There's a new guy in the galactic neighborhood — a small, ultra-faint satellite galaxy has been discovered orbiting the Milky Way.
Currently named Virgo 1, this cosmic groupie isn't alone: There are about 50 known satellite galaxies orbiting our own (the largest of these are the Large Magellanic Cloud and Small Magellanic Cloud, which are visible from Earth with the naked eye).
But Virgo 1 is special because it is extremely faint — finding it required a very large telescope equipped with a very specialized instrument. There should be many more of these faint satellite galaxies around the Milky Way, according to computer simulations that predict how the galaxy formed. And finding these galaxies will help scientists better understand a major ingredient in those computer models: dark matter, a material that makes up most of the mass in the universe. [Dark Matter and Dark Energy: The Mystery Explained (Infographic)]
Virgo 1 "may well be the faintest satellite galaxy yet found," according to a statement from the National Astronomical Observatory of Japan (NAOJ), which owns and operates the 8.2-meter Subaru Telescope located on Mauna Kea in Hawaii. The new galaxy (so named because it lies in the direction of the constellation Virgo) was discovered using the Hyper Suprime-Cam (HSC) instrument on Subaru, as part of the Subaru Strategic Survey.
The discovery of an ultra-faint galaxy like Virgo 1 may demonstrate that, as telescopes improve, scientists will be able to spot more of these faint galactic groupies, researchers said in the statement.
"Our theories and simulations of how the Universe evolves tell us that the Milky Way should have many of these small, faint satellite galaxies," Jason Rhodes, an astrophysicist at NASA's Jet Propulsion Laboratory, told Space.com in an email. "However, they have been notoriously difficult to detect, prompting some people to say that they may not exist. This is called the 'missing satellite problem.'"
Modeling the formation of the Milky Way galaxy is kind of like going home and trying to recreate a dish you ate at a restaurant. You assemble the likely ingredients, and do your best to duplicate the steps to cook them. The more complex the dish is, the harder it will be to recreate the recipe.
The universe is an extremely complicated dish, and one of the major ingredients is dark matter. There is ample evidence that this mysterious substance exists, even though it doesn't radiate or reflect light (its gravitational effects, however, are quite visible). Some computer models of the Milky Way's formation suggest there should be many more satellite galaxies around it — perhaps even hundreds, according to the statement — along with clumps of dark matter. It is possible the ideas about dark matter contained in those models is incorrect – that scientists have failed to recreate the universe on their own. But if scientists can't see those galaxies, they can't be sure if their models are correct or not.
"However, we are now able, with instruments like HSC (Hyper Suprime-Cam) and the ongoing Dark Energy Survey, to find these previously missing satellites," Rhodes said. "This may mean that our theories of the formation of structures like the Milky Way are indeed correct."
These ultra-faint galaxies don't necessarily resemble the Milky Way, a galaxy containing hundreds of billions of stars that form massive spiral arms. At its center is a supermassive black hole that binds all the stars and other material together.
These satellite galaxies are smaller clusters of stars that are gravitationally bound together. (The Milky Way is also orbited by even smaller groups of stars known as globular clusters). Because the structure of these satellite galaxies isn't as distinct as that of large galaxies like the Milky Way, they can be tricky to identify. Scientists have to first observe a cluster of stars on the sky, and then make sure those stars are actually close together, and don't just appear that way from Earth. (Two stars that appear to be right next to each other in the sky could actually be located at very different distances from Earth.)
More of these faint satellite galaxies have been discovered recently by the Dark Energy Survey and the Sloan Digital Sky Survey, although the statement from NAOJ notes that those telescopes have a smaller aperture than the Subaru Telescope, "so only satellites relatively close to the Sun or those with higher magnitudes were identified. Those that are more distant or faint ones in the halo of the Milky Way are yet to be detected."
"The combination of the large aperture of 8.2-meter Subaru Telescope and the large field-of-view Hyper Suprime-Cam (HSC) instrument is very powerful in this study," researchers said in the statement. "It enables an efficient search for very faint dwarf satellites over large areas of the sky."
Rhodes said the HSC instrument and other telescopes set to come online soon will further expand the search for these faint satellite galaxies.
"Virgo 1, the structure these authors found, is just at the limit of the sensitivity of their instrument," he said. "As their survey gets bigger, more of these formerly missing satellites should be found. Likewise, with a new generation of instruments and telescopes in the 2020s, we will be able to push the sensitivity limits even further and find smaller and fainter 'missing satellites.'"