Scientists Discover Ultramassive Galaxy “Gone Dark” | Unveiled

Scientists Discover Ultramassive Galaxy “Gone Dark”

Galaxies are already some of the biggest singular structures in our entire universe; with stars, planets, asteroids and black holes all existing inside them. But in early 2020, astronomers observed a galaxy which truly shook our understanding of space.

This is Unveiled, and today we’re uncovering the extraordinary strangeness of ultramassive galaxies.

Just like all other celestial objects, galaxies can vary hugely in size. One of the lowest mass galaxies out there is Segue 2, which “only” holds 1,000 stars. Meanwhile the catchily-named ESO 146-IG 005 galaxy has ten times the mass of our own, the Milky Way… And the largest observed galaxy by diameter is thought to be Malin 1, which is more than half a million lightyears across, making it five times as wide as the Milky Way. But scientists still don’t categorize either Malin 1 or ESO 146-IG 005 as “ultramassive”.

In terms of galaxy types, “ultramassive” is quite a new grouping. The term “ultramassive” was initially used to describe black holes with a solar mass higher than ten billion - creating a “next stage up” from “supermassive”. After all, the difference between a black hole with a mass a mere one hundred times greater than the sun and, say, the largest black hole ever discovered (which has a mass sixty-six billion times that of the sun), is almost incomprehensibly vast. Still, “ultramassive” as an astrophysical adjective is only beginning to be widely used. And some scientists still aren’t keen to use it in relation to galaxies.

Even so, the galaxy XMM-2599 has found itself christened as “ultramassive”. But its story is actually much more interesting than just; “it’s really big”. XMM-2599 was discovered by a team of astronomers at the Keck Observatory in Hawaii. It’s situated around 12 billion lightyears away from us… and It is big - to the point that it’s also been labelled a “monster galaxy”. But what’s especially unusual about it isn’t its size or the number of stars it has right now… but rather how quickly it once made its stars and how swiftly it apparently stopped making them.

In the first 800 million years of its lifetime, XMM-2599 produced some 300 billion stars. For contrast, some higher estimates put the Milky Way as having a similar number today - only the Milky Way is one of the universe’s oldest galaxies and has had 13.5 billion years to make that many. So, in just a fraction of the time, this one ancient galaxy managed to create as many stars as the Milky Way has made across its entire existence. Even within the infinite reaches of space, that’s impressive!

So, if this galaxy is so big and brilliant, why are we only just learning about it? Surely, with all of our ever-advancing technology, we’d have uncovered this cosmic colossus long ago? Well, the reason why we didn’t mostly rests with the other unusual aspect of XMM-2599; that it stopped making stars before the universe was even two billion years old - more than eleven billion years ago. No new stars for billions of years means that this galaxy - despite its size and mass - produces very little light. In astronomical circles it is described as having “gone dark”, which is why it took us so long to find it. Ultimately, the discovery required a highly specialised instrument much more catchily named the MOSFIRE. Amongst other things, the MOSFIRE - or Multi-Object Spectrograph for Infrared Exploration - can see beyond the visible light spectrum humans are familiar with.

So, how can a galaxy just “go dark” and stop making new stars? It’s not something that’s yet happened to the Milky Way, which is still creating new stars right now. It’s a mystery that scientists don’t currently have a solid answer to. A “galaxy gone dark” is a new phenomenon for us, but the basic, assumed explanation is that (for whatever reason) it simply ran out of fuel.

Importantly, XMM-2599 isn’t losing any of its mass – everything that was inside it is still inside it - it’s just that all of its brightest stars have died. The most massive stars are often the brightest, but they also tend to have the shortest lifespans. Perhaps this unseen, ultramassive entity was once powered by a higher than average number of these types of star. In contrast, long-lived stars like red dwarfs likely still exist as they always have done within the parameters of XMM-2599. From our vantage point, though, these stars are so dim that they’re incredibly difficult to detect - so, kudos to MOSFIRE.

Interpretations of this discovery and “what it means” are only just starting to form… But there are some conclusions to be had. First, just because this galaxy is dark right now doesn’t mean it will be dark forever. Some of the brightest galaxies in our sky are made that way because of galactic collisions - the like of which the Milky Way and Andromeda are scheduled to experience in around 4.5 billion years’ time. Like all matter, XMM-2599 is constantly moving through space, so one day it, too, could consume another galaxy; which could re-energise it and light the whole thing up. This galaxy is so big, though, that if this were to happen, astronomers have said it could create a “city of galaxies” in the night sky.

Elsewhere, the discovery has further ramifications for current models of early cosmology. Scientists don’t yet understand how XMM-2599 made so many stars so quickly, or how it gained such a huge amount of mass at the beginning of the universe when everything was smaller and lighter. In general, notably big galaxies only get that way by merging with others… but, based on most contemporary ideas on how the universe works, this galaxy shouldn’t have had time enough to merge and gain its size. And, seeing as because it’s “dark” we know it hasn’t grown in the billions of years since it went “dark”, it’s a head-scratcher of the highest degree! Which isn’t to say that it destroys everything we thought we understood about early cosmology; only that it changes it. We now know that this ancient galaxy does exist, and we might predict that there are others still undetected but just like it. It’s a problem, yes, but really it only shows us what we already knew; that, when it comes to the universe, we still have plenty left to learn.

And, in truth, XMM-2599 isn’t completely off on its own. There are other strange and unusual galaxies out there, some of which share characteristics with it. The galaxy known as Dragonfly 44, for example, is shaped like a conventional galaxy, but is organized around dark matter instead of typical stars. Meanwhile, there are galaxies like Messier 82 which produce stars at an incredible rate much like XMM-2599 did at the dawn of the universe. Messier 82 is today making stars so quickly, in fact, that it’s emitting two enormous, signature plumes of red hydrogen.

Nevertheless, the discovery of XMM-2599 has thrown open the doors of possibility once again. Because, if something as massive as that could go unnoticed for billions of years, then what else is out there? With at least 100 billion galaxies in just the observable universe, we still have lots to learn about what they all look like and how they all function… but isn’t it always exciting when “something new” threatens to rewrite everything we thought we knew before!