Gemini North spies on an ultra-weak fossil galaxy discovered on the outskirts of Andromeda

An unusually ultra-weak dwarf galaxy has been discovered on the outer edges of the Andromeda Galaxy thanks to the sharp eyes of an amateur astronomer examining archival data processed by NSF’s NOIRLab Community Science and Data Center. Tracking professional astronomers using the Gemini International Observatory, a program from NSF’s NOIRLab, has revealed that the dwarf galaxy – Pegasus V – contains very few heavier elements and is probably a fossil of early galaxies.

An unusually ultra-weak dwarf galaxy has been discovered at the edge of the Andromeda Galaxy using several facilities at NSF’s NOIRLab. The galaxy, called Pegasus V, was first discovered as part of a systematic search for Andromeda dwarfs coordinated by David Martinez-Delgado of the Instituto de Astrofísica de Andalucía, Spain, when amateur astronomer Giuseppe Donatiello found an interesting ‘spot’ in data from a DESI Legacy Imaging Survey image [1]. The image was taken with the Dark Energy Camera made by the US Department of Energy on the 4 meter long Víctor M. Blanco telescope at the Cerro Tololo Inter-American Observatory (CTIO). The data were processed through the Community Pipeline, which is operated by NOIRLabs Community Science and Data Center (CSDC).

Further follow-up observations by astronomers using the larger 8.1-meter Gemini North Telescope with the GMOS instrument revealed faint stars in Pegasus V, confirming that it is an ultra-dwarf galaxy, faintly on the edge of the Andromeda Galaxy . Gemini North in Hawai’i is half of the Gemini International Observatory.

Observations with Gemini have revealed that the galaxy appears to be extremely deficient in heavier elements compared to similar dwarf galaxies, meaning that it is very old and probably a fossil of early galaxies in the universe.

“We have found an extremely faint galaxy whose stars were formed very early in the history of the universe,” commented Michelle Collins, an astronomer at the University of Surrey, UK and lead author of the newspaper, announcing this discovery. “This discovery marks the first time that such a faint galaxy has been discovered around the Andromeda Galaxy using an astronomical study that was not specifically designed for this task.”

The weaker galaxies are thought to be fossils of the very first galaxies that were formed, and these galactic relics have traces of the formation of the first stars. While astronomers expect the universe to be filled with faint galaxies like Pegasus V. [2], they have not yet discovered as many as their theories predict. If there really are fewer faint galaxies than expected, it would pose a serious problem with astronomers’ understanding of cosmology and dark matter.

Discovering examples of these faint galaxies is therefore an important task, but also a difficult one. Part of the challenge is that these faint galaxies are extremely difficult to spot, and they appear as a pair of sparse stars hidden in huge images of the sky.

“The problem with these extremely weak galaxies is that they have very few bright stars, which we typically use to identify them and measure their distances,” explained Emily Charles, a PhD student at the University of Surrey who also attended in the study. examination. . “The twin’s 8.1 meter large mirror allowed us to find faint old stars so we could both measure the distance to Pegasus V and determine that its star population is extremely old. »

The high concentration of ancient stars that the team found in Pegasus V suggests that the object is probably a fossil of early galaxies. Compared to the other faint galaxies around Andromeda, Pegasus V appears particularly old and metal-poor, indicating that its star formation actually ceased very early.

“We hope that further study of the chemical properties of Pegasus V will provide clues to the earliest periods of star formation in the universe,” Collins concluded. “This small fossil galaxy from the early universe can help us understand how galaxies are formed and whether our understanding of dark matter is correct.”

“The publicly available Gemini North Telescope offers a range of opportunities for community astronomers,” said Martin Still, Gemini Program Manager at the National Science Foundation. “In this case, Gemini supported this international team to confirm the presence of the dwarf galaxy, physically associate it with the Andromeda galaxy, and determine the metal-deficient nature of its evolved star population.”

Upcoming astronomical facilities should shed more light on faint galaxies. Pegasus V witnessed a period in the history of the universe known as reionization, and other objects from this time will soon be observed with NASA’s James Webb Space Telescope. Astronomers also hope to discover other faint galaxies in the future using the Vera C. Rubin Observatory, a program from NSF’s NOIRLab. The Rubin Observatory will conduct an unprecedented decade-long study of the optical sky called the Legacy Survey of Space and Time (LSST).


[1] The DESI Legacy Imaging Survey was conducted to identify targets for Dark Energy Spectroscopic Instrument (DESI) operations. These studies include a unique blend of three projects that observed a third of the night sky: the Dark Energy Camera Legacy Survey (DECaLS), observed by the Dark Energy Camera (DECam) built by the DOE on the Víctor M. Blanco 4 Meter Telescope at Cerro Tololo Inter- American Observatory (CTIO) in Chile; Mayall z-band Legacy Survey (MzLS), by the Mosaic3 camera on the Nicholas U. Mayall 4-meter telescope at the Kitt Peak National Observatory (KPNO); and the Beijing-Arizona Sky Survey (BASS) of the 90Prime camera on the 2.3-meter-long Bok telescope, owned and operated by the University of Arizona and located at KPNO. CTIO and KPNO are programs in NSF’s NOIRLab.

[2] Pegasus V is so named because it is the fifth discovered dwarf galaxy in the constellation Pegasus. The separation in the sky between Pegasus V and the Andromeda Galaxy is approximately 18.5 degrees.

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