Astronomers have discovered a stealthy giant hidden right next door, a dwarf galaxy called Antlia 2 that is about one-third the size of the Milky Way. Antlia 2 was only able to avoid detection for this long because it is 10,000 times fainter than usual even if it’s the same size as the Large Magellanic Cloud, our galaxy’s largest companion. This dim lurker has challenged models of galaxy formation and dark matter, the stuff that pulls all galaxies together.

The hidden galaxy and RR Lyrae stars

The galaxy was discovered using data from the European Space Agency’s Gaia satellite, a space telescope measuring the motions and properties of more than 1 billion stars in and around the Milky Way. RR Lyrae are old stars that are often found in dwarf galaxies and are known to shine with a blue light that pulses at a rate signaling their inherent brightness.

ESO/VVV Survey/D. Minniti

This allows astronomers to determine their distance from Earth. When three are found together like what happened here, it’s a signal that there may be a cluster of stars in that location.

Gaia satellite sheds new light

The team was able to see past the foreground stars using the Gaia data provided. Objects in our galaxy’s disk are close enough for Gaia to measure their parallax, the shift in their apparent position as Earth moves around the Sun.

NASA/ESA Hubble Space Telescope

After removing the parallax-bearing stars, the team focused on more than 100 red giant stars moving as a group in the Antlia constellation. These giants marked out a companion galaxy 100 times less massive than anything of similar size, with fewer stars.

How could this happen?

Vasily Belokurov, an astronomer at the University of Cambridge in the United Kingdom, suggested that early in Antlia’s history, a group of young stars exploded as violent supernovae that would have blown gas and dust out of the galaxy. The team has also speculated that Antlia 2 may have been born from another dustier, faster-moving type of dark matter than current models have predicted.

Nick Rose, ESA/Hubble & NASA

This discovery shows that the objects in our galaxy interact with each other more often than we originally thought. It’s a new step into understanding what is happening around us.