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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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.
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.
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.
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?
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 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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