So it is with the galaxy triplet Arp 248, an arrangement of interacting galaxies that is both visually and scientifically fascinating. Arp 248 is a trio of small interacting galaxies about 200 million light-years away in the constellation Virgo. The image shows two of Arp 248’s galaxies framing another smaller unrelated galaxy in the background. Galaxies are connected by a stream of stars, gas and dust, created as galaxies pull each other gravitationally. Remove all ads on Universe Today Join our Patreon for just $3! Get the ad-free experience of a lifetime Arp 248 is also known as Wild’s Triplet, after astronomer Paul Wild (1923–2008), who studied the triplet in the early 1950s. Image credit: Credit line and copyright Adam Block/Mount Lemmon SkyCenter/University of Arizona – CC BY-SA 3.0 us, https://commons.wikimedia.org/w/index.php?curid=20540032 Astronomers call the currents “tidal tails.” When dusty and gas-rich galaxies like Arp 248 merge, the merger often forms tails. The tails are made of material from the outer spiral disks of merging galaxies and host active star formation indicated in blue. The top image is from an observational project looking at two collections of unusual galaxies that include Halton Arp. Arp was an American astronomer who created the Atlas of Strange Galaxies in 1966. The Atlas contains 338 galaxies selected for their unusual shapes. He intended to highlight the variety of strange structures that galaxies take on. The Universe is full of galaxies whose shape has been altered by interactions and mergers. This is Centaurus A, an elliptical galaxy that merged with a spiral galaxy about 300 million years ago. The merger created the dark dust lane, which is not usually a feature of elliptical galaxies. The merger also formed a gas spiral in the core of Centaurus A. Image credit: ESA We now know that these galaxies take on such strange shapes because they are interacting and potentially merging. Arp disagreed with this interpretation and said that the unusual forms were due to ejections. But in any case, Arp realized that astronomers didn’t know much about how galaxies change over time, and he intended for astronomers to use his atlas to study the evolution of galaxies. The second collection of unusual galaxies in the observing project is called the Catalog of Southern Peculiar Galaxies and Associations. It was published in 1987 by Arp and his colleague Barry Madore. The Catalog contains 25 different varieties of objects, including comets. Astronomers have expanded their knowledge of interacting galaxies and galaxy mergers since the Atlas and Catalog were published. We know that mergers play an important role in the evolution of galaxies. Interacting galaxies are found throughout the Universe, sometimes as dramatic collisions that trigger bursts of star formation, at other times as stealthy mergers that result in new galaxies. These images are from a series of 59 images of colliding galaxies released from archived raw images from the NASA/ESA Hubble Space Telescope. Image credit: NASA/ESA/STScI As astronomers study interacting galaxies in more detail, they are discovering a new class of objects they call “intergalactic star-forming objects” (ISFOs). ISFOs are a broad class of objects that capture the different types that form when galaxies interact. ISFOs can form due to tidal interactions and the scavenging of material from interacting galaxies. They can also develop due to the inflow of gas and dust into the tails and through a combination of all these processes. ISFOs can range in mass from super star clusters to what astronomers call “tidal dwarf galaxies” (TDGs.) A 2012 paper based on the Sloan Digital Sky Survey estimated that about 6% of dwarf galaxies could have tidal origin. This image shows NGC7252, a strange galaxy formed by a merger of two galaxies a billion years ago. The white circles highlight the positions of two tidal dwarf galaxies (TDGs) forming in the tails. Researchers believe that about 6% of dwarf galaxies are TDGs. Image credit: Frederic Bournaud/Pierre-Alain Duc. ISFOs are often gravitationally bound to galaxies, but how many remain bound and for how long is still an open question. Sometimes material from the tidal currents will flow back into the galaxies, causing more star formation. The material left over from all this interaction enriches the interstellar medium with dust and metals. Astronomers now believe that about 25% of galaxies are currently merging with other galaxies. Even more of them are gravitationally interacting, if not merging, according to the Harvard Center for Astrophysics. Our Milky Way is proof of this, as it cannibalized gas and even stars from the Magellanic Clouds and the Sagittarius Dwarf Galaxy. And in several billion years, the Milky Way and the Andromeda Galaxy will merge. Who knows what behemoth may emerge from this event? This series of photos shows the predicted merger between our Milky Way and the neighboring Andromeda Galaxy. Credit: NASA; ESA? Z. Levay and R. van der Marel, STScI; T. Hallas and A. Mellinger How supermassive black holes (SMBHs) become so massive is also an open field of research. Astrophysicists know that mergers are part of the SMBH’s growth process, but there’s a lot they don’t know. The James Webb Space Telescope captured this image of Stephan’s Quintet. It’s a quintet of galaxies, four are interacting and the fifth is only optically connected. The leftmost galaxy is NGC7320 which is well in the foreground of the other four. The image is a composite of nearly 1,000 separate images. The four stars and their interactions produce tails, regions of active star formation, glowing regions containing millions of young stars, and shock waves from NGC 7318B as it bullies its way through the cluster. NGC 7318B is the leader of the closest pair of galaxies. Image credit: NASA, ESA, CSA and STScI The Hubble Space Telescope’s Advanced Camera for Surveys (ACS) examined this collection of unusual interacting galaxies to lay the groundwork for more detailed study in the future. Hubble will examine some of these targets with its other instruments, as will the James Webb Space Telescope and ALMA. Observing time in these telescopes is always in high demand, so this project will help astronomers to allocate time better.
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