From The Editors Science

Unusually Large Ghost Galaxy Discovered Orbiting the Milky Way

A team of astronomers from different universities around the world have discovered a bizarre dwarf galaxy, named Antlia 2 (or Ant 2), orbiting the Milky Way in the constellation Antlia.

Not only is Antlia 2’s enormous proportions atypical of a dwarf galaxy, but it is also so dim and pale that it had gone undetected until recently – thanks to the European Space Agency’s Gaia satellite that provided the necessary data for the research team to sift through.

The researchers, basically, trawled the Gaia data, looking for Milky Way satellites with the help of RR Lyrae stars, which are old with low levels of metal, much like a dwarf galaxy.

“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,” said study co-author Vasily Belokurov from the Institute of Astronomy at the University of Cambridge.

“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,” Belokurov added.

“This is a ghost of a galaxy,” the paper’s lead author Gabriel Torrealba 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,” Torrealba added.

Antlia 2’s dimness, low density and the fact that it is hiding behind the Milky Way’s bright central disk are the reasons why it went undetected for as long as it did.

One of the earliest galactic structures to form after the big bang were dwarf galaxies and, therefore, most of the stars within them are old with low metal levels.

While our Milky Way has anywhere between 200 billion and 400 billion stars, dwarf galaxies are home to around 100 million to several billion stars, which is way fewer but something to be expected, as they are way smaller, as well.

The relatively smaller size of most dwarf galaxies makes them defenseless against the gravitational forces of larger and more massive galaxies in their vicinity.

However, being larger than is normal for a dwarf galaxy and the fact that it barely emits any light, Antlia 2 is kind of bizarre for a dwarf galaxy.

To give you a better perspective, Antlia 2 is one third the size of our Milky Way – about the same size as the Large Magellanic Cloud (LMC) orbiting the Milky Way.

It is also 10,000 times less bright than the LMC, making it unusually dark for a galaxy as large as it is.

Researchers attribute Antlia 2’s low luminosity to the gravitational tides of the Milky Way.

Even though the Phantom galaxy is distant enough from the Milky Way to be ripped apart by its gravity, it does get influenced by the huge mass of the larger parent galaxy.

What the researchers have not been able to explain, though, is the disparity in Antlia 2’s mass and size.

With a relatively low mass, the ghost galaxy is vulnerable to the gravitational forces around it, because of which its size should have also been small, which is not the case – something the researchers have, so far, been unable to explain.

Under normal circumstances, the powerful forces of a much larger galaxy would cause the smaller galaxy to lose mass as well as condense, rather than grow.

“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.

“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,” Koposov added,

However, two plausible theories explaining the mismatch between Antlia 2’s mass and size have been floated:

One, it started off as a relatively large galaxy, growing even larger because of the expanding gasses caused by the vigorous birth of new stars, which in turn weakened the gravitational pull to the center of Antlia 2.

“Even if star formation could re-shape the dark matter distribution in Ant 2 as it was put together, it must have acted with unprecedented efficiency,” said co-author Jason Sanders, from Cambridge.

Two, contrary to the belief that dark matter is prone to clustering, and hence accumulate at the center of a galaxy, it could instead be spreading out throughout it.

For now, Antlia 2 may appear to be an “oddball” dwarf galaxy, but if researchers are able to locate more such galaxies, the oddity of Antlia 2 could possibly be better explained.

“Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball,” said co-author Matthew Walker, from 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,” Walker added.

The universities involved in the research include:

  • Academia Sinica, Taipei, Taiwan, University of Cambridge, Cambridge, UK
  • Flatiron Institute, New York, NY, USA
  • Carnegie Mellon University, Pittsburgh, PA, USA
  • Fermi National Accelerator Laboratory, Batavia, IL, USA
  • University of Chicago, Chicago, IL, USA
  • Imperial College London, London, UK
  • Macquarie University, Sydney, NSW, Australia
  • Macquarie University, North Ryde, NSW, Australia
  • University of Leicester, Leicester, UK
  • Sternwarte der Ludwig-Maximilians-Universit¨at (Ludwig Maximilian University of Munich) Munich, Germany

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