Milky Way rare, massive, special, embedded in wall, influences rotation, Copernican bias, IllustrisTNG simulation, too big for wall.
An international team of astronomers has discovered that the Milky Way is too big for its “cosmological wall,” something yet to be seen in other galaxies. This wall environment, the Local Sheet, influences how 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 a wall. The findings are based on a state-of-the-art computer simulation, part of the IllustrisTNG project.
Typically, galaxies tend to be significantly smaller than this so-called wall. The Milky Way is found to be surprisingly massive in comparison to its cosmological wall, a rare cosmic occurrence. The team simulated a volume of the universe nearly a billion light-years across that contains millions of galaxies, with only a handful—about a millionth of all the galaxies in the simulation—being as “special” as the Milky Way, both embedded in a cosmological wall like the Local Sheet, and as massive as our home galaxy.
According to the team, it may be necessary to take into account the special environment around the Milky Way when running simulations, to avoid a so-called “Copernican bias” in making scientific inference from the galaxies around us. This bias, describing the successive removal of our special status in the nearly 500 years since Copernicus demoted the Earth from being at the center of the cosmos, would come from assuming that we reside in a completely average place in the universe.
Joe Silk, another of the researchers, said, “The Milky Way doesn’t have a particularly special mass, or type. There are lots of spiral galaxies that look roughly like it. But it is rare if you take into account its surroundings. If you could see the nearest dozen or so large galaxies easily in the sky, you would see that they all nearly lie on a ring, embedded in the Local Sheet. That’s a little bit special in itself. What we newly found is that other walls of galaxies in the universe like the Local Sheet very seldom seem to have a galaxy inside them that’s as massive as the Milky Way.”
Miguel Aragón, the research lead, added, “You might have to travel a half a billion light years from the Milky Way, past many, many galaxies, to find another cosmological wall with a galaxy like ours. That’s a couple of hundred times farther away than the nearest large galaxy around us, Andromeda.” This indicates that the Milky Way is, in a way, special, and it may be important to use precise locations to make such measurements, instead of assuming that any point in a simulation is as good as any.