Gemini North spy on ultra-faint fossil galaxy discovered on the outskirts of Andromeda

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

An unusual ultra-faint 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 detected as part of a systematic search for Andromeda dwarfs coordinated by David Martinez-Delgado of the Instituto de Astrofísica de Andalucía, Spain, when the amateur astronomer Giuseppe Donatiello found an interesting “blotch” in the data from a DESI Legacy Imaging Surveys image [1]. The image was taken with the Dark Energy Camera manufactured by the US Department of Energy on the 4-meter Víctor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory (CTIO). The data was processed through the Community Pipeline which is operated by NOIRLab’s 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. faint on the outskirts 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 it is very old and likely to be a fossil of early galaxies in the Universe.

“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, UK and lead author of the paper. announcing this discovery. “This discovery marks the first time such a faint galaxy has been discovered around the Andromeda Galaxy using an astronomical survey that was not specifically designed for the task.”

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

Discovering examples of these faint galaxies is therefore an important undertaking, but also a difficult one. Part of the challenge is that these faint galaxies are extremely difficult to spot, appearing as a few sparse stars hidden in vast images of the sky.

“The problem with these extremely faint 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 took part in the study. ‘study. . “Gemini’s 8.1-meter mirror allowed us to find faint old stars, allowing us to both measure the distance to Pegasus V and determine that its stellar population is extremely old.”

The high concentration of ancient stars the team found in Pegasus V suggests the object is likely 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 indeed 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 tiny fossil galaxy from the early Universe may help us understand how galaxies form and whether our understanding of dark matter is correct.”

“The publicly available Gemini North Telescope offers an array of capabilities 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 stellar 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 of NSF’s NOIRLab. The Rubin Observatory will conduct an unprecedented decade-long survey of the optical sky called the Legacy Survey of Space and Time (LSST).


[1] DESI Legacy Imaging Surveys were conducted to identify targets of Dark Energy Spectroscopic Instrument (DESI) operations. These surveys include a unique blend of three projects that observed one-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 the 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 Kitt Peak National Observatory (KPNO); and the Beijing-Arizona Sky Survey (BASS) by the 90Prime camera on the 2.3-meter Bok Telescope, which is owned and operated by the University of Arizona and located at KPNO. CTIO and KPNO are programs of NSF’s NOIRLab.

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