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Title: Deep in the Fields of the Andromeda Halo: Discovery of the Pegasus VII Dwarf Galaxy in UNIONS
Authors: Simon E. T. Smith et al.
First Author’s Institution: University of Victoria
Status: Published in ApJ
Picture a galaxy. It’s big, right? Unfathomably big, huge even! But galaxies aren’t all extremely massive; they come in a variety of sizes and luminosities, and some galaxies are so small and so dim that astronomers may not even know they’re there, even when they’re basically in our backyard. The hunt for the smallest galaxies is an important one, as these systems give us extreme environments where we can test theories of dark matter and galaxy evolution. The authors of this article present a newly discovered ultra-faint dwarf galaxy dubbed Pegasus VII (Peg VII), found next to our nearest major galactic neighbour: Andromeda.
How Do You Find a Dwarf Galaxy?
Finding ultra-faint dwarf galaxies requires deep, long-exposure imaging across wide swaths of the night sky, something that is only possible through observational surveys. Some telescopes, like JWST, require astronomers to submit an application to point the telescope at a specific, known target and, if approved, will have some amount of “telescope time” to do their imaging. Others, like the Euclid space telescope, are part of large surveys in which an area of the sky is selected and systematically imaged over several years, regardless of if there are known objects there or not. Because of this, surveys are our best bet to discover faint systems like ultra-faint dwarf galaxies.
The survey these authors used is UNIONS (Ultraviolet Near Infrared Optical Northern Survey), which covers the same area of the sky as the Euclid space telescope but is done with three ground-based telescopes: the Canada-France-Hawaii Telescope, Pan-STARRS, and Subaru, all located on Maunakea in Hawaiʻi. The authors were originally focused on finding satellite galaxies in the outskirts of Andromeda’s gravitational influence and during their search were able to identify an overdensity of bright stars. They then applied for and were awarded telescope time with both the Canada-France-Hawaii Telescope and the Gemini Observatory to perform deeper follow-up imaging of the system to figure out what exactly it was.
Optical Illusion or Optical Observation?
With this deeper follow-up imaging they could confirm if these stars were really one coherent system or if they just appeared to be clumped together on the sky. When they plotted the stars on a colour–magnitude diagram (Figure 1) they were able to identify a main sequence, horizontal branch, and red giant branch, as would be expected if these stars all formed at roughly the same time and were allowed to evolve over billions of years. Congratulations, it’s a dwarf galaxy!
Figure 1: Left: The colour–magnitude diagram for all stars within 2 half-light radii of Peg VII. Those inside the orange dashed lines are identified members of Peg VII, and the red lines are the best-fit isochrones. The vertical red line is the main sequence, the curve at the top is the red giant branch, and the horizontal line is the horizontal branch. Below the black dashed lines are stars that are too dim to be reliably used in the analysis. Right: The colour–magnitude diagram for stars that appear to be around Peg VII that are not actually members. These stars don’t follow the main sequence at all. [Adapted from Smith et al. 2025]
Figure 2: The spatial distribution of the Andromeda (M31) satellite galaxy system. Galaxies are coloured based on how bright they are (darker means dimmer). The dashed grey line denotes the area covered by PAndAS, a survey focused on Andromeda satellites that are more centrally concentrated. The dashed green line denotes the area covered by the UNIONS survey, with Peg VII on the right-hand side, more than 978,000 light-years from Andromeda. [Smith et al. 2025]
How Are Baby Dwarf Galaxies Made?
The big question surrounding Peg VII is how exactly it got like this. Did it originally form with so few stars, or was it originally bigger and has since had stars stripped away due to tidal forces exerted on it by Andromeda? It’s difficult to answer this right now because the imaging the authors obtained doesn’t allow them to determine Peg VII’s orbital path and speed around Andromeda. But, they can look for indicators of tidal disruption, like spatial elongation of the stars’ distribution, or how “stretched out” the galaxy is.
They found that Peg VII is quite elliptical (oval-like) and its major axis is roughly pointed at Andromeda (Figure 3). This could indicate that Peg VII has interacted with Andromeda in the past, and this could have affected its mass and size. However, there are lots of other possibilities for Peg VII’s shape that wouldn’t involve Andromeda at all. Peg VII could have naturally formed like this in isolation and has only just recently been brought into Andromeda’s orbit. It’s also possible Peg VII is the byproduct of a merger between two even smaller dwarf galaxies, which has been shown to result in elliptical distributions.
Figure 3: The spatial distribution of Peg VII member stars (dark black dots). The dashed lines show where 1, 2, and 3 half-light radii are, showing how elliptical Peg VII is. The blue arrow shows the direction Andromeda is in, which roughly lines up with Peg VII’s major axis. [Adapted from Smith et al. 2025]
In the meantime, keep your eye on the sky for more news about Peg VII and its other ultra-faint dwarf galaxy buddies. As big surveys designed for finding faint systems like these start to ramp up, like the Euclid space telescope and the Vera C. Rubin Observatory, we may be hearing more big things about these little galaxies.
Original astrobite edited by Caroline Von Raesfeld.
About the author, Veronika Dornan:
Veronika is a postdoctoral research associate at the University of Edinburgh. Her research is in observations of globular star clusters and how they can be used to study the evolution of their host galaxies.