While examining intricate optical images from the IAC Stripe 82 Legacy Project, a group of international astronomers unexpectedly uncovered a virtually dark galaxy. Named “Nube,” this galaxy features an exceptionally faint surface brightness, comparable in mass to the Small Magellanic Cloud (SMC).
This discovery was documented in a paper released on October 18 on the arXiv pre-print server.
Generally, “almost dark galaxies” refer to those with a central surface brightness dimmer than 26 mag/arcsec2. Often overlooked in optical catalogs of broad field surveys, they possess no clear optical match. Nevertheless, with deeper imaging, these elusive galaxies might reveal ultra-faint optical emissions.
Mireia Montes, from the University of La Laguna in Spain, along with her team, has identified yet another galaxy fitting this elusive category. They stumbled upon it during a review of a survey field from the IAC Stripe 82 Legacy Project. This project delves deep into Stripe 82—a celestial band spanning 2.5 degrees along the Celestial Equator in the Southern Galactic Cap.
Situated roughly 350 million light-years away, Nube boasts an effective surface brightness of around 26.75 mag/arcsec2. Estimated to be around 10 billion years old, its metal content was found to be at a -1.1 level.
Diving deeper into Nube’s primary characteristics, the research highlighted its expansive nature, sporting a half-mass radius of 22,500 light years. The galaxy’s stellar mass approximates 390 million times that of our Sun, while its total halo mass might reach 26 billion solar masses. This suggests an effective surface density close to 0.9 solar masses/parsec2.
Based on the data, the researchers deduced that Nube stands out as the largest and most expansive galaxy of its type discovered to date. Remarkably, it’s ten times dimmer, and its radius is triple that of typical ultra-diffuse galaxies (UDGs) with similar stellar masses. UDGs, by nature, have a very low star density; though their size is akin to the Milky Way, they only host about 1% of its star count.
Given Nube’s extraordinary attributes, the team delved into discussions about its origins and characteristics. They explored whether its unique features stemmed from its initial formation or if it evolved later, influenced by its surrounding environment.