An international team of astronomers analyzing deep optical images from the IAC Stripe 82 Legacy Project accidentally discovered a new, near-dark galaxy. The newly discovered Nube galaxy has a very low surface brightness and a mass as large as the Small Magellanic Cloud (SMC). The discovery was reported in an article published on the preprint server on October 18. arXiv.

Galaxies with central surface luminosity less than 26 mag/s²They are commonly known as “almost dark galaxies”. They have no precise optical equivalent and are not usually found in optical catalogs of large-scale surveys. However, these faint galaxies can show extremely weak optical emission when imaged at greater depth.

Now, a team of astronomers led by Mireia Montes of Spain’s University of La Laguna has discovered another galaxy of this rare type. They determined this during a visual inspection of one of the fields of view of the IAC Stripe 82 Legacy project, a large-scale survey of faint surface brightness astronomy. The survey examines Band 82, a 2.5-degree-wide band along the celestial equator in the Southern Galactic Cap.

Nube is approximately 350 million light-years away and has an effective surface luminosity of approximately 26.75 mag/s.². The age of the galaxy is estimated to be 10 billion years and its metallicity is measured as -1.1.

As for Nube’s other key parameters, the study found that it is quite large, with a half-mass radius of 22,500 light-years. The stellar mass of the galaxy is approximately 390 million solar masses, and the total halo mass is estimated to be 26 billion solar masses. These results indicate an effective surface density of approximately 0.9 solar maparsec.².

Based on the results obtained, the authors of the paper concluded that Nube is the largest and most extensive galaxy of this type discovered to date. The galaxy was also 10 times fainter and three times larger in radius than typical extremely diffuse galaxies (UDGs) with similar stellar masses. In general, UDGs are extremely low-density galaxies, comparable in size to the Milky Way, but with only 1% more stars than our own galaxy.

Given Nube’s extraordinary properties, researchers are debating the origin and nature of this galaxy. They investigated whether these features were a result of the galaxy’s initial formation or a subsequent evolution caused by its environment.

“To this end, and under the hypothesis that the distribution of stars in Nube corresponds to the distribution of dark matter halos, we find that the soliton-like profile (a typical example of fuzzy dark matter) reproduces the observed distribution of stars very well,” the study’s authors concluded: