Andromeda XXXVI: An Amateur Astronomer's 12.5-Billion-Year-Old Fossil Galaxy and What It Means for Dark Matter

The Missing Satellites Problem

The standard cosmological model, Lambda Cold Dark Matter (ΛCDM), makes a straightforward prediction: large galaxies like the Milky Way and Andromeda should be surrounded by hundreds or even thousands of dark-matter subhalos, each capable of hosting a dwarf galaxy. For years, however, observers found only a few dozen luminous satellites around each galaxy. This glaring mismatch became known as the "missing satellites problem" .

Ultra-faint dwarf galaxies (UFDs) like Andromeda XXXVI are key to resolving this tension. As surveys push deeper, each new UFD detection adds to the census of luminous satellites, steadily closing the gap between theory and observation .

How Andromeda XXXVI Tests Dark Matter and Galaxy Formation

Andromeda XXXVI and its fellow UFDs serve as unique laboratories for testing cosmological models in several ways:

• Filling the predicted abundance. The discovery of Andromeda XXXVI, along with others such as Andromeda XXXV and Pegasus VII, incrementally raises the known satellite count around Andromeda, helping to alleviate the predicted-versus-observed discrepancy .
• Probing the nature of dark matter. UFDs are overwhelmingly dominated by dark matter. Measuring the motions of their stars allows astronomers to infer the masses and spatial distribution of their parent subhalos, providing direct tests of ΛCDM predictions for subhalo abundance and clustering .
• Studying galaxy formation at the smallest scales. Because UFDs are extremely old and metal-poor, they are essentially fossils from the epoch of reionization. Their properties constrain how baryonic processes—like reionization and supernova feedback—suppressed star formation in the smallest dark matter halos. This helps determine whether simulated ΛCDM subhalos would actually host visible galaxies .
• Comparing two independent hosts. The Milky Way and Andromeda offer two separate censuses of UFDs around similar-mass galaxies. Finding consistent satellite populations around both is a critical test of the ΛCDM principle that galaxy-halo connections are universal .

An Amateur Astronomer's Contribution

The story of Andromeda XXXVI is also a reminder that discovery in astronomy is not solely the domain of professionals. Giuseppe Donatiello, an astrophotographer and amateur astronomer, has now been involved in the discovery of multiple ultra-faint satellites of Andromeda, including Pegasus V (Andromeda XXXIV) in 2022 . His method is painstakingly simple: he visually inspects archival survey images for the subtle overdensities of stars that indicate a previously unknown galaxy. Automated pipelines can miss these faint, sparse systems, but the trained human eye, when systematically applied, can still find them .

The Bottom Line

Andromeda XXXVI is a 12.5-billion-year-old, extremely metal-poor, ultra-faint dwarf galaxy that an amateur astronomer spotted in public survey data. It is one of the faintest known satellites of the Andromeda Galaxy, and its discovery—along with the broader search for Andromeda's UFD population—provides crucial observational data for testing dark matter models and understanding galaxy formation in the early universe. Each new discovery like Andromeda XXXVI helps close the gap between the hundreds of predicted dark-matter subhalos and the small but growing number of observed luminous satellites, offering a clearer picture of how galaxies form and evolve at the smallest scales.