What’s hiding near Andromeda?
First there was an amateur astronomer: Giuseppe Donatiello found an interesting “spot” in the Dark Energy Camera data on the 4-meter Víctor M. Blanco telescope. Then, using the larger 8.1-meter Gemini North telescope, the professionals took a closer look and confirmed that the object, then named Pegasus V, was an ultra-faint dwarf galaxy on the edge of the Andromeda Galaxy. The observations revealed that the galaxy appears to be extremely poor in heavier elements compared to similar dwarf galaxies, which means that it is very old and probably represents a fossil of the first galaxies in the universe. In addition, star formation in this galaxy must have stopped very early.
“We have found an extremely faint galaxy whose stars formed very early in the history of the universe,” commented Michelle Collins, an astronomer at the University of Surrey, United Kingdom, and lead author of the paper announcing the discovery. The faintest galaxies are thought to be fossils of the very first specimens that formed after the Big Bang. As relics, they contain interesting clues about the formation of the earliest stars. Although astronomers believe that the universe is teeming with faint galaxies like Pegasus V, they have not yet discovered nearly as many as their theories predict. If there are indeed fewer faint galaxies than predicted, this would pose a serious problem for our understanding of cosmology and dark matter.
Discovering examples of these faint galaxies is therefore an important but difficult undertaking. Part of the challenge is that these faint galaxies are extremely difficult to detect, appearing only as a few sparse stars hidden in large areas of the sky. “The problem with these extremely faint galaxies is that they have very few of the bright stars we normally use to identify them and measure their distances,” explained Emily Charles, a doctoral student at the University of Surrey who was also involved in the study. “Gemini’s 8.1-meter mirror allowed us to find faint, old stars that we could use to both measure the distance to Pegasus V and determine that its stellar population is extremely old.