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Bad astronomy | Ultra-faint galaxy Pegasus V discovered near the Andromeda galaxy

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Astronomers have found very much a faint and tiny galaxy in the backyard of our cosmic neighbor, and despite its diminutive nature, it is of great importance for our understanding of the universe.

The galaxy is called Pegasus V, and all signs point to it being a satellite of the Andromeda galaxy, a massive spiral much like our own Milky Way. Andromeda lies about 2.5 million light-years away 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 with a small telescope, you can see M32 and NGC 205, a pair of dwarf elliptical galaxies near its center.

Pegasus V many, a lot of however, weaker. It was discovered by amateur astronomer Giuseppe Donatiello during the DESI Legacy Imaging Survey, a huge survey of the northern hemisphere sky that covers 14,000 square degrees: about 1/3.rd all over the sky*! It looked like very faint, ultradense radiation, so a team of astronomers turned to the huge 8.1-meter Gemini telescope to get deeper images.

What they found is amazing [link to paper]. Pegasus V is really miniature, probably only a few hundred light-years wide – by comparison, the Milky Way is 120,000 light-years across. It is also very faint, glowing with light about 26,000 times greater than that of the Sun. Keep in mind that one massive star can shine brighter!

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

The stars in it are extremely old, so old that there is an acute shortage of heavy elements in them. When the universe was young, it was composed almost entirely of hydrogen and helium, and heavier elements such as oxygen and iron formed inside stars that exploded at the end of their lives and scatter those elements, which were then incorporated into later generations of stars. By measuring the amount of these elements, we can determine the age of the stars, and their low number shows that the stars of Pegasus V are old.

The other technique was given an age of about 12.5 billion years, so the stars we see in it didn’t form that long after the Big Bang itself! This is really important. When the universe was very young, the gas in it was so dense that it was opaque to light, but then the first stars were born, and their huge stream of ultraviolet light stripped electrons from hydrogen atoms, making the universe transparent. We call this the epoch of reionization, and it has played a huge role in the evolution of the universe.

This explosion of ultraviolet radiation at that time would have cleared the small galaxies of gas, and we think that this would have extinguished any star formation. So the discovery of 12.5 billion year old stars in Pegasus V means it could be a relic of that era, a fossil from reionization. This makes it a very tempting target for astronomers who want to understand this difficult-to-observe period of cosmic history.

In addition, the distance of the Pegasus V also turns out to 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 of Andromeda have been discovered, some even as dim as Pegasus V, but all much closer to the center of the larger galaxy found in targeted research. This is the first time such a weak one has been seen in a review that didn’t specifically look for them.

But more importantly, the existence of the Pegasus V implies that there are probably many more than far from Andromeda just waiting to be found. If so, it could help solve a big problem: the problem of the dwarf galaxy. Models of how galaxies form predict that there should be hundreds of small, faint galaxies around Andromeda, but only a few dozen have been discovered. The fact is that the faintest ones are really hard to detect, especially far from Andromeda, so the discovery of Pegasus V strongly indicates that much more can be found, easing the tension of predictions and observations. If true, this means that models of how the early universe forms galaxies could be much better than current observations indicate.

It’s kind of funny how a slightly shapeless, dim blob in an image can have such a profound effect on our understanding, but sometimes the key to many problems is simply being able to find that first example. If Pegasus V is in fact a sample of a huge but dim population of nearby galaxies, many astronomers will breathe a sigh of relief.


* This is not Donatiello’s first discovery of a faint galaxy; he found another one a few years ago, too.

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