Our galaxy would be "too large for its environment", making it extremely rare in the Universe
Our galaxy , the Milky Way, is just one of billions of galaxies in the known universe. Within it, there are more than 100 billion stars, accompanied by dust and gas. The Milky Way, like more than 1200 billion others in the observable Universe (about 60% of the total), is a spiral galaxy. Nothing surprising so far. But recently, researchers have shown that it would be much more imposing than we thought compared to its galactic environment. A very rare fact in the Universe, perhaps explaining part of our history?
Our sun is one of 100 billion stars in the Milky Way , a spiral galaxy about 100,000 light-years in diameter, whose stars are arranged in a windmill pattern, with four main arms. The Sun sits near a small arm called the Orion arm, or Orion spur, located between the arms of Sagittarius and Perseus.
At the center of the Milky Way is a supermassive black hole called Sagittarius A* . Discovered in 1974, its mass is equivalent to that of four million suns. In other words, the Milky Way is only one of the 2000 billion galaxies in the observable universe, and at first glance seems quite ordinary.
But recently, an international team of astronomers led by Miguel Aragon from the National Autonomous University of Mexico, highlighted the fact that our galaxy seems too big for its galactic environment, known as the Local Sheet. A very rare fact in the Universe. The results of this new study are published in the journal Monthly Notices of the Royal Astronomical Society .
The Milky Way "exceeds" its cosmological wall
The Milky Way is part of the small cluster of galaxies “Local Sheet”, or “local group”, which is located on the edge of the Virgo cluster, a conglomeration of several thousand galaxies. Our local group is considered “a cosmological wall”.
Concretely, a cosmological wall is a flattened arrangement of galaxies surrounding other galaxies, characterized by particularly empty regions, and areas of avoidance on either side of it. These voids seem to constrain the galaxies to create the flattened arrangement. This “wall” environment influences the way the Milky Way and nearby galaxies rotate around their axes, in a more organized way than if we were in a random place in the Universe, without this type of “wall”. In fact, as a general rule, galaxies tend to be much smaller than their walls.
Surprisingly, the Milky Way is an exception. The new findings are based on a computer simulation that is part of the IllustrisTNG project. It aims to shed light on the physical processes that govern galaxy formation: understanding when and how galaxies evolve. The simulations use state-of-the-art digital code that includes a full physical model and runs on some of the largest supercomputers in the world.
Specifically, the project consists of three volumes and 18 simulations in total. The large physical volume associated with the largest simulation box (TNG300) allows the study of galaxy aggregation, the analysis of rare objects such as galaxy clusters and provides the largest sample of galaxies.
From the project's largest simulation (TNG300), of a volume of the Universe nearly a billion light-years large containing millions of galaxies, the authors found that only a handful — about one millionth of all the galaxies in the simulation — galaxies as massive as the Milky Way could be located within a cosmological wall structure such as the Local Group.
The background image shows the distribution of dark matter (green and blue) and galaxies (here seen as tiny yellow dots) in a thin slice of the cubic volume in which lies one of these rare massive galaxies, such as the Milky Way.
Miguel A. Aragon-CalvoJoe Silk, from the Institut d'Astrophysique de Paris and co-author, said in a statement : “The Milky Way has no particular mass or type. There are many spiral galaxies that look pretty much like it. But this is rare if you take into account its environment. If you could easily see the nearest dozen large galaxies in the sky, you would see that almost all of them rest on a ring, embedded in the Local Group. It's a bit special in itself. What we have recently discovered is that other galaxy walls in the Universe like the local sheet very rarely seem to have a galaxy inside as massive as the Milky Way .”
Understand the evolution of the Universe and the Milky Way, without subjective bias
According to the team, it may be necessary to take into account the particular environment around the Milky Way when running simulations, to avoid a so-called "Copernican bias" in understanding the formation and evolution. evolution of the Milky Way and the neighboring universe.
This bias, describing the removal of our special status since Copernicus demoted Earth from the center of the cosmos, is believed to stem from the assumption that we reside in a completely average place in the Universe. To simulate observations, astronomers sometimes assume that any point in a simulation such as IllustrisTNG is as good as any, but the team's findings indicate that it may be important to use specific locations to perform such measurements.
Not to mention that the simulations in the present study did not take into account Andromeda , the Milky Way's largest galactic neighbor. Since the team's simulations only considered the context of the Milky Way within a cosmological wall, future work will need to consider more galaxies within the local group.
The researchers also note that the environmental context could help explain some previously unexplained phenomena, such as the unusual arrangement of satellite galaxies around Andromeda, and their particular absence around the Milky Way.
Mark Neyrinck, from the Basque Foundation for Science in Spain, explains: “ You have to be careful… in choosing properties that are called 'special' ”. He concludes: “ If we were to add a ridiculously restrictive condition, like the one highlighted by this study, on any galaxy, we would certainly be the only galaxy in the observable Universe. But this property is physically significant and observationally relevant enough to be considered truly rare .”
Source: websites