Just as the moons orbit the planets, the planets orbit the stars and the stars orbit around the galactic nuclei, the galaxies can be orbits from other much smaller galaxies. The Milky Way has several of these hangers, in particular the large and small magellanic clouds, the only two satellites of our galaxy visible to the naked eye.
Now, thanks to the data of Gaia – the most complete map of our sky ever compiled – the astronomers have just found one another. And it's absolutely enormous – as big as the Great Magellanic Cloud, or about a third of the Milky Way's size.
In the picture above, is the dim glow in the upper left part of the image, which is located adjacent to the Milky Way (bottom left), with the large Magellan cloud depicted in the lower right corner.
Since it is located in the southern constellation of Antlia (The Pump), it has been given the name of Antlia 2.
So, how did Antlia 2 manage to escape detection for so long, especially since we first met the Great Magellanic Cloud since 964 CE?
Some ways The first is that it was well hidden behind the Milky Way disc. The second is that it has an extremely low density, which means it does not emit a lot of light. In fact, it's 10,000 times weaker than the Big Magellanic Cloud – its representation in the image above is lit, so we can see it.
It is, by far, the most widespread galaxy ever found. It is also much weaker – about 100 times – compared to incredibly weak ultra-diffused galaxies, which lack stars-forming gas, and therefore the ability to create new stars.
This could mean that Antlia 2 is only the remains of a long-dead galaxy. Or, as the astronomer Gabriel Torrealba of Academia Sinica in Taipei said: "This is a ghost of a galaxy".
The team found the galaxy hunting for the Milky Way satellites based on a type of star called RR Lyrae variables. These stars are very old and metal-poor and are commonly found in dwarf galaxies and globular clusters.
As for the name, they are also variable stars, which means that their light varies in a very regular time – about half a day on Earth. This means that they can be used as standard candles to calculate the precise distances between the Earth and the star, as discovered by Henrietta Leavitt at the beginning of the 20th century.
The team found a group of these stars in the Gaia data, but when they checked the position against the databases of known objects, there was nothing. Then they conducted further observations and managed to get the spectra of 100 red giant stars just before the position of Antlia 2 was obscured by the Sun, where it was expected to remain for several months.
All the stars they studied were moving together, which is how they confirmed the existence of the enormous previously unknown Antlia 2 lurking beyond the Milky Way.
Based on the team's observations, Antlia 2 is about 424,000 light years from Earth and is about 11.2 billion years old. The simulations suggest that much of his material has been engulfed by the Milky Way, the same fate that awaits the Magellanic Clouds.
"The simplest explanation of why Ant 2 seems to have so little mass today is that it is broken up by the Milky Way galaxy tides," said the astronomer Sergey Koposov of Carnegie Mellon University.
"What remains unexplained, however, is the giant dimension of the object: normally, when galaxies lose mass in the Milky Way tides, they shrink, they do not grow".
This means that it probably should have started much bigger than now, even if this has yet to be determined. And there's also some questions about the size of Antlia 2.
Such as New Scientist according to the astronomer and the expert of RR Lyrae Gisella Clementini of the National Institute of Astrophysics in Italy, the calculation used by the team to determine the distance from the RR Lyrae group included an error. If calculated correctly, they are only 260,000 light years away.
But, according to Torrealba, that calculation error only changes the distance of the RR Lyrae stars, not the red giants studied by the team, who have confirmed using two separate methods.
So, the RR Lyrae stars are in front of Antlia 2, which means that the discovery of the galaxy was the result of a miscalculation and therefore extremely lucky, or are part of the galaxy, but simply the nearest edge.
However, there is no doubt that the team has discovered something strange in our neighborhood.
"Compared to the rest of the more than 60 Milky Way satellites, Ant 2 is a strange game," said astronomer Matthew Walker, also of Carnegie Mellon University.
"We are wondering if this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of almost invisible dwarves similar to this one."
The team card has been accepted in the diary Monthly notices from the Royal Astronomical Societyand can be read on the preprint arXiv resource.