An international research team led by scientists from Poland (Dr. Oliver Newton, CTP PAS Warsaw), Spain (Dr. Arianna Di Cintio and Dr. Salvador Cardona-Barrero, IAC/ULL Tenerife) and Germany (Dr. Noam Libeskind, AIP Potsdam) has made exciting predictions that could shed new light on our understanding of the universe. They concern ultra-scattered galaxies: faint galaxies with masses of "only" up to one billion suns (about one-thousandth the mass of the Milky Way), but scattered over an area comparable to the size of our Galaxy. Their properties make them shine very faintly and difficult to observe, and as a result they remain little understood.

Researchers believe that more discoveries await us in the Local Group - a small group in which we have so far identified about 60 galaxies, including our Milky Way and the Andromeda Galaxy. So far, only two ultra-scattered galaxies have been found in our group of galaxies; however, scientists believe that understanding the total number of galaxies in our area is crucial to understanding the entire cosmos.

How many such ultra-scattered galaxies are still lurking in our cosmic backyard? To find out, an international team has studied state-of-the-art computer simulations of our cosmic neighborhood. Named after the ancient Greek goddess of the house, the HESTIA numerical simulations yield the most accurate computer model to date of the Milky Way and its immediate vicinity. The simulations predict that there may be as many as 12 ultra-scattered galaxies in the Local Group, waiting to be discovered. Based on analysis of the properties of such galaxies in the HESTIA data, the team predicts that several of these galaxies may be detected in already collected data from sky surveys such as the Sloan Digital Sky Survey.

The discovery of these new galaxies could have far-reaching implications for our understanding of galaxy formation and evolution. Current models suggest that up to half of the small-mass galaxies in the universe may have this diffuse form. Most of them we are unable to observe with our current technological capabilities. Since the number of galaxies in the universe strongly depends on its global properties at the largest scales (i.e. the cosmological model), the number of ultra-scattered galaxies in the Local Group can be used to rule out some of these models.

The research, titled "The undiscovered ultra-diffuse galaxies of the Local Group," will be published in The Astrophysical Journal Letters on 30th March 2023.

ApJL: https://iopscience.iop.org/article/10.3847/2041-8213/acc2bb

ApJL DOI: https://doi.org/10.3847/2041-8213/acc2bb

arXiv: https://arxiv.org/abs/2212.05066