It is an extremely rare type of galaxy with something very peculiar located at its center.
A new type of galaxy has just been discovered by astronomers. It’s small, compact, and old — hence the name ‘compact elliptical galaxy’ — but this one shows signs of a monster black hole in its center, and astronomers are puzzled.
Compact elliptical galaxies are not a new discovery, but they are an extremely rare early-type of dwarf galaxy. They are a very close-packed collection of ancient stars, which exhibit high surface brightness, but aren’t actively forming stars.
Most compact elliptical galaxies are found in dense environments, particularly around massive galaxies, leading astronomers to believe that they may form by the tidal stripping of a once-large galaxy through interactions with another massive galaxy. In other words, when stars and other material are pulled from a system, this material is often left behind as a compact elliptical galaxy.
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However, the recent discovery of a few isolated compact ellipticals has flipped this idea on it’s head, and this newly discovered galaxy, SDSS J085431.18+173730.5, is confusing things even further. Why? It is the first-ever compact elliptical to also display signs of an active black hole at its center.
View of the ancient compact elliptical galaxy SDSS J085431.18+173730.5 (top center). Photo credit: Paudel et. al.
Ancient compact ellipticals are supposed to be free of gas, with no fuel left to trigger new activity. However, a team of scientists, led by Sanjaya Paudel of the Korea Astronomy and Space Science Institute, used Sloan Digital Sky Survey data coupled with data from the Canada-France-Hawaii Telescope, to show that SDSS J085431.18+173730.5 has emission lines indicating accretion onto a supermassive black hole roughly the mass of two million suns.
What’s more, SDSS J085431.18+173730.5 has no nearby neighbors, which really throws a wrench into the tidal forcing hypothesis. So how was this ancient galaxy born?
The authors of the study find support for a previously proposed “flyby” scenario, where isolated compact ellipticals are tidally stripped systems that either ran away or were flung out by their host galaxy. For example, Paudel and his team suggest that SDSS J085431.18+173730.5 may have interacted with NGC 2672 — a galaxy that is about 6.5 million light-years away — before being tossed out to its current location.
The researchers will continue to monitor the galaxy to determine the origin of this bizarre system, but they also hope that SDSS J085431.18+173730.5 will help them understand how all compact ellipticals form and evolve.
The results have been published in The Astrophysical Journal Letters.
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