NASA's X-ray observatory Chandra has released an incredible image of a pair of colliding galaxies known as Arp 299. The observation revealed much more than just a stunning picture, the entire system itself has intriguing sources aplenty!   

The international research team was able to spot 25 individual sources, 14 of which are emitting so many X-rays that they are categorized as “ultra-luminous X-ray sources” (ULXs). The whole system is also bathed in the soft glow of intergalactic hot gas and the intense emissions from the cores of the two merging galaxies.

This extreme setup makes for some fascinating physics. These ULXs are either black holes or neutron stars stealing material from a companion. As the material rains down on the compact objects, it is under incredible gravitational strain, which heats it up until it begins to shine in X-rays. Arp 299’s ULXs are found in the middle of regions where stars are forming at a high rate.

While this is one of the few locations with that many ULXs in the local universe, there are slightly fewer than the expected number from theoretical arguments. According to the research paper, published in the Monthly Notices of the Royal Astronomical Society, the galaxy is rich in heavy elements, which leads to smaller stars on average and could explain why we see fewer sources.

The image is a mixture of X-ray data from Chandra and NuSTAR (pink and purple, respectively) and optical from Hubble (white and brown). But that’s not the only thing that researchers have been looking at – the galaxy also emits a significant amount of infrared light. This is unsurprising as infrared is a good tracer of star formation.

The combined X-ray and infrared emissions are remarkably similar to what we observe in the very early universe, where collisions between galaxies happened often and the star-formation rate of galaxies was much higher than today. Arp 299 is a great local proxy for all those distant galaxies.

Arp 299 is located 140 million light-years from us and is in an advance stage of merging. The whole merging process will probably take another billion years to complete. At that point, the two galaxies will become a single elliptical-looking object.