A team of astronomers led by David Sand, a professor of astronomy at the University of Arizona Steward Observatory, has uncovered three faint and ultra-faint dwarf galaxies in the vicinity of NGC 300, a galaxy approximately 6.5 million light-years from Earth. These rare discoveries – named Sculptor A, B and C – offer an unprecedented opportunity to study the smallest galaxies in the universe and the cosmic forces that halted their star formation billions of years ago. 

Sand presented the findings, which are published in The Astrophysical Journal Letters, during a press briefing at the 245th Meeting of the American Astronomical Society in National Harbor, Maryland, on Wednesday.

Ultra-faint dwarf galaxies are the faintest type of galaxy in the universe. Typically containing just a few hundred to thousands of stars – compared to 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 the Milky Way.

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

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"Small galaxies like these are remnants from the early universe," Sand said. "They help us understand what conditions were like when the first stars and galaxies formed, and why some galaxies stopped creating new stars entirely."

A manual search by eye was needed to discover three faint and ultra-faint dwarf galaxies. Sand saw them when reviewing images taken for the DECam Legacy Survey, or DECaLS, one of three public surveys known as the DESI Legacy Imaging Surveys, which jointly imaged about a third of the sky to provide targets for the ongoing Dark Energy Spectroscopic Instrument, or DESI, Survey. 

"It was during the pandemic," Sand recalled. "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 Sculptor galaxies 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 these 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.

Gemini South's Gemini Multi-Object Spectrograph 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 long 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 we would expect for such tiny objects," Sand said. "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 on to this all-important ingredient, and it is easily lost when they are affected by nearby, massive galaxies."

Because the Sculptor galaxies are far from any larger galaxies, their gas could not have been removed by giant neighbors. An alternative explanation is what astronomers call 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 (about 20 million miles) per hour and pushing the gas out of their own hosts from within.

Dwarf galaxies could open a window into studying the very early universe, according to the research team, because the Epoch of Reionization potentially connects the current-day structure of all galaxies with the earliest formation of structure on a cosmological scale. 

"We don't know how strong or uniform this reionization effect was," Sand explained. "It could be that reionization is patchy, not occurring everywhere all at once."

To answer that question, astronomers need to find more objects like the Sculptor galaxies. By enlisting machine learning tools, Sand and his team hope to automate and accelerate discoveries, in hopes that astronomers can draw stronger conclusions.

Funding for this work was provided by the National Science Foundation. Gemini Observatory, a program of NSF's NOIRLab, is managed by the Association of Universities for Research in Astronomy, or AURA, under a cooperative agreement with the NSF, on behalf of the Gemini partnership of Argentina, Brazil, Canada, Chile, the Republic of Korea, and the U.S.