A study on the formation of star clusters following galaxy mergers

6 galaxy merger systems: at the top are the galaxies NGC 3256, NGC 1614, and NGC 4194; at the bottom are the galaxies NGC 3690, NGC 6052, and NGC 34.
An article published in the journal “Monthly Notices of the Royal Astronomical Society” reports a research on the star formation rate in galaxy merger systems. A team of researchers used observations carried out with the Hubble Space Telescope within the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC) survey to study the influence of a galaxy merger on star formation, particularly of entire star clusters. ESA has published a composition of six such systems. The image (NASA & ESA) shows those six galaxy merger systems: at the top are the galaxies NGC 3256, NGC 1614, and NGC 4194; at the bottom are the galaxies NGC 3690, NGC 6052, and NGC 34.

Mergers between galaxies are very common and in cases where there’s a remarkable difference in mass between two galaxies merging the more massive one absorbs the other. For example, the Milky Way absorbed some dwarf galaxies. Such events can go on for many millions of years and dramatically alter the appearance of the galaxies involved.

The space occupied by a galaxy is very large, so a galaxy merger hardly leads to a collision between stars within it. The situation is different for the supermassive black holes that galaxies generally host at their center and, in a merging event, they attract each other until they merge.

While existing stars in merging galaxies may not be affected, the clouds of gas and dust within them may undergo major transformations. The decidedly out of the ordinary gravitational interactions can generate compressions and shock waves in those clouds that trigger a new star formation.

In such a situation, entire star clusters can form that are much larger than the average size. The star clusters in the Milky Way have masses that can be around 10,000 times the Sun’s. Clusters that form during a galaxy merger can reach masses of up to millions of times the Sun’s.

Those dense star clusters are also very bright, a great advantage for astronomers, who can study them in detail to better understand the processes that took place or are still taking place in cosmic environments that can be extreme.

The Hubble Space Telescope can detect many of those details. The ultraviolet and near-infrared observations made it possible to estimate the age, masses, star formation, and even the extinctions of star clusters in the six merging galaxy systems object of this study.

The HiPEEC survey made it possible to discover that populations of star clusters undergo changes in their morphological, physical, and chemical properties that are dramatic and very rapid in astronomical terms. One of the conclusions reached by the researchers is that the most massive star clusters form towards the end of the galaxy merger process.

Current instruments offer very interesting details on these star cluster formation processes, but their history is still incomplete. It will take next-generation instruments to obtain even more detailed observations that can go beyond interstellar dust to take this type of study a step further.

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