Bad Astronomy | Ultrafaint galaxy Pegasus V found near the Andromeda galaxy

Astronomers found one extremely faint and tiny galaxy in our cosmic neighbor’s backyard, and despite its tiny nature, it has major implications for our understanding of the Universe.

The galaxy is called Pegasus V, and all indications are that it is a companion to the Andromeda Galaxy, a massive spiral very similar to our own Milky Way. Andromeda is about 2.5 million light-years distant and, along with the Milky Way, dominates our small cluster of galaxies we call the Local Group. Both Andromeda and we are surrounded by dozens of much smaller companion galaxies; Even through a small telescope it is possible to see M32 and NGC 205, a pair of dwarf elliptical galaxies near their center.

Pegasus V is a lot, a lot of however weaker. It was discovered by amateur astronomer Giuseppe Donatiello while surveying the DESI Legacy Imaging Survey, a vast survey of the Northern Hemisphere sky covering 14,000 square degrees: about 1/3approx of the whole sky*! It appeared as a very faint, overdense light, so a team of astronomers went to the massive Gemini 8.1-meter telescope to get deeper images.

What they found is amazing [link to paper]. Pegasus V is really tiny, probably only a few hundred light-years wide – compared to the Milky Way, which is 120,000 light-years across. It is also very faint, glowing with light only about 26,000 times that of the Sun. Mind you, a single massive star can shine brighter!

The Gemini data allowed astronomers to study individual stars in the galaxy, giving them two important characteristics: their age and distance.

The stars in it are extremely old, so old that they lack heavy elements. When the universe was young it consisted almost entirely of hydrogen and helium, and heavier elements like oxygen and iron were made in stars that exploded at the end of their lives, scattering these elements, which were then incorporated into later generations of stars. By measuring the amounts of these elements, we can estimate the ages of the stars, and the lack of them shows that the stars of Pegasus V are old.

Another technique has an age of about 12.5 billion years, so the stars we see in it formed not too long after the Big Bang itself! That’s actually important. When the universe was very young, the gas it contained was so dense that it was opaque, but then the first stars were born and their huge tide of ultraviolet light stripped electrons from the hydrogen atoms, making the universe transparent. We call this the epoch of reionization, and it played a major role in the evolution of the universe.

That burst of UV light would have erased the gas right out of small galaxies back then, and we believe it would have erased all star formation. So if you see stars in Pegasus V that are 12.5 billion years old, it could be a relic from that era, a reionization fossil. That makes it a very tempting target for astronomers looking to understand this elusive time in cosmic history.

The distance from Pegasus V will also be interesting. It lies about 2.3 million light-years from Earth and about 850,000 light-years from the center of the Andromeda galaxy. Many faint satellite galaxies have been found for Andromeda, some even as faint as Pegasus V, but they are all much closer to the center of the larger galaxy spotted in targeted surveys. This is the first time such swoons have been seen in a poll that didn’t specifically look for them.

But more importantly, the existence of Pegasus V implies that there are likely many more so far from Andromeda just waiting to be found. If so, it could help solve a big problem: the dwarf galaxy problem. Galaxy formation models predict that there should be hundreds of small, faint galaxies orbiting Andromeda, but only a few dozen have been found. The faintest ones are really hard to spot, especially far outside of Andromeda. So when you find Pegasus V, it strongly suggests there is much more to find, which takes the tension out of the predictions and observations. If true, it means the models of how the early Universe formed galaxies may be much better than current observations suggest.

It’s kind of funny that a slightly amorphous, faint spot in an image can have such a profound impact on our understanding, but sometimes the key to many problems lies in just finding the first example. If Pegasus V is indeed the example of a huge but faint population of nearby galaxies, many astronomers will breathe a sigh of relief.


* This isn’t Donatiello’s first faint galaxy discovery; he also found one a few years ago.

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