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Galactic mergers were extremely common billions of years ago and have shaped the Universe as we know it. But not all of these unions are long-lived. For some galaxies, such a union will force them to wither and die.

Though only a few galaxies seem to be currently undergoing mergers, the process was much more common 6 – 10 billion years ago and the process has shaped the very Universe around us.



Three new images from NASA’s Spitzer Space Telescope show pairs of galaxies on the cusp of cosmic consolidations. Though the galaxies appear separate now, gravity is pulling them together, and soon they will combine to form new, merged galaxies.



Some merged galaxies will experience billions of years of growth. For others, however, the merger will kick off processes that eventually halt star formation, dooming the galaxies to wither prematurely.

This image shows the galactic mergerof galaxies known as NGC 6786 (right) and UGC 11415 (left), also collectively called VII Zw 96. It is composed of images from three Spitzer Infrared Array Camera (IRAC) channels: IRAC channel 1 in blue, IRAC channel 2 in green and IRAC channel 3 in red. Credit: NASA/JPL-Caltech



For more than 10 years, scientists working on the Great Observatories All-sky LIRG Survey (GOALS) have been using nearby galaxies to study the details of galaxy mergers and to use them to study the universe’s history.

The survey focused on 200 nearby objects, including many galaxies in various stages of merging. The images above show three of those targets, imaged by Spitzer.

One of the primary processes thought to be responsible for a sudden halt in star formation inside a merged galaxy is an overfed black hole. At the centre of most galaxies lies a supermassive black hole— millions to billions of times more massive than the Sun.

During a galactic merger, gas and dust are driven into the centre of the galaxy, where they help make young stars and also feed the central black hole.

This sudden burst of activity can create an unstable environment. Shockwaves or powerful winds produced by the growing black hole can sweep through the galaxy, ejecting large quantities of gas and shutting down star formation. Sufficiently powerful or repetitive outflows can hinder the galaxy’s ability to make new stars.

This image shows two galaxies in a galactic merger known as Arp 302, also called VV 340. In these images, different colors correspond to different wavelengths of infrared light. Blue and green are wavelengths both strongly emitted by stars. Red is a wavelength mostly emitted by dust. Credit: NASA/JPL-Caltech



The relationship between mergers, bursts of star formation, and black hole activity is complex, and scientists are still working to understand it fully.



One of the newly merged galaxies is the subject of a detailed study with the W.M. Keck Observatory in Hawaii, in which GOALS scientists searched for galactic shockwaves driven by the central active galactic nucleus, an extremely bright object powered by a supermassive black hole feeding on material around it.



The lack of shock signatures suggests that the role of active galactic nuclei in shaping galaxy growth during a galactic merger may not be straightforward.

Galactic mergers in the nearby universe appear especially bright to infrared observatories like Spitzer. GOALS studies have also relied on observations of the target galaxies by other space-based observatories, including NASA’s Hubble and Chandra space telescopes, the European Space Agency’s Herschel satellite, as well as facilities on the ground, including the Keck Observatory, the National Science Foundation’s Very Large Array and the Atacama Large Millimeter Array.

Main image: The merger of two galaxies, known as NGC 7752 (larger) and NGC 7753 (smaller), also collectively called Arp86. In these images, different colors correspond to different wavelengths of infrared light. Blue and green are wavelengths both strongly emitted by stars. Red is a wavelength mostly emitted by dust. Credit: NASA/JPL-Caltech





















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