Just as lunar orbital planets, planets of orbiting stars and stars of galactic nuclei, orbital galaxies can rotate in the orbits of other, much smaller galaxies. In the Milky Way there are several such hangers, primarily the Large and Small Magellanic clouds, the only two satellites of our galaxy visible to the naked eye.
Now, thanks to the Gaia data — the most complete map of our sky ever made — the astronomers have just found another one. And it’s absolutely huge — as big as the Large Magellanic Cloud, or about one third the size of the Milky Way.
In the image above, this dim glow is in the upper left corner of the image, located next to the Milky Way (bottom left), with the Large Magellanic Cloud, shown at the bottom right.
Since it is located in the southern constellation of Anthony (The Pump), it was given the name Antlia 2.
Since Antlia 2 was able to avoid detection for so long, especially since we know about the Large Magellanic Cloud with at least 964 CE?
Several ways. First, it was neatly hidden behind the disk of the Milky Way. Secondly, 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 Large Magellanic Cloud – its image in the image above is enlightened, so we can see it.
This is by far the most common galaxy. It is even much weaker – about 100 times – than incredibly weak ultra-dimensional galaxies that do not have star-forming gas and, therefore, the ability to create new stars.
This may mean that Antlia 2 is just the remains of a galaxy long dead. Or, as astronomer Gabriel Torrealba of Academia Sinica in Taipei said: "This is a ghost of the galaxy."
The team found a galaxy on the hunt for satellites of the Milky Way, based on a type of star called RR Lyrae. These stars are very old and poor metals and are commonly found in dwarf galaxies and globular clusters.
According to the name, they are also variable stars, which means that their light changes on a very regular time scale – at about half the Earth’s day. This means that they can be used as standard candles to calculate exact distances between the Earth and a star, as was discovered by Henrietta Leavitt in the early 20th century.
The team found a group of these stars in the Gaia data, but when they checked the location against a database of known objects, there was nothing there, they say. Therefore, they conducted further observations and managed to obtain the spectra of 100 red giant stars shortly before the position of Antlia 2 became covered by the Sun, where it is expected that it will remain for several months.
All the stars with which they studied, moved together, and that is how they confirmed the existence of a huge, previously unknown Antalya 2, hiding behind the Milky Way.
Based on the observations of the team, Antlia 2 is located at 424,000 light years from Earth and is about 11.2 billion years old. The simulation suggests that most of its material was absorbed by the Milky Way – the same fate that awaits the Magellanic Clouds.
“The simplest explanation for why Ant 2 seems to be so low today is that it is separated by the galactic tides of the Milky Way,” said astronomer Sergey Koposov of Carnegie Mellon University.
“However, what remains inexplicable is the gigantic size of the object. Usually, when galaxies lose mass before the tides of the Milky Way, they shrink, not grow. ”
This means that it probably should have started much more than now, although this has yet to be determined. And there is also the question of the size of Antlia 2.
how New scientist reports, according to astronomer and expert RR Lyrae Gisela Clementini from the National Astrophysical Institute in Italy, the calculation used by the team to determine the distance to the RR Lyrae group included an error. When properly calculated, they are only 260,000 light years away.
But, according to Torrealba, this miscalculation changes the distance to the stars of RR Lyrae, and not the red giants who studied the team, which they confirmed in two different ways.
Thus, either the stars of RR Lyrae are in front of Antlia 2, which means that the discovery of the galaxy was the result of miscalculation and, therefore, extremely lucky, or they are part of the galaxy, but only the nearest edge.
In any case, of course, the team discovered something strange in our area.
“Compared to the other 60 satellites of the Milky Way, Ant 2 is a strange game,” said astronomer Matthew Walker, also from Carnegie Mellon University.
"We are wondering if this galaxy is only the tip of the iceberg, and the Milky Way is surrounded by a large population of almost invisible dwarfs like this one."
Team document has been logged. Monthly Notices of the Royal Astronomical Society, and can be read on the preprint resource arXiv.