A New Discovery in the Andromeda Halo: The Faint Dwarf Galaxy Pegasus VII

Dwarf galaxies are some of the smallest and faintest members of the universe but provide crucial insights into dark matter, galaxy formation, and the evolution of the cosmos. In this study, Simon E.T. Smith and collaborators present the discovery of Pegasus VII (Peg VII), a newly identified dwarf galaxy near the Andromeda Galaxy (M31). Using data from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) and follow-up imaging from the Canada-France-Hawaii Telescope (CFHT) and Gemini-North Telescope, the team confirms that Peg VII is a legitimate member of the Local Group of galaxies.

How Was Pegasus VII Discovered?

The UNIONS survey, a large-scale imaging project covering the northern sky, played a key role in the discovery. By analyzing photometric data in different filters (r and i bands), the researchers detected an excess of faint stars in the constellation Pegasus, hinting at a hidden stellar system. However, this initial detection alone was not enough to confirm the presence of a dwarf galaxy.

To verify their findings, the team obtained deeper images using CFHT's MegaCam and the Gemini-North Telescope's GMOS instrument. These images provided a clearer view of the stars in Peg VII, allowing the researchers to measure its size, brightness, and stellar population.

Properties of Pegasus VII

Peg VII is an ultra-faint dwarf galaxy with an absolute V-band magnitude of -5.7 (meaning it is much dimmer than most known dwarf galaxies). It has a half-light radius of 177 parsecs (pc), making it significantly larger than the most extended globular clusters in M31. The galaxy is located approximately 331 kiloparsecs (kpc) from M31, meaning it lies beyond the expected boundary of Andromeda's dark matter halo. Peg VII also has a noticeable elongation (ellipticity of ~0.5), which could suggest a past gravitational interaction with M31.

By comparing the color and brightness of Peg VII's stars to known models of stellar evolution, the team estimates its age to be between 7 and 13 billion years, with a metallicity of -2.0, indicating that its stars contain very few heavy elements—typical of old and faint dwarf galaxies.

Has Peg VII Interacted with Andromeda?

One of the biggest questions surrounding Peg VII is whether it has ever interacted with M31 or if it is on its first infall toward the galaxy. Because no velocity measurements exist yet, it is difficult to determine its exact orbit. However, its elongated shape and alignment toward M31 suggest that it may have experienced tidal forces from Andromeda’s gravity. If so, Peg VII might be at the farthest point in its orbit (apocenter) or even moving away after a previous encounter.

Alternatively, if Peg VII is falling toward Andromeda for the first time, it could offer a rare glimpse into the conditions of an isolated dwarf galaxy before it undergoes interactions with a massive host.

What Does This Mean for the Andromeda Satellite System?

Peg VII joins a growing list of newly discovered dwarf galaxies around M31, suggesting that many more faint satellite galaxies remain undiscovered. The study highlights how modern deep-sky surveys like UNIONS are expanding our knowledge of faint, extended objects in the Local Group. Future observations, such as spectroscopy to measure Peg VII's velocity and deep imaging to study its star formation history, will be critical to understanding its origins and role in the evolution of M31’s satellite system.

Conclusion

Pegasus VII represents a significant addition to the known dwarf galaxy population around Andromeda, being one of its faintest and most distant satellite candidates. Its discovery reinforces the idea that many more ultra-faint galaxies may still be hidden in wide-field astronomical surveys. Further observations will determine whether Peg VII is a relic of the early universe, an orphaned system drifting through space, or a satellite that has already felt the pull of Andromeda’s gravity.

Source: Smith