The Amateur Astronomer Who Found a Lost Galaxy: Andromeda XXXVI Is 12.5 Billion Years Old and Testing the Limits of Cosmology

The Galaxy: Andromeda XXXVI (And XXXVI)

The ultra-faint dwarf galaxy recently discovered orbiting the Andromeda Galaxy is Andromeda XXXVI (And XXXVI). It was spotted by amateur astronomer Giuseppe Donatiello during a systematic visual inspection of public images from the Pan-Andromeda Archaeological Survey (PAndAS), and confirmed via deep imaging with the OSIRIS instrument on the Gran Telescopio Canarias (GTC) . The discovery was announced in a study published in March 2026 led by Joanna Sakowska .

Key Characteristics

• Age: ~12.5 billion years old — its stars formed within the first ~1 billion years after the Big Bang .
• Heavy-element content (metallicity): Extremely metal-poor, with [Fe/H] ≈ –2.5 (less than 1% of the Sun's iron abundance), making it a near-primordial object . One source reports [Fe/H] = –2.0 ; the most recent preprint states [Fe/H] = –2.5 and notes the galaxy is "remarkably poor in heavy elements" .
• Number of identified stars: Not given as an exact count in published sources, but the galaxy is extremely sparsely populated — its absolute magnitude is only M_V ≈ –6.0, placing it among the faintest known satellite galaxies .
• Size and distance: About 200–400 light-years across, located ~2.5 million light-years from Earth .

How This Discovery Tests Dark Matter Models and Cosmological Predictions

Standard cold dark matter (ΛCDM) cosmology predicts that large galaxies like the Milky Way and Andromeda should be surrounded by hundreds or even thousands of dark-matter subhalos, but until recently only a few dozen luminous satellite galaxies were observed in each system. This long-standing discrepancy is called the "missing satellites problem" .

Ultra-faint dwarf galaxies (UFDs) like Andromeda XXXVI help resolve this tension in several key ways:

• Filling the predicted abundance. Each new UFD detection adds to the census of luminous satellites around Andromeda. Earlier work showed that as surveys push deeper, the number of known UFDs substantially alleviates the discrepancy between predicted and observed satellite counts .
• Testing the nature of dark matter. UFDs are dark-matter-dominated systems — their stellar motions can be used to measure subhalo masses and compare with ΛCDM predictions for subhalo abundance and spatial distribution. The observed spatial distribution of UFDs can discriminate between different models of how dwarf galaxies populate dark matter subhalos .
• Probing galaxy formation physics at the lowest mass scale. Because UFDs are extremely old and metal-poor, they are effectively "fossils" from the early universe, formed before or during the epoch of reionization. Their properties (luminosity, metallicity, velocity dispersion) constrain how baryonic processes like reionization and supernova feedback suppressed star formation in the smallest dark matter halos — a key test for whether simulated ΛCDM subhalos actually host luminous galaxies .
• Comparing two independent hosts. The Milky Way and Andromeda provide two independent censuses of UFDs around similar-mass galaxies. Finding consistent satellite populations around both is a critical test of the ΛCDM prediction that galaxy-halo connections are universal .

In short, Andromeda XXXVI and other recently discovered UFDs (such as Andromeda XXXV and Pegasus VII) help bridge the gap between the hundreds of predicted dark-matter subhalos and the small number of known luminous satellites, while also providing pristine laboratories for studying early galaxy formation and the particle properties of dark matter .