“Whispers of the Cosmos: Unveiling the Secrets of Elusive Dwarf Galaxies”

A group of astronomers led by David Sand, an astronomy professor at the University of Arizona’s Steward Observatory, has discovered three faint and ultra-faint dwarf galaxies located near NGC 300, a galaxy about 6.5 million light-years away from our planet. These uncommon findings – designated Sculptor A, B, and C – provide a rare chance to examine the tiniest galaxies in the cosmos and the cosmic forces that impeded their star formation billions of years ago.

Sand showcased the discoveries, which are documented in The Astrophysical Journal Letters, during a press conference at the 245th Meeting of the American Astronomical Society in National Harbor, Maryland, on Wednesday.

Ultra-faint dwarf galaxies are regarded as the most obscure type of galaxy in existence. Generally housing only a few hundred to thousands of stars – in contrast to the hundreds of billions found in the Milky Way – these small, dispersed formations typically blend into the multitude of brighter celestial bodies. Consequently, astronomers have mostly encountered them in close proximity to the Milky Way.

This presents a challenge for understanding; the gravitational forces and elevated temperatures in the outer regions of the Milky Way deplete the gas from these dwarf galaxies and disrupt their natural development. Furthermore, beyond the Milky Way, ultra-faint dwarf galaxies become increasingly too indistinct and challenging for astronomers and conventional computer algorithms to identify.

“Small galaxies like these are vestiges from the early universe,” stated Sand. “They assist us in comprehending the conditions present when the first stars and galaxies emerged and why certain galaxies entirely ceased the formation of new stars.”

The discovery of the three faint and ultra-faint dwarf galaxies necessitated a manual visual search. Sand noticed them while reviewing photos taken for the DECam Legacy Survey, or DECaLS, which is one of three public surveys collectively known as the DESI Legacy Imaging Surveys, targeted at imaging about a third of the sky to aid the ongoing Dark Energy Spectroscopic Instrument, or DESI, Survey.

“It occurred during the pandemic,” Sand recounted. “I was watching television and navigating through the DESI Legacy Survey viewer, concentrating on sections of the sky that I was aware had not been previously explored. After a few hours of casual inspection – and then voila! They simply emerged.”

The Sculptor galaxies rank among the inaugural ultra-faint dwarf galaxies identified in a pristine, isolated setting devoid of the Milky Way’s or other significant structures’ influence. To further examine these galaxies, Sand and his team employed the Gemini South telescope, which constitutes one half of the International Gemini Observatory, partly sponsored by the NSF and operated by NSF NOIRLab.

The Gemini South’s Gemini Multi-Object Spectrograph captured all three galaxies in remarkable detail. An evaluation of the data indicated that they seem to be void of gas and contain exclusively very old stars, implying that their star formation was hindered eons ago. This reinforces existing theories that ultra-faint dwarf galaxies serve as stellar “ghost towns” where star formation was halted in the early universe.

“This aligns perfectly with what we would anticipate for such diminutive entities,” Sand stated. “Gas serves as the essential raw component required for coalescing and igniting the fusion of a new star. But ultra-faint dwarf galaxies simply lack sufficient gravity to retain this vital ingredient, making it easily lost when affected by nearby, massive galaxies.”

The Sculptor galaxies, being distanced from any larger galaxies, likely had their gas preserved from removal by colossal neighbors. An alternate theory is what astronomers refer to as the Epoch of Reionization – a timeframe not long after the Big Bang when energetic ultraviolet photons permeated the cosmos, possibly vaporizing the gas in the smallest galaxies. Another conjecture is that some of the earliest stars within the dwarf galaxies experienced energetic supernova outbursts, ejecting material at speeds up to 35 million kilometers (approximately 20 million miles) per hour and driving the gas out of their hosts from within.

Dwarf galaxies could provide insights into the study of the very early universe, according to the research team, as the Epoch of Reionization potentially links today’s galaxy structure with the earliest formation of structures on a cosmological scale.

“We lack knowledge on how strong or uniform this reionization effect has been,” Sand clarified. “It’s possible that reionization occurred unevenly, rather than simultaneously across all regions.”

To resolve this question, astronomers must identify more objects similar to the Sculptor galaxies. By utilizing machine learning technology, Sand and his team aspire to streamline and expedite discoveries, with the aim of allowing astronomers to draw more robust conclusions.

Support for this research was supplied by the National Science Foundation. The Gemini Observatory, a program of NSF’s NOIRLab, is administered by the Association of Universities for Research in Astronomy, or AURA, under a cooperative agreement with NSF, on behalf of the Gemini partnership comprising Argentina, Brazil, Canada, Chile, the Republic of Korea, and the U.S.