How do we know that a galaxy recently collided with another?

This new NASA Hubble Space Telescope image shows the luminous blue compact galaxy (BCG) called ESO 185-IG013. Located about 260 million light-years away, the stars in this system have a combined mass more than 7 billion times that of our Sun.

BCGs are nearby galaxies that show an intense burst of star formation. What sets them apart from other high-starburst galaxies that emit more infrared light is their unusually blue appearance in visible light. BCGs are of interest because they provide a relatively close-by equivalent for galaxies from the early universe, meaning they can help scientists learn about galaxy formation and evolution that may have been happening billions of years ago.

Hubble imaged ESO 185-IG013 in ultraviolet, visible, and infrared wavelengths to reveal details about its past. The galaxy is populated by hundreds of young star clusters, many of which are younger than 100 million years while a large number of star clusters are only 3.5 million years old - relatively young compared to the timescale of our universe.

How do we know that ESO 185-IG013 recently collided with another?

The large number of young star clusters in the galaxy indicates that it was a part of a recent galaxy collision and merger. Its perturbed structure, which is believed to have occurred from the violent interactions of gas and dust during the collision, is another sign of a merger, which supplied the system with lots of fuel for star formation.

ESO 185-IG013 also boasts a tidal shell - the diffuse glow surrounding its bright center - which is also a common signal of galaxy mergers. In addition to the shell, the galaxy has a tail of gas in the northeast.

Show us a sign, ESO 185-IG013!How do we know this galaxy recently collided with another? It's full of young star clusters! The disturbed structure of the galaxy is another sign of a merger, which supplied the system with lots of fuel for star formation: https://t.co/WIJlSQ9x0W pic.twitter.com/JY7eTxlkyb

— Hubble (@NASAHubble) February 2, 2024