noirlab2502 — Science Release

DECam and Gemini South Discover Three Tiny ‘Stellar-Ghost-Town’ Galaxies

Rare ultra-faint dwarf galaxies beyond the influence of other galaxies show evidence that star formation was stifled long ago

15 January 2025

By combining data from the DESI Legacy Imaging Surveys and the Gemini South telescope, astronomers have investigated three ultra-faint dwarf galaxies that reside in a region of space isolated from the environmental influence of larger objects. The galaxies, located in the direction of NGC 300, were found to contain only very old stars, supporting the theory that events in the early Universe cut star formation short in the smallest galaxies.

Ultra-faint dwarf galaxies are the faintest type of galaxy in the Universe. Typically containing just a few hundred to a thousand stars — compared with the hundreds of billions that make up the Milky Way — these small diffuse structures usually hide inconspicuously among the many brighter residents of the sky. For this reason, astronomers have previously had the most luck finding them nearby, in the vicinity of our own Milky Way Galaxy.

But this presents a problem for understanding them; the Milky Way’s gravitational forces and hot corona can strip away the dwarf galaxies’ gas and interfere with their natural evolution. Additionally, further out beyond the Milky Way, ultra-faint dwarf galaxies increasingly become too diffuse and unresolvable for astronomers and traditional computer algorithms to detect.

That’s why a manual, by-eye search by University of Arizona astronomer David Sand was needed to discover three faint and ultra-faint dwarf galaxies located in the direction of spiral galaxy NGC 300 and the Sculptor constellation. “It was during the pandemic,” recalls Sand. “I was watching TV and scrolling through the DESI Legacy Survey viewer, focusing on areas of sky that I knew hadn't been searched before. It took a few hours of casual searching, and then boom! They just popped out.”

The images uncovered by Sand were taken for the DECam Legacy Survey (DECaLS), one of three public surveys, known as the DESI Legacy Imaging Surveys [1], that jointly imaged 14,000 square degrees of sky to provide targets for the ongoing Dark Energy Spectroscopic Instrument (DESI) Survey. DECals was conducted using the 570-megapixel Department of Energy-fabricated Dark Energy Camera (DECam), mounted on the U.S. National Science Foundation (NSF) Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile, a Program of NSF NOIRLab.

The Sculptor galaxies, as they are referred to in the paper, are among the first ultra-faint dwarf galaxies found in a pristine, isolated environment free from the influence of the Milky Way or other large structures. To investigate the galaxies further, Sand and his team used the Gemini South telescope, one half of the International Gemini Observatory, partly funded by the NSF and operated by NSF NOIRLab. The results from their study are presented in a paper appearing in The Astrophysical Journal Letters, as well as at a press conference at the AAS 245 meeting in National Harbor, Maryland.

Gemini South’s Gemini Multi-Object Spectrograph (GMOS) captured all three galaxies in exquisite detail. An analysis of the data showed that they appear to be empty of gas and contain only very old stars, suggesting that their star formation was stifled a long time ago. This bolsters existing theories that ultra-faint dwarf galaxies are stellar ‘ghost towns’ where star formation was cut off in the early Universe.

This is exactly what astronomers would expect for such tiny objects. Gas is the crucial raw material required to coalesce and ignite the fusion of a new star. But ultra-faint dwarf galaxies just have too little gravity to hold onto this all-important ingredient, and it is easily lost when they are buffeted by the dynamic Universe they are part of.

But the Sculptor galaxies are far from any larger galaxies, meaning their gas could not have been removed by giant neighbors. An alternative explanation is an event called the Epoch of Reionization — a period not long after the Big Bang when high-energy ultraviolet photons filled the cosmos, potentially boiling away the gas in the smallest galaxies. Another possibility is that some of the earliest stars in the dwarf galaxies underwent energetic supernova explosions, emitting ejecta at up to 35 million kilometers per hour (about 20 million miles per hour) and pushing the gas out of their own hosts from within.

If reionization is responsible, these galaxies would open a window into studying the very early Universe. “We don’t know how strong or uniform this reionization effect is,” explains Sand. “It could be that reionization is patchy, not occurring everywhere all at once. We’ve found three of these galaxies, but that isn’t enough. It would be nice if we had hundreds of them. If we knew what fraction was affected by reionization, that would tell us something about the early Universe that is very difficult to probe otherwise.”

“The Epoch of Reionization potentially connects the current day structure of all galaxies with the earliest formation of structure on a cosmological scale,” says Martin Still, NSF program director for the International Gemini Observatory. “The DESI Legacy Surveys and detailed follow-up observations by Gemini allow scientists to perform forensic archeology to understand the nature of the Universe and how it evolved to its current state.”

To speed up the search for more ultra-faint dwarf galaxies, Sand and his team are using the Sculptor galaxies to train an artificial intelligence system called a neural network to identify more. The hope is that this tool will be able to automate and accelerate discoveries, offering a much vaster dataset from which astronomers can draw stronger conclusions.

Notes

[1] The DESI Legacy Imaging Surveys data are served to the astronomical community via the Astro Data Lab at NSF NOIRLab’s Community Science and Data Center (CSDC).

More information

This research was presented in a paper entitled “Three Quenched, Faint Dwarf Galaxies in the Direction of NGC 300: New Probes of Reionization and Internal Feedback” to appear in The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/ad927c

The team is composed of David J. Sand (University of Arizona), Burçin Mutlu-Pakdil (Dartmouth College), Michael G. Jones (University of Arizona), Ananthan Karunakaran (University of Toronto), Jennifer E. Andrews (International Gemini Observatory/NSF NOIRLab), Paul Bennet (Space Telescope Science Institute), Denija Crnojević (University of Tampa), Giuseppe Donatiello (Unione Astrofili Italiani), Alex Drlica-Wagner (Fermi National Accelerator Laboratory, Kavli Institute for Cosmological Physics, University of Chicago), Catherine Fielder (University of Arizona), David Martínez-Delgado (Unidad Asociada al CSIC), Clara E. Martínez-Vázquez (International Gemini Observatory/NSF NOIRLab), Kristine Spekkens (Queen’s University), Amandine Doliva-Dolinsky (Dartmouth College, University of Tampa), Laura C. Hunter (Dartmouth College), Jeffrey L. Carlin (AURA/Rubin Observatory), William Cerny (Yale University), Tehreem N. Hai (Rutgers, the State University of New Jersey), Kristen B.W. McQuinn (Space Telescope Science Institute, Rutgers, the State University of New Jersey), Andrew B. Pace (University of Virginia), and Adam Smercina (Space Telescope Science Institute)

Data for DECaLS were obtained at the Víctor M. Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, a Program of NSF NOIRLab. Pipeline processing and analyses of the data were supported by NOIRLab and the Lawrence Berkeley National Laboratory (LBNL). The Legacy Surveys project is supported by the Director, Office of Science, Office of High Energy Physics of the U.S. Department of Energy; by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility; and by the U.S. National Science Foundation, Division of Astronomical Sciences.

NSF NOIRLab, the U.S. National Science Foundation center for ground-based optical-infrared astronomy, operates the International Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), NSF Kitt Peak National Observatory (KPNO), NSF Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and NSF–DOE Vera C. Rubin Observatory (in cooperation with DOE’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona.

The scientific community is honored to have the opportunity to conduct astronomical research on I’oligam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence of I’oligam Du’ag (Kitt Peak) to the Tohono O’odham Nation, and Maunakea to the Kanaka Maoli (Native Hawaiians) community.