“Cosmic Whispers: DECam and Gemini South Unveil Three Elusive ‘Ghost Town’ Galaxies”

By integrating data from the DESI Legacy Imaging Surveys alongside the Gemini South telescope, astronomers have examined three ultra-faint dwarf galaxies situated in a region of space free from the environmental impacts of larger entities. The galaxies, found toward NGC 300, were observed to contain predominantly very ancient stars, reinforcing the hypothesis that phenomena in the early universe curtailed star formation in the smallest galaxies.

Ultra-faint dwarf galaxies rank as the dimmest type of galaxy in existence. Usually comprising only a few hundred to a thousand stars—compared to the hundreds of billions that constitute the Milky Way—these diminutive, diffuse formations often remain inconspicuous amongst the many more luminous inhabitants of the night sky. Consequently, astronomers have primarily succeeded in locating them in proximity to our own Milky Way galaxy.

However, this poses a challenge for comprehending these galaxies; the gravitational forces and heated corona of the Milky Way can strip away the gas of dwarf galaxies and disrupt their natural progression. Furthermore, further away from the Milky Way, ultra-faint dwarf galaxies have increasingly become too diffuse and indistinguishable for astronomers and conventional computer algorithms to identify.

This is precisely why a manual, visual search by University of Arizona astronomer David Sand was essential in uncovering three dim and ultra-faint dwarf galaxies positioned in the direction of the spiral galaxy NGC 300 and the Sculptor constellation.

“It was during the pandemic,” Sand reminisces. “I was watching television and browsing through the DESI Legacy Survey viewer, concentrating on areas of the sky that I was aware hadn’t been previously explored. After several hours of relaxed searching, suddenly, they just emerged.”

The images discovered by Sand were captured for the DECam Legacy Survey (DECaLS), one of three public surveys collectively known as the DESI Legacy Imaging Surveys, which have mapped 14,000 square degrees of sky to supply targets for the ongoing Dark Energy Spectroscopic Instrument (DESI) Survey.

DECaLS was conducted utilizing the 570-megapixel Dark Energy Camera (DECam), designed by the Department of Energy, 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 galaxies in the Sculptor constellation, as referred to in the discussion, represent some of the initial ultra-faint dwarf galaxies discovered in a pristine and isolated environment devoid of the influences from the Milky Way or other massive structures. To delve deeper into these galaxies, Sand and his associates employed the Gemini South telescope, which is a part of the International Gemini Observatory. The findings from their investigation are detailed in a publication featured in The Astrophysical Journal Letters, along with a presentation during a press conference at the AAS 245 conference in National Harbor, Maryland.

Gemini South’s Gemini Multi-Object Spectrograph (GMOS) captured detailed imagery of all three galaxies. A thorough examination of this data indicated that they appear devoid of gas and are composed solely of ancient stars, hinting that their star formation was halted long ago. This supports existing hypotheses that ultra-faint dwarf galaxies function as stellar ‘ghost towns’ where stellar formation ceased in the primordial universe.

This aligns perfectly with what astronomers anticipate for such diminutive entities. Gas represents the essential raw material required to coalesce and ignite the fusion in a new star. However, ultra-faint dwarf galaxies possess insufficient gravitational strength to retain this critical component, which is readily lost when subjected to the tumultuous dynamics of the universe they inhabit.

However, the Sculptor galaxies are situated far from any more massive galaxies, indicating that their gas could not have been stripped away by colossal neighbors. An alternative theory is an occurrence known as the Epoch of Reionization—a phase not long following the Big Bang when high-energy ultraviolet photons permeated the universe, possibly vaporizing the gas in the smallest galaxies.

An additional option is that some of the primordial stars within the dwarf galaxies experienced vigorous supernova explosions, ejecting material at speeds of up to 35 million kilometers per hour (approximately 20 million miles per hour) and expelling the gas from their own galaxies from the inside.

Should reionization be the cause, these galaxies would provide insights into investigating the very nascent universe. “We lack clarity on how intense or uniform this reionization phenomenon is,” states Sand.

“It might be that reionization is uneven, not occurring everywhere simultaneously. While we’ve discovered three of these galaxies, that number isn’t sufficient. It would be advantageous to locate hundreds of them. Understanding what proportion was influenced by reionization could reveal information about the early universe that is quite challenging to explore otherwise.”

“The Epoch of Reionization potentially links the present-day structure of all galaxies with the initial formation of structure on a cosmic scale,” comments Martin Still, NSF program director for the International Gemini Observatory. “The DESI Legacy Surveys, along with detailed follow-up observations by Gemini, enable scientists to conduct forensic archaeology to comprehend the universe’s nature and its evolution to its current state.”

To expedite the search for additional ultra-faint dwarf galaxies, Sand and his group are utilizing the Sculptor galaxies to train an artificial intelligence framework called a neural network to detect more. The expectation is that this tool will automate and hasten discoveries, providing a significantly larger dataset from which astronomers can derive more robust conclusions.