Our galactic neighborhood is a diverse place , filled with galaxies of many different types, shapes, and sizes. From the gigantic and spiral Andromeda, located almost two and a half million light years away, through the Magellanic Clouds , two satellite galaxies of our own, considerably smaller and misshapen but still containing millions of stars, to dwarf galaxies as Triangulum II , a satellite galaxy of the Milky Way that contains barely a thousand stars.
The discovery of a new satellite galaxy of Andromeda , one of the dimmest and most difficult to detect galaxies known to date, has recently been published. This ultra-faint dwarf galaxy, named Pegasus V , was discovered by an amateur astronomer examining archival material from the US National Science Foundation’s NOIRLab . After alerting this Foundation, professional astronomers investigated the matter using the Gemini Observatory , confirming the presence of this galaxy on the outskirts of Andromeda’s halo, the largest member of our local group. This galaxy appears to have a very small proportion of elements other than helium , which are known in astronomy as heavy metals or elements. This seems to indicate that the galaxy could be a fossil of the first galaxies in the universe .
The images that made up the archive material in which the dwarf galaxy was first located were taken during a systematic search for Andromeda satellite dwarf galaxies coordinated by a Spanish researcher, David Martínez Delgado , from the Institute of Astrophysics of Andalusia. Later the amateur Giuseppe Donatiello identified a “stain” or “blot” . These images had been taken from the 4 meter diameter telescope of the Cerro Tololo Observatory in Chile. Subsequent observations with the 8.1-meter Gemini North telescope revealed the presence of this new galaxy.
The object’s name, Pegasus V , is due to it being the fifth satellite galaxy of Andromeda to be discovered in the constellation of Pegasus . This constellation is adjacent to Andromeda, but even so the fact that the satellite galaxy was discovered here already indicates that it will be located at a great distance from the main galaxy . Specifically, it is located about 850,000 light years away , about 4 times the diameter of Andromeda itself. This is therefore one of the most distant satellite galaxies of Andromeda detected. In fact, we think that it had not been detected before because the search efforts for this type of object have focused mainly on the region closest to Andromeda . Galaxies do not have well-defined boundaries, so it is always difficult to establish an exact size. Pegasus V contains most of its mass in a size of about 300 light years , so it could host several thousand stars .
This dwarf galaxy contains a very low proportion of elements heavier than helium, so it is thought to be a relic from the early universe. After the Big Bang , mainly hydrogen and helium were formed, in a ratio of 75% for hydrogen and 25% for helium. The next most abundant element after the first few minutes of the universe’s life was lithium, which was formed in quantities thousands of times smaller than helium. The rest of the elements did not begin to be synthesized until the arrival of the first stars . These created elements such as oxygen, carbon or silicon through nuclear fusion and other heavier ones such as germanium, gold or uranium in more violent processes, such as the death of a star, supernova explosions or the collision of stars. of neutrons .
Therefore, for all these heavy elements to form, the presence of various objects and, especially, time is required. This is why when we look at younger galaxies , with stars that have been created from clouds that were enriched with the ashes of earlier stars, we see a higher proportion of elements beyond helium . Pegasus V, however, does not show large amounts of these elements, which indicates that it could have been created during the first couple of million years of the life of the universe, not having enough interstellar gas to allow the existence of several stellar generations inside it. . Therefore, the few massive stars it contained have already reached the end of their cycle and today only the smallest and longest-lived stars survive.
Cosmological models predict the existence of objects like Pegasus V, very dim or massive, scattered throughout the universe. However, experimental searches have, to date, yielded fewer results than expected . If there really were fewer dwarf and ultra-faint galaxies than predicted, this could change our understanding of cosmology and dark matter. But we cannot say anything yet, because discovering these galaxies is especially complicated. Containing only small, old stars, they do not contain any of the especially luminous stars that we often use to identify other similar objects and to measure their distances.
These types of objects can be studied with better results with new telescopes and observatories, such as the James Webb Space Telescope, which has recently begun its first observations, and the Vera C. Rubin Observatory, also belonging to NOIRLab.
Reference:
Michelle LM Collins et al, 2022, Pegasus V — a newly discovered ultra-faint dwarf galaxy on the outskirts of Andromeda, Monthly Notices of the Royal Astronomical Society, https://doi.org/10.48550/arXiv.2204.09068
