Categories: galaxiesObserving

Spiral Galaxies Could Eat Dwarfs All Across the Universe

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For years, astronomers have seen evidence that – at least in our own local neighborhood — spiral galaxies are consuming smaller dwarf galaxies. As they are digested, these dwarf galaxies are severely distorted, forming structures like strange, looping tendrils and stellar streams that surround the cannibalistic spirals. But now, for the first time, a new survey has detected such tell-tale structures in galaxies more distant than our immediate galactic neighborhood, providing evidence that this galactic cannibalism might take place on a universal scale. Remarkably, these cutting-edge results were obtained with small, amateur-sized telescopes.


Since 1997, astronomers have seen evidence that spirals in our local group of galaxies are swallowing dwarfs. In fact, our own Milky Way is currently in the process of eating the Canis Major dwarf galaxy and the Sagittarius dwarf galaxy. But the Local group with its three spiral galaxies and numerous dwarfs is much too small a sample to see whether this digestive process is happening elsewhere in the Universe. But an international group of researchers led by David Martínez-Delgado from the Max Planck Institute for Astronomy recently completed a survey of spiral galaxies at distances of up to 50 million light-years from Earth, discovering the tell-tale signs of spirals eating dwarfs.

For their observations, the researchers used small telescopes with apertures between 10 and 50 cm, equipped with commercially available CCD cameras. The telescopes are located at two private observatories — one in the US and one in Australia. They are robotic telescopes that can be controlled remotely.

During the “eating” process, when a spiral galaxy is approached by a much smaller companion, such as a dwarf galaxy, the larger galaxy’s uneven gravitational pull severely distorts the smaller star system. Over the course of a few billions of years, tendril-like structures develop that can be detected by sensitive observation. In one typical outcome, the smaller galaxy is transformed into an elongated “tidal stream” consisting of stars that, over the course of additional billions of years, will join the galaxy’s regular stellar inventory through a process of complete assimilation. The study shows that major tidal streams with masses between 1 and 5 percent of the galaxy’s total mass are quite common in spiral galaxies.

One of the galaxies in the survey, NGC 4651, sports a remarkable umbrella-like structure. It is composed of tidal star streams, the remnants of a smaller satellite galaxy which NGC 4651 has attracted and torn apart. This galaxy's distance from Earth is 35 million light-years.Credit: R. Jay Gabany (Blackbird Obs.) in collaboration with D. Martínez-Delgado (MPIA and IAC) et al.

Detailed simulations depicting the evolution of galaxies predict both tidal streams and a number of other distinct features that indicate mergers, such as giant debris clouds or jet-like features emerging from galactic discs. Interestingly, all these various features are indeed seen in the new observations – impressive evidence that current models of galaxy evolution are indeed on the right track.

Smaller satellite galaxies caught by a spiral galaxy are distorted into elongated structures consisting of stars, which are known as tidal streams, as shown in this artist's impression. Credit: Jon Lomberg

The ultra-deep images obtained by Delgado and his colleagues open the door to a new round of systematic galactic interaction studies. Next, with a more complete survey that is currently in progress, the researchers intend to subject the current models to more quantitative tests, checking whether current simulations make the correct predictions for the relative frequency of the different morphological features.

While larger telescopes have the undeniable edge in detecting very distant, but comparatively bright star systems such as active galaxies, this survey provides some of the deepest insight yet when it comes to detecting ordinary galaxies that are similar to our own cosmic home, the Milky Way. The results attest to the power of systematic work that is possible even with smaller instruments.

For more images see this page from the Max Planck Institute for Astronomy

*Note: Originally the lead image image was credited incorrectly, and is actually a product of R. Jay Gabany, an astrophotographer whose work has been featured quite often here on Universe Today. See more of his amazing handiwork at his website, Cosmotography.

Source: Max Planck Institute for Astronomy

Nancy Atkinson

Nancy has been with Universe Today since 2004, and has published over 6,000 articles on space exploration, astronomy, science and technology. She is the author of two books: "Eight Years to the Moon: the History of the Apollo Missions," (2019) which shares the stories of 60 engineers and scientists who worked behind the scenes to make landing on the Moon possible; and "Incredible Stories from Space: A Behind-the-Scenes Look at the Missions Changing Our View of the Cosmos" (2016) tells the stories of those who work on NASA's robotic missions to explore the Solar System and beyond. Follow Nancy on Twitter at https://twitter.com/Nancy_A and and Instagram at and https://www.instagram.com/nancyatkinson_ut/

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