ESA’s Gaia spots a ‘ghost’ galaxy next to Milky Way
London: Using the European Space Agency’s (ESA) Gaia telescope, an international team of astronomers discovered an enormous ‘ghost’ galaxy lurking on the outskirts of the Milky Way.
Behind the Milky Way’s disc, Gaia spotted a massive object with an extremely low density, named Antlia 2 (or Ant 2).
Ant 2 is known as a dwarf galaxy. But compared to the other known dwarf satellites of our Galaxy, Ant 2 is immense. It is as big as the Large Magellanic Cloud (LMC), and a third of the size of the Milky Way itself, the astronomers said.
It is also 10,000 times fainter, in terms of light. In other words, it is either far too large for its luminosity or far too dim for its size.
“This is a ghost of a galaxy,” lead author Gabriel Torrealba, from the University of Cambridge, said in a statement.
“Objects as diffuse as Ant 2 have simply not been seen before. Our discovery was only possible thanks to the quality of the Gaia data.”
For the study, the team measured the spectra of more than 100 red giant stars just before the Earth’s motion around the Sun rendered Ant 2 unobservable for months.
This enabled them to confirm that the ghostly object they spotted was real: all the stars were moving together.
Ant 2 never comes too close to the Milky Way, always staying at least 40 kiloparsecs (about 130,000 light-years) away. The researchers were also able to obtain the galaxy’s mass, which was much lower than expected for an object of its size.
“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 co-author Sergey Koposov from Carnegie Mellon University in Pennsylvania, US.
While it is been speculated that some dwarfs could be inflated by vigorous star formation, the team is yet to figure out the exact process that made Ant 2 so extended.
Solving the Ant 2 puzzle may help researchers understand how the first structures in the early Universe emerged and finding more such objects will show just how common such ghostly galaxies are, the astronomers said.