Darker than the night sky: Scientists discover biggest "near-dark" galaxy ever!

The introduction of imaginary numbers was a surprising moment for many of us in school. Dealing with regular maths daily was already challenging, but now we had to grasp the concept of the truly incomprehensible.

Astronomers, who are known for their penchant for tackling challenging problems, grapple with this concept regularly. Their journey to unveil the secrets of the cosmos often leads them to confront dark matter, a theoretical form of matter that is essentially unobservable!

However, one should not underestimate the significance of this. According to scientific models, dark matter constitutes 26% of the universe, which is more than five times the amount of ordinary matter that we can see. This enigmatic substance has existed since the dawn of time, influencing the universe's structure and the creation of galaxies.

Remarkably, there are entire galaxies predominantly composed of dark matter. In 2016, astronomers discovered the Dragonfly 44 Ultra Diffuse Galaxy (UDG). Dragonfly 44 is as massive as our Milky Way, but it lacks the usual bright stars and galactic structure. Further investigation revealed that most of this galaxy's mass comes from dark matter.

Throughout history, scientists have encountered dark galaxies when they observed simple gas clouds exerting an inexplicable gravitational force on other celestial structures. While confirming a dark galaxy is challenging, there are several candidates that meet the criteria.

Moreover, there are galaxies that, while not entirely dark, come close to that definition due to their limited luminosity. These "near-dark galaxies" are rare and typically contain a substantial amount of dark matter. Scientists may have just identified the largest one to date!

The "Nube" galaxy, which is 10 billion years old, was found through an analysis of data from the IAC Stripe 82 Legacy Project, a focused study of a specific region of the sky imaged by the SDSS Telescope. With a half-mass radius (an imaginary boundary where the galaxy encompasses half of its estimated mass) of 22,500 light years, it is three times larger than a typical UDG!

Additionally, its very low optical emissions give it a surface brightness of only 26.75 mag/arcsec2, which is ten times fainter than regular UDGs, already among the dimmest galaxies in the observable universe. To put it in perspective, deep-sky objects above 22 mag/arcsec are considered faint, with a completely dark sky having a surface brightness of 21.8 mag/arcsec.

Nube also has a stellar mass of approximately 390 million times that of our Sun, meaning the Milky Way, with all its luminosity, is about 3,800 times more massive than this near-dark galaxy. However, comparing these two distinct galaxy types may not be entirely fair, given that even typical UDGs typically have only 1% as many stars as our home galaxy.

The discovery of Nube has sparked intriguing discussions about the origin and nature of such galaxies: did these peculiar properties emerge during its formation, or did something occur later? Exciting discoveries are on the horizon!

The results of this research have been published in a preprint journal and can be accessed here.