The Perfect Tidal Tail Connects These two Galaxies Seen by Hubble

Sometimes it’s tempting to imagine a supernatural hand behind the arrangement of celestial bodies. But the Universe is big, huge even, and nature’s flow presents many fascinations.

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

Arp 248 is a trio of small interacting galaxies around 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 as the galaxies tug on one another gravitationally.

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 –, CC BY-SA 3.0 us,

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

The top image is from an observing project examining two collections of unusual galaxies 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 intended for it to highlight the variety of peculiar structures that galaxies take.

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 around 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 spiral of gases in Centaurus A’s core. Image Credit: ESA

We now know these galaxies take such strange shapes because they’re interacting and potentially merging. Arp disagreed with that interpretation and said the unusual forms were due to ejections. But in any case, Arp realized astronomers weren’t very knowledgeable about how galaxies change over time, and he intended that astronomers could use his Atlas to study galaxy evolution.

The second collection of unusual galaxies in the observing project is called A Catalogue of Southern Peculiar Galaxies and Associations. It was published in 1987 by Arp and his colleague Barry Madore. The Catalogue contains 25 different varieties of objects, including galaxies with tails.

Astronomers have expanded their knowledge of interacting galaxies, and galaxy mergers since the Atlas and the Catalogue were published. We know that mergers play an important role in galaxy evolution.

Interacting galaxies are found throughout the Universe, sometimes as dramatic collisions that trigger bursts of star formation, on other occasions 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’re uncovering a new class of objects that 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 ram-sweeping of material from interacting galaxies. They can also develop due to the inflow of gas and dust to 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 origins.

This image shows NGC7252, a peculiar galaxy formed from 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 think that about 6% of dwarf galaxies are TDGs. Image Credit: Frederic Bournaud/Pierre-Alain Duc.

ISFOs are often bound gravitationally to the galaxies, but how many stay bound and for how long is still an open question. Sometimes material from the tidal streams will flow back into the galaxies, triggering more star formation. The leftover material from all this interaction enriches the interstellar medium with dust and metals.

Astronomers now think that about 25% of galaxies are currently merging with other galaxies. Even more of them are interacting gravitationally, if not merging, according to the Harvard Center for Astrophysics. Our Milky Way galaxy is evidence of this, as it cannibalized gas and even stars from the Magellanic Clouds and the Sagittarius Dwarf Galaxy. And in several billions of years, the Milky Way and the Andromeda Galaxy will merge. Who knows what behemoth might arise from that event?

This series of photo illustrations shows the predicted merger between our Milky Way galaxy and the neighbouring Andromeda galaxy. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas, and A. Mellinger

How supermassive black holes (SMBHs) grow so massive is also an open area of inquiry. 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’s a fivesome of galaxies, four interacting, and the fifth is only visually associated. The left-most galaxy is NGC7320 which is well in the foreground of the other four. The image is a composite of almost 1,000 separate images. The four stars and their interactions produce tails, regions of active star formation, glittering regions containing millions of young stars, and shock waves from NGC 7318B as it bullies its way through the cluster. NGC 7318B is the top one in the pair of galaxies closest together. Image Credit: NASA, ESA, CSA, and STScI

The Hubble Space Telescope’s Advanced Camera for Surveys (ACS) scrutinized this assortment of unusual interacting galaxies to lay the groundwork for more detailed study in the future. The Hubble will examine some of these targets with its other instruments, and so will the James Webb Space Telescope and ALMA. Observing time on these telescopes is always in high demand, so this project will help astronomers allocate time better.


Evan Gough

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