Gran Telescopio Canarias Confirms And XXXVI as Andromeda’s Faintest Satellite

Astronomers have confirmed a new dwarf galaxy orbiting Andromeda (M31), built from as few as 46 detectable stars and estimated to be 12.5 billion years old. Designated Andromeda XXXVI, the object sits at a projected distance of roughly 119 kiloparsecs (about 388,000 light-years) from Andromeda’s center, making it one of the faintest satellite galaxies ever confirmed around the Milky Way’s largest neighbor. The discovery, published in Astronomy & Astrophysics on June 25, 2026, traces back to an amateur astronomer’s visual search of public survey images.

A Galaxy Built From 46 Stars

And XXXVI first appeared as a faint overdensity of stars in the Pan-Andromeda Archaeological Survey (PAndAS), a wide-field imaging survey of Andromeda’s halo captured with the MegaCam camera on the Canada-France-Hawaii Telescope (CFHT). Amateur astronomer Giuseppe Donatiello spotted it while manually scanning the survey’s full footprint for diffuse, star-like overdensities, flagging 11 candidate dwarf galaxies. And XXXVI was one of two candidates selected for deeper follow-up.

Using the 10.4-meter Gran Telescopio Canarias (GTC) at the Roque de los Muchachos Observatory in La Palma, Spain, a team led by Joanna D. Sakowska of the Instituto de Astrofísica de Andalucía (IAA-CSIC) resolved the galaxy into individual stars. Their structural fit, based on 46 (+14/−11) stars identified in a color-magnitude diagram, gives And XXXVI a half-light radius of 64 (+30/−19) parsecs — about 209 light-years — and an ellipticity of 0.015, making it nearly circular. Its absolute magnitude, M_V ≈ −5.9 ± 0.1, places it among the faintest satellites known around M31.

Why Ultra-Faint Dwarfs Are Hard to Find

Ultra-faint dwarf galaxies are defined as systems fainter than absolute magnitude −7.7 — dim enough that their light is easily lost against foreground stars and survey noise. They are also among the most dark matter-dominated objects known, with stellar populations that formed in the early universe and have changed little since.

That combination makes them scientifically valuable. Because they formed before their host galaxies grew large and retain primitive chemistry, ultra-faint dwarfs serve as fossil records of the first galaxies. Their numbers and locations also test predictions of the Lambda Cold Dark Matter (ΛCDM) model, which forecasts that massive galaxies like Andromeda should be orbited by hundreds of faint companions.

What the Data Actually Shows

Sakowska’s team fit And XXXVI’s color-magnitude diagram against the PARSEC stellar isochrone library, using deep imaging in the g′ and r′ optical bands. The best fit favors an old, metal-poor population: an age of 12.5 billion years and a metallicity of [Fe/H] = −2.5, meaning the galaxy holds more than 300 times less iron, relative to hydrogen, than the sun.

“Our study suggests that And XXXVI is an extremely old galaxy, around 12.5 billion years old, and remarkably poor in heavy elements,” Sakowska said. No horizontal branch stars — a feature normally used to pin down distance independently — turned up in the data, so the team assumed And XXXVI sits at Andromeda’s own distance of 776 kiloparsecs (about 2.5 million light-years).

Gran Telescopio Canarias and How OSIRIS+ Made the Detection Possible

The follow-up imaging relied on a single instrument setup over one night:

• Telescope: 10.4-meter Gran Telescopio Canarias, La Palma, Spain
• Instrument: OSIRIS+ imager, field of view 7.8 × 7.8 arcminutes
• Observation date: January 22, 2026
• Exposure time: 3,600 seconds in g′, 2,980 seconds in r′
• Seeing: approximately 0.9 arcseconds in both bands

Photometry was processed through the DAOPHOT/ALLFRAME pipeline and calibrated against Pan-STARRS data, while structural parameters — size, shape, and orientation — were fit using the EMCEE Markov Chain Monte Carlo sampler.

What This Means for Dark Matter Models

“We currently know of around 40 dwarf satellite galaxies around Andromeda, of which only about 15 are classified as ultra-faint,” Sakowska said. “Each new discovery, such as Andromeda XXXVI, is important because it suggests that we may still be seeing only the tip of the iceberg of a much larger population of extremely faint galaxies.”

Co-author Isabel M. E. Santos-Santos, of Durham University and the Leibniz Institute for Astrophysics Potsdam (AIP), placed the find in cosmological context: “Within the framework of the standard cosmological model, the so-called Lambda Cold Dark Matter model (ΛCDM), we expect galaxies like Andromeda to be surrounded by hundreds of such small companions — yet many of them have remained hidden until now due to their low luminosity.”

Earlier modeling had predicted that M31 should host roughly 92 dwarf satellites brighter than M_V = −5.5; only about 43 are currently confirmed. Discoveries like And XXXVI narrow that gap between prediction and observation.

What Comes Next

“However, observations with space telescopes such as Hubble will be needed to determine its distance, age and chemical composition with greater precision,” Sakowska said. Without an independent distance measurement, And XXXVI’s true luminosity, size, and age all carry added uncertainty tied to its assumed distance. The research team notes that deeper space-based photometry reaching the galaxy’s oldest main-sequence turnoff stars would let researchers pin down when star formation in And XXXVI ended.

And XXXVI adds to the faint end of Andromeda’s satellite luminosity function — the catalog of known companion galaxies ranked by brightness. Each new ultra-faint dwarf identified around M31 narrows the gap between the satellite population ΛCDM predicts and the one astronomers have actually confirmed, while adding another data point to how the smallest, oldest galaxies in the universe formed and survived.

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SOURCE ATTRIBUTION
Source: Instituto de Astrofísica de Andalucía (IAA-CSIC) and Leibniz Institute for Astrophysics Potsdam (AIP)
Published in: Astronomy & Astrophysics, Vol. 710, A342 (2026)
Primary source: https://www.aanda.org/articles/aa/full_html/2026/06/aa60151-26/aa60151-26.html