Ultra-faint galaxy discovered near Andromeda may be 12.5 billion years old

A new ultra-faint dwarf galaxy has been discovered in the vicinity of Andromeda (M31), the Milky Way’s large neighboring galaxy. The new study, published in Astronomy & Astrophysics suggests that the galaxy, named And XXXVI, is one of the faintest satellite galaxies discovered around Andromeda to date.

Ultra-faint dwarf galaxies are among the smallest and dimmest galaxies known. Formed during the earliest stages of the universe, they are considered fossil records of the first galaxies and are thought to be dominated by dark matter. As such, they provide a unique window into galaxy formation in the early universe and offer valuable tests of dark matter models.

“Our study suggests that And XXXVI is an extremely old galaxy, around 12.5 billion years old, and remarkably poor in heavy elements,” says Joanna Sakowska, a researcher at the Instituto de Astrofísica de Andalucía (IAA-CSIC) and lead author of the study. “However, observations with space telescopes such as Hubble will be needed to determine its distance, age and chemical composition with greater precision.”

A sparse census around Andromeda

Located approximately 2.5 million light-years from Earth, the Andromeda Galaxy is the closest giant spiral galaxy to the Milky Way. Like our own galaxy, it is surrounded by numerous dwarf satellite galaxies that orbit under its gravitational influence.

“The discovery of Andromeda XXXVI offers a new perspective on the smallest galaxies in the universe. 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,” says Isabel Santos-Santos of the Leibniz Institute for Astrophysics Potsdam (AIP), a co-author of the study. “Each newly discovered ultra-faint dwarf galaxy helps us explore the limits of galaxy formation and put our cosmological models to the test.”

“We currently know of around 40 dwarf satellite galaxies around Andromeda, of which only about 15 are classified as ultra-faint,” explains Sakowska. “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.”

From survey images to deeper follow-up

Andromeda XXXVI was first identified by the astrophotographer and amateur astronomer Giuseppe Donatiello while examining images from the Pan-Andromeda Archaeological Survey (PAndAS), carried out with the Canada-France-Hawaii Telescope (CFHT). The object appeared as a faint, diffuse feature in which individual stars could already be distinguished. It was subsequently included in a list of candidate galaxies for further investigation.

The team secured director’s observing time on the Gran Telescopio Canarias (GTC), where they used the OSIRIS+ instrument to obtain much deeper images. These observations allowed them to distinguish individual stars within the galaxy’s faint, diffuse light. However, Andromeda XXXVI proved to be an exceptionally faint object: The research team was able to identify only about 46 stars associated with it.

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