Giant ‘Ghost’ Galaxy Found Tucked Behind The Milky Way

Astronomers have discovered a new satellite galaxy of the Milky Way, hidden some 130,000 light-years away behind the galactic disk.

A ghost galaxy as seen in the Milky Way.
muratart / Shutterstock

Astronomers have discovered a new satellite galaxy of the Milky Way, hidden some 130,000 light-years away behind the galactic disk.

Astronomers have made an outstanding discovery on the fringes of the Milky Way, reports Phys.org. Unbeknownst to us, a “hidden giant” had been lurking all along on the outskirts of our galaxy, concealed by the shroud of our galactic disk.

This gargantuan apparition is nothing more than a dwarf galaxy orbiting the Milky Way — one of a few dozen satellites that swarm around our galaxy, something similar to the two Magellanic Clouds.

Some of the tiny galaxies circling the Milky Way are among the oldest in the entire universe, as the Inquisitr previously reported. Now, a new one has been added to the list and it goes by the name of Antlia 2 — Ant 2, for short.

The newfound dwarf galaxy is located some 42,000 light-years away — in the constellation of Antlia (“The Pump”) — and was only recently spotted camouflaged behind the disk of the Milky Way.

According to the team that sighted the dwarf galaxy, Ant 2 is about the size of the Large Magellanic Cloud — and roughly a third the size of the Milky Way itself, making it truly enormous compared with the other satellite galaxies. However, despite its mammoth size, Ant 2 gives off very little light — and is actually 10,000 times less luminous than the Large Magellanic Cloud.

Image of the Large Magellanic Cloud, a satellite galaxy of the Milky Way.
The Large Magellanic Cloud, a satellite galaxy of the Milky Way located some 160,000 light-years away, in the constellation Dorado (‘The Swordfish’). Alexcpt_photography / Shutterstock

“While similar in extent to the Large Magellanic Cloud, Antlia 2 is orders of magnitude fainter… making it by far the lowest surface brightness system known,” the astronomers write in a paper, published today on the preprint server arXiv.

This means that Ant 2 “is either far too large for its luminosity or far too dim for its size,” notes the University of Cambridge in the U.K. — which participated in the discovery alongside researchers from Taiwan, the U.S., Australia, and Germany.

Moreover, this dwarf has such a low density that it’s actually “100 times more diffuse than the so-called ultra-diffuse galaxies,” the authors write in the study.

For this reason, the scientists are calling it a “ghost galaxy,” as stated by study lead author Gabriel Torrealba, a researcher at the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei, Taiwan.

“This is a ghost of a galaxy. Objects as diffuse as Ant 2 have simply not been seen before.”

As his team points out, this unusual trait — coupled with the satellite galaxy’s excellent hiding place — explains why Ant 2 hasn’t been found until now.

The astronomers stumbled upon the dwarf galaxy after combing through the second wave of data from the European Space Agency’s Gaia satellite. The same slew of Gaia information has led to a spectacular catalog of around 1.7 billion stars in our galaxy — and has also captured the fingerprint of the Large Magellanic Cloud, the Inquisitr reported earlier this year.

“Our discovery was only possible thanks to the quality of the Gaia data,” explained Torrealba.

The giant satellite galaxy was spotted after the team searched the Gaia data looking for a special type of stars, ones that are typically found in dwarf galaxies. Known as RR Lyrae stars, these celestial objects are old and have a low metal content — exactly the type of low-mass, metal-poor stars that sparked into existence in the early days of the universe. It is believed that, at this time, all galaxies were dwarf galaxies and the larger ones had yet to appear.

The newfound dwarf galaxy was confirmed by follow-up observations with the Anglo-Australian Telescope (AAT) in Australia, which captured the spectra of more than 100 red giant stars in Ant 2. These observations revealed that the satellite galaxy has a strikingly low mass — much lower than expected for an object of such giant proportions.

“The simplest explanation of why Ant 2 appears to have so little mass today is that it is being taken apart by the galactic tides of the Milky Way,” said study co-author Sergey Koposov from Carnegie Mellon University in Pittsburgh, Pennsylvania. “What remains unexplained, however, is the object’s giant size. Normally, as galaxies lose mass to the Milky Way’s tides, they shrink, not grow.”

This has prompted team member Matthew Walker, also from Carnegie Mellon University, to characterize Ant 2 as an “oddball.”

“Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball. 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.”