In the same way as orbital planets, planets orbit stars and stars orbiting galactic nuclei, galaxies can be orbited by other, much smaller galaxies. The Milky Way has many of these suspensions, including large and small magellanic clouds, the only two satellites in our galaxy visible to the naked eye.
Now, thanks to Gaia's data – the most complete map of our sky ever compiled – astronomers have just found another one. And it's absolutely huge – as big as the Big Magellan Cloud, or about a third of the size of the Milky Way.
In the picture above, it is the faint glow at the top left of the image, located next to the Milky Way (bottom left), with the Big Cloud of Magellan on the picture on the bottom right.
As it is located in the southern constellation Antlia (The Pump), it bears the name Antlia 2.
So, how did Antlia 2 escape detection for so long, especially since we've known the Great Magellanic Cloud since at least 964 CE?
Some ways. The first is that it was perfectly hidden behind the disk of the Milky Way. The second is that it has an extremely low density, which means that it does not emit much light. In fact, it is 10,000 times weaker than the Great Magellanic Cloud – its representation in the above image is brightened, so we can see it.
It is by far the most diffuse galaxy ever found. It's even much weaker – about 100 times – than the ultra-diffuse extremely diffuse galaxies, which lack gas forming stars, and thus the ability to create new stars.
This could mean that Antlia 2 is only the rest of a dead galaxy for a long time. Or, as astronomer Gabriel Torrealba of the Academia Sinica told Taipei: "It's a galaxy ghost".
The team discovered the galaxy looking for Milky Way satellites from a star type called RR Lyrae variables. These stars are very old and poor in metals, and are usually found in dwarf galaxies and globular clusters.
As their name indicates, they are also variable stars, which means that their light varies with a very regular interval of time, about half a day on the Earth. This means that they can be used as standard candles to calculate the precise distances between the Earth and the star, as they were discovered by Henrietta Leavitt in the early twentieth century.
The team found a group of these stars in the Gaia data, but when they checked the location against known object databases, there was apparently nothing . So they made further observations and managed to get the specter of 100 giant red stars just before the position of Antlia 2 was obscured by the Sun, where it should stay for several months.
All the stars they were studying were moving together, thus confirming the existence of the huge Antlia 2, hitherto unknown, which was hiding beyond the Milky Way.
According to the observations of the team, Antlia 2 lies some 424 000 light years from Earth and is about 11.2 billion years old. The simulations suggest that much of his material was engulfed by the Milky Way – the same fate that lurks Magellan's clouds.
"The simplest explanation of why Ant 2 seems to have so little mass today is that it is being demolished by the galactic tides of the Milky Way," said astronomer Sergey Koposov of the University Carnegie Mellon.
"What remains unexplained, however, is the giant size of the object.Normally, as the galaxies lose mass against the tides of the Milky Way, they contract and do not grow. "
This means that he probably should have started as much as he is today, although that remains to be determined. And there is also a question as to the size of Antlia 2.
As New scientist reports, according to Gisella Clementini, astronomer and expert for RR Lyrae, of the National Institute of Astrophysics in Italy, the calculation used by the team to determine the distance to the RR Lyrae group included an error. When they are correctly calculated, they are only at 260,000 light-years.
But, according to Torrealba, this miscalculation only changes the distance to the stars of RR Lyrae, not to the red giants studied by the team, which they confirmed by two distinct methods.
So, the stars of RR Lyrae are in front of Antlia 2, which means that the discovery of the galaxy was the result of a miscalculation and was therefore extremely lucky, that they were part of the galaxy, but that they represented the nearest edge.
Whatever the case may be, there is no doubt that the team has discovered something strange in our neighborhood.
"Compared to the rest of the sixty or so satellites of the Milky Way, Ant 2 is a funny balloon," said astronomer Matthew Walker, also of Carnegie Mellon University.
"We wonder if this galaxy is only the tip of an iceberg and if the Milky Way is surrounded by a large population of almost invisible dwarfs, similar to this one."
The paper of the team was accepted in the newspaper Monthly Notices from the Royal Astronomical Society, and can be read on the arXiv preprint resource.