A ghost galaxy orbiting Andromeda has been hiding for 12.5 billion years — and we just found it

A new ultra-faint dwarf galaxy has been discovered in the outskirts of the Andromeda galaxy, offering astronomers a rare window into the conditions of the universe when it was barely a billion years old. Designated Andromeda XXXVI, the object was identified through data from the Pan-Andromeda Archaeological Survey (PandAS) and confirmed with deep imaging at the Roque de los Muchachos Observatory in the Canary Islands. The findings appear in a paper submitted to arXiv on March 30, 2026, led by Joanna D. Sakowska of the Institute of Astrophysics of Andalusia in Spain.

What ultra-faint dwarfs reveal about the early universe

Ultra-faint dwarf galaxies (UFDs) occupy a unique and scientifically invaluable position in the cosmic landscape. They are the least luminous, the most dark matter-dominated, and the least chemically enriched galaxies known. Their stellar populations are ancient — formed when the universe was young and metal-poor — and their internal dynamics have remained largely undisturbed by subsequent cosmic events. For these reasons, UFDs are considered among the best preserved relics of the epoch of reionization, effectively functioning as fossil records of galaxy formation in its earliest stages.

Andromeda XXXVI adds to the growing catalog of these objects around Messier 31 (M31), the Andromeda galaxy, which at approximately 2.537 million light years is the nearest large spiral galaxy to the Milky Way.

Discovery and confirmation

The initial detection of Andromeda XXXVI was made not by an automated pipeline, but by amateur astronomer Giuseppe Donatiello during a systematic visual inspection of public PandAS images. This is not an isolated case — Donatiello has a track record of identifying faint stellar structures that automated algorithms overlook, underscoring the continued value of the human eye in modern survey astronomy.

Following this identification, Sakowska and her team conducted dedicated deep imaging of the candidate using the Roque de los Muchachos Observatory, resolving individual stars in Andromeda XXXVI and constructing its color-magnitude diagram (CMD). The CMD analysis confirmed the object’s nature as a genuine ultra-faint dwarf galaxy and allowed the team to constrain its stellar population and structural parameters.

Physical properties of Andromeda XXXVI

Andromeda XXXVI sits at a distance of roughly 388,000 light years from the center of Andromeda, well within M31’s virial radius of approximately 850,000 light years, firmly establishing it as a satellite. Its visual magnitude of −6.0 places it among the intrinsically faintest galaxies in the known universe, and its half-light radius of 208 light years makes it one of the most compact UFD satellites associated with Andromeda — potentially the second most compact of its kind around M31.

The galaxy’s metallicity, measured at [Fe/H] = −2.5, reflects an extremely metal-poor stellar population, consistent with a system that ceased star formation long before the universe had time to enrich its interstellar medium through multiple generations of stellar evolution. Its estimated age of approximately 12.5 billion years reinforces this picture, placing the bulk of its star formation squarely within the first billion years after the Big Bang.

An incomplete census of Andromeda’s satellites

Current models of structure formation under the standard ΛCDM cosmological framework predict that large galaxies like Andromeda should host a large number of satellite dwarf galaxies. Estimates suggest Andromeda may harbor close to 100 dwarf satellites, yet only roughly half have been identified to date. The missing satellites problem — the discrepancy between the number of predicted and observed subhalos — remains one of the open tensions in near-field cosmology.

Discoveries like Andromeda XXXVI demonstrate that the census of M31’s satellite population is still incomplete, and that some of the missing objects may simply be too faint and too diffuse to be captured by automated detection methods. The authors highlight the complementary role of visual inspection alongside machine learning approaches, arguing that both are necessary to construct a complete picture of Andromeda’s satellite system.

Why this matters

Each newly confirmed UFD around Andromeda is not merely another entry in a catalog. These galaxies carry encoded within their stars the chemical and dynamical conditions of the primordial universe. Their dark matter content, measured through stellar kinematics, provides constraints on the nature of dark matter itself — whether it behaves as cold, warm, or self-interacting. Their metallicities and age distributions trace the reionization history of the universe. And their spatial distribution around M31 tests the predictions of cosmological simulations at the smallest scales.

Andromeda XXXVI, faint as it is, may yet have much to say.

The full study is available on arXiv: Joanna D. Sakowska et al., «Andromeda XXXVI: discovery of a new ultra-faint dwarf galaxy towards M31,» arXiv (2026). DOI: 10.48550/arxiv.2603.26492.

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