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Gaia Satellite Finds Strange 'Ghost' Galaxy Right Outside Milky Way

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A photo of the Milky Way Galaxy. Scientists discovered from Gaia's second data release a ghost dwarf hiding behind the far side of the disk of the Milky Way. In a new study, scientists discussed Antlia 2, a galaxy that escaped detection because of its low density.   ( Hans Braxmeier | Pixabay )

The European Space Agency's Gaia, a space telescope, has spotted a faint dwarf galaxy that has been lurking around the Milky Way.

Named the Antlia 2, scientists revealed that the dwarf galaxy is only one-third the size of the Milky Way but as big as the Large Magellanic Cloud. It has eluded detection until now because of its extremely low density and a perfect hiding place.

Scientists have published the discovery in the online pre-print site arXiv.

Ghost Galaxy Haunting The Milky Way

The study used the data collected by Gaia, which has produced the richest star catalog to date. It monitors and measures nearly 1.7 billion stars in and across the Milky Way.

A team of international researchers discovered the dwarf galaxy while trawling through the troves of new data recently released by the ESA Gaia. They searched for RR Lyrae stars, old stars typically found in dwarf galaxies and used by astronomers as standard candles to pin down distance.

They found three sitting above the Milky Way's galactic disc. The scientists took it as an "overwhelming signal" of a cluster of stars, but finding the rest became a challenge. The dwarf galaxy was hiding behind the far side of the disk of the Milky Way, obstructed from view.

Using the Anglo-Australian Telescope, the team was able to measure the spectra of about 100 red giant stars. They also found that the Antlia 2 never comes too close to Earth. It is always at least 130,000 light-years away from Earth.

"RR Lyrae had been found in every known dwarf satellite, so when we found a group of them sitting above the Galactic disc, we weren't totally surprised," explained co-author Vasily Belokurov of Cambridge University. "But when we looked closer at their location on the sky it turned out we found something new, as no previously identified object came up in any of the databases we searched through."

Explaining Milky Way's Unusual Companion

The discovery of Antlia 2 challenges the current models of galaxy formation today. The researchers believe that the dwarf galaxy has so little mass because of the constant pull of the galactic tides of the Milky Way. However, this does not explain its impressive size.

The team theorized that Antlia 2 was born huge but an explosion of stars early in its history blew dust and gas out of the galaxy, weakening gravity and allowing dark matter to drift away.

The team thinks that the discovery of Antlia 2 could make scientists rethink of the properties of dark matter. The current theory claiming that dark matter is tightly packed in the centers of galaxies does not appear to apply to the newly-discovered ghost dwarf.

"Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball," added study co-author Matthew Walker of Carnegie Mellon University. "We are wondering whether this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of nearly invisible dwarfs similar to this one."

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