Assembling the largest galaxies

The galaxy featured in this week’s Hubble Picture of the Week is the dwarf irregular galaxy NGC 5238, located 14.5 million light-years from Earth in the constellation Canes Venatici. Its unexciting, blob-like appearance, resembling more an oversized star cluster than a galaxy, belies a complicated structure which has been the subject of much research by astronomers. Here, the NASA/ESA Hubble Space Telescope is able to pick out the galaxy’s countless stars, as well as its associated globular clusters — the glowing spots both inside and around the galaxy that are swarmed by yet more stars.

NGC 5238 is theorised to have recently — here meaning no more than a billion years ago! — had a close encounter with another galaxy. The evidence for this is the tidal distortions of NGC 5238’s shape, the kind produced by two galaxies pulling on each other as they interact. There’s no nearby galaxy which could have caused this disturbance, so the hypothesis is that the culprit is a smaller satellite galaxy that was devoured by NGC 5238. Traces of the erstwhile galaxy might be found by closely examining the population of stars in NGC 5238, a task for which the Hubble Space Telescope is an astronomer’s best tool. Two tell-tale signs would be groups of stars with properties that look out of place compared to most of the galaxy’s other stars, indicating that they were originally formed in a separate galaxy, or stars that look to have all formed abruptly at around the same time, which would occur during a galactic merger. The data used to make this image will be put to use in testing these predictions.

Despite their small size and unremarkable appearance, it’s not unusual for dwarf galaxies like NGC 5238 to drive our understanding of galaxy formation and evolution. One main theory of galaxy evolution is that galaxies formed ‘bottom-up’ in a hierarchical fashion: star clusters and small galaxies were the first to form out of gas and dark matter, and they gradually were assembled by gravity into galaxy clusters and superclusters, explaining the shape of the very largest structures in the Universe today. A dwarf irregular galaxy like NGC 5238 merging with an even smaller companion is just the type of event that might have begun this process of galaxy assembly in the early Universe. So, it turns out that this tiny galaxy may serve as a test of some of the most fundamental predictions in astrophysics!

[Image Description: A dwarf irregular galaxy. It appears as a cloud of bluish gas, filled with point-like stars that also spread beyond the edge of the gas. A few glowing red clouds sit near its centre. Many other objects can be seen around it: distant galaxies in the background, four-pointed stars in the foreground, and star clusters that are part of the galaxy - shining spots surrounded by more tiny stars.]