Deep (Space) Dives is going to be a type of article where I just focus in on a particular thing in astrophysics. I’ll give you super in-depth, hardcore, raw, uncensored stuff to do with this thing. I’ll try and cover literally all the things I can about that thing. It’s going to be fun, and we’re going to start big. Really big.
What is a supercluster? Well, that’s a good question. A supercluster is generally defined as a collection of galaxy clusters. The thing that sticks out the most about superclusters is that they’re huge. Like, really really big. The supercluster we are defined as being in is the Laniakea supercluster, which contains the entire local group easily at its outskirts, as well as the supercluster we used to think we were part of, the Virgo supercluster. It is about 160 mega parsecs, or about 520 million light years across.
Superclusters are difficult to define, as galaxies are usually spread out in space along “filaments”, so it is difficult to decide where one supercluster ends and another begins. That is, it was difficult until 2014, when R. Brent Tully and Helene Courtois published an article in the scientific journal Nature entitled “The Laniakea Supercluster of Galaxies”. In this article, they had taken the speeds of the recession of galaxies (the speed they’re all moving apart) and subtracted the speed of recession that would be caused by the expansion of the universe. The regions that then had negative velocities (i.e. the regions that were now moving together rather than apart) were now defined as part of the same supercluster. In this case, Laniakea.
Galaxy filaments are generally accepted to follow strings of dark matter, as this is where it would be easiest to form galaxies because of the concentration of mass. These filaments are classified in four different ways: filaments (yes, I know, a subtype fo filaments is filaments), galaxy walls, large quasar groups and supercluster complexes.
These are generally defined as having roughly circular cross sections, and be reasonably long, about 50-80 mega parsecs, or 163 to 261 million light years. There are about 5 we know of so far: Coma Filament, Perseus-Pegasus Filament, Ursa Major Filament, Lynx-Ursa Major Filament and the z (redshift) =2.38 filament around protocluster ClG J2143-4423.
These have a significant difference in the shortest length (minor axis) across the cross section and the longest length (major axis) across the cross section. There are about 6 well-defined great walls, including the famous Sloan Great Wall and the Hercules-Corona Borealis Great Wall, which is the biggest known structure in the universe.
Large Quasar Groups
These are large groups of quasars (or “active galaxies”, supermassive black holes that are feeing on lots of matter and are very luminous) that stretch to larger than any of the other types of filaments, and are generally assumed to be precursors to galaxy walls, sheets and filaments. Also, the biggest one is literally just called “the Huge-LQG”, and that’s honestly such a mood.
There is only one know one of these: the Pisces-Cetus Supercluster Complex, discovered in 1987 by R. Brent Tully (the same guy from the 2014 study). This complex contains about 60 clusters and is about 1 billion light years across at its largest axis. This subgroup is actually a bit controversial, as it doesn’t really make sense to classify this specific group as its own superstructure, as it lies inside what we have already decided (in that 2014 paper) was the Laniakea Supercluster.
Voids are areas of space that don’t contain a lot of galaxies or generally matter. They also tend to be slightly cooler than space in galaxy filaments. We are actually inside the biggest void yet detected, the KBC, which means that the value for the Hubble constant inside the void will be slightly higher, as the galaxies inside it are being pulled outwards by the denser space around the void.
So that concludes this week’s deep (space) dive. Check back next week to see me start another long term project I’ll never finish! Ok, fine, next week I’ll try doing something on Planck Stars, as part of the Exotic Stellar Remnant series.