The perfect tidal tail connects these two galaxies seen by Hubble

The perfect tidal tail connects these two galaxies seen by Hubble

It is sometimes tempting to imagine a supernatural hand behind the arrangement of celestial bodies. But the Universe is big, immense even, and the flow of nature presents many fascinations.

So is the galactic triplet Arp 248, an arrangement of interacting galaxies that is both visually and scientifically fascinating.

Arp 248 is a trio of small, interacting galaxies located about 200 million light-years away in the constellation Virgo. The image shows two of Arp 248’s galaxies flanking another, smaller, unrelated galaxy in the background. The galaxies are connected by a stream of stars, gas and dust, created when the galaxies pull each other gravitationally.

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Arp 248 is also known as Wild's Triplet, after astronomer Paul Wild (1923–2008), who studied the trio in the early 1950s. Image credit: by credit line and copyright Adam Block /Mount Lemmon SkyCenter/University of Arizona - http://www.caelumobservatory.com/gallery/wilds.shtml, CC BY-SA 3.0 us, https://commons.wikimedia.  org/w/index.php?curid=20540032
Arp 248 is also known as Wild’s Triplet, after astronomer Paul Wild (1923–2008), who studied the trio in the early 1950s. Image credit: by credit line and copyright Adam Block /Mount Lemmon SkyCenter/University of Arizona – http://www.caelumobservatory.com/gallery/wilds.shtml, CC BY-SA 3.0 us, https://commons.wikimedia. org/w/index.php?curid=20540032

Astronomers call the currents “tidal tails”. When dusty, gas-rich galaxies like Arp 248 merge, the merger frequently forms tails. The tails are made of material from the outer spiral disks of merging galaxies, and they host active star formation shown in blue.

The top image is from an observational project examining two unusual galaxy collections involving Halton Arp. Arp was an American astronomer who created the Atlas of Peculiar Galaxies in 1966. The Atlas contains 338 galaxies chosen for their unusual shapes. He wanted it to highlight the variety of particular 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 fusion also formed a gas spiral in the core of Centaurus A. Image credit: ESA
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 fusion 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 interact and potentially merge. Arp disagreed with this interpretation and said the unusual shapes were due to ejections. But either way, Arp realized astronomers weren’t very familiar with how galaxies change over time, and he wanted astronomers to be able to use his Atlas to study galaxy evolution.

The Observing Project’s second collection of unusual galaxies is called A 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 galaxies with tails.

Astronomers have expanded their knowledge of interacting galaxies and galaxy mergers since the publication of the Atlas and Catalog. 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 star-forming outbursts, 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
Interacting galaxies are found throughout the Universe, sometimes as dramatic collisions that trigger star-forming outbursts, 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 are calling “intergalactic star-forming objects” (ISFOs). ISFOs are a large class of objects that capture the different types that form when galaxies interact. ISFOs can form due to tidal interactions and ram-sweeping materials from interacting galaxies. They can also develop due to the influx of gas and dust into the tailings and by a combination of all these processes. ISFOs can range in mass from clusters of superstars to what astronomers call “tidal dwarf galaxies” (TDGs). A 2012 paper based on the Sloan Digital Sky Survey estimated that around 6% of dwarf galaxies may have tidal origins.

This image shows NGC7252, a peculiar galaxy that formed as a result of a merger between two galaxies over a billion years ago.  The white circles highlight the locations of two tidal dwarf galaxies (TDGs) forming in the tails.  Researchers believe that about 6% of dwarf galaxies are TDGs.  Image credit: Frédéric Bournaud/Pierre-Alain Duc.
This image shows NGC7252, a peculiar galaxy formed by the merger of two galaxies over a billion years ago. The white circles highlight the locations of two tidal dwarf galaxies (TDGs) forming in the tails. Researchers believe that about 6% of dwarf galaxies are TDGs. Image credit: Frédéric Bournaud/Pierre-Alain Duc.

ISFOs are often gravitationally bound to galaxies, but how much stay bound and for how long remains an open question. Sometimes materials from tidal currents flow back into galaxies, triggering more star formation. The material left over from all this interaction enriches the interstellar medium with dust and metals.

Astronomers today believe that about 25% of galaxies currently merge with other galaxies. According to the Harvard Center for Astrophysics, even more of them interact gravitationally, or even merge. Our galaxy, the 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 juggernaut could arise from this event?

This series of photographic illustrations shows the predicted merger between our Milky Way Galaxy and the neighboring Andromeda Galaxy.  Credit: NASA;  ESA;  Z. Levay and R. van der Marel, STScI;  T. Hallas and A. Mellinger
This series of photographic illustrations shows the predicted merger between our Milky Way Galaxy 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) get so massive is also an open area of ​​research. Astrophysicists know that mergers are part of 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 is a group of five galaxies, four of which interact and the fifth is only visually associated.  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, twinkling regions containing millions of young stars, and shock waves from NGC 7318B, the tallest of the closest pair of galaxies, so let it make its way through the heap.  Image credit: NASA, ESA, CSA and STScI
The James Webb Space Telescope captured this image of Stephan’s Quintet. That’s five galaxies, four interacting, and the fifth is only visually associated. 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, twinkling regions containing millions of young stars, and shock waves from NGC 7318B as it smashes its way through the cluster. NGC 7318B is the tallest of the closest pair of galaxies. Image credit: NASA, ESA, CSA and STScI

The Hubble Space Telescope’s Advanced Camera for Surveys (ACS) has examined this assortment 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. Observation time on these telescopes is always in high demand, so this project will help astronomers better allocate time.

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