence rather than a settled conclusion. Whether rapid, flyby-triggered disk growth like this is common across the universe—or whether M74 is simply unusual—remains an open question for future work.