Scientists studying role galactic collisions have in star formation
The larger NGC 7752 galaxy and smaller NGC 7753, collectively called Apr86, are in the process of merging. The 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. Image Credit: NASA/JPL-Caltech
Scientists working on the Great Observatories All-sky Luminous Infrared Galaxies Survey (GOALS) survey have spent the last 10 years studying galaxies in the process of merging with one another in an effort to learn why some mergers trigger star formation while others shut it down.
The study combined data collected by NASA’s Hubble Space Telescope, Spitzer Space Telescope, Chandra X-ray Observatory, and Galaxy Evolution Explorer (GALEX) the European Space Agency’s Herschel satellite and several ground-based telescopes, including the Atacama Large Millimeter Array, the Keck Observatory, and the National Science Foundation’s (NSF) Very Large Array.
This photo shows the merger of two galaxies, the one on the right known as NGC 6786 and the one on the left known as UGC 11415, also collectively called VII Zw 96. It is composed of images from three Spitzer Infrared Array Camera channels: IRAC channel 1 in blue, IRAC channel 2 in green and IRAC channel 3 in red. Photo and Caption Credit: NASA/JPL-Caltech
According to NASA, galactic mergers happened much more frequently between six and 10 billion years ago than they do now. To understand these ancient processes, which shaped the current universe, researchers studied nearby pairs of galaxies in the process of merging as substitutes for the ancient ones. In total, scientists on the GOALS project studied 200 pairs of galaxies, all in different stages of merging.
In three separate images of merging galaxies released by GOALS, colors are used to indicate various wavelengths of light beyond the optical range, with blue and green representing light emitted by stars and red representing light emitted by dust. Blue light emits at 3.6 microns, green light at 4.5 microns, and red light at 8.0 microns.
Nearby merging galaxies in the study appeared especially bright in infrared wavelengths.
Most galaxies have central supermassive black holes, whose nature could be the determining factor as to whether a merger generates or terminates star formation.
When two galaxies merge, their gas and dust are pushed to both of their centers, often setting off a wave of star formation and feeding the central black hole. Eventually, the two central supermassive black holes in merging galaxies themselves merge into one even-larger central black hole.
Star formation triggered by galactic mergers can go on for billions of years, causing the newly-merged galaxy to grow significantly larger.
However, when gas and dust fall into and feed these supermassive black holes, they often cause instability, generating shockwaves that travel throughout each galaxy. These shockwaves, especially if they occur repeatedly, eject gases from the galaxy, thereby shutting down the star formation process and causing the new galaxy to weaken and fade.
Yet when scientists looked for these shockwaves in a merger where one of the galaxies hosts an active central supermassive black hole, they failed to detect any.
Active supermassive black holes are extremely bright, as they feed on infalling material. According to NASA, the researchers, who observed this pair using the Keck Observatory in Hawaii, are uncertain as to why the active supermassive black hole did not generate shockwaves and recognize they do not yet fully understand the role these types of central black holes in galactic mergers.
This photo shows two merging galaxies 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. Photo and Caption Credit: NASA/JPL-Caltech
Laurel Kornfeld
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.
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