But the Universe is big, vast and the flow of nature presents many charms. 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. Known as Wild’s Triplet, Arp 248 has two galaxies connected by a tidal tail. A third, smaller unrelated spiral galaxy can be seen in the background. (ESA/Hubble & NASA, Dark Energy Survey/Department of Energy/Fermilab Cosmic Physics Center/Dark Energy Camera/Cerro Tololo Inter-American Observatory/NOIRLab/National Science Foundation/AURA Astronomy; J. Dalcanton) 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 Peculiar 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. 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. A composite image of Centaurus A, revealing the lobes and jets emanating from the active galaxy’s central black hole. (ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimeter), NASA/CXC/CfA/R.Kraft et al. (X-ray)) 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. 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 clusters of stars to what astronomers call “tidal dwarf galaxies” (TDGs.) A 2012 paper based on the Sloan Digital Sky Survey estimated that about 6 percent of dwarf galaxies could have tidal origin. 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. In this image of Stefan’s Quintet, we see five galaxies, four of which are interacting. Galaxies pull and stretch each other. (NASA, ESA, CSA and STScI) Astronomers now believe that about 25 percent 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? 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 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. This article was originally published by Universe Today. Read the original article.
