Astronomers have explored three ultra-faint dwarf galaxies by merging data from the DESI Legacy Imaging Surveys and the Gemini South telescope. These galaxies, positioned towards NGC 300, are noteworthy for containing only ancient stars, which lends weight to the hypothesis that early cosmic events hindered star formation in the smallest galaxies.
Ultra-faint dwarf galaxies represent the dimmest variety of galaxies in the Universe. They typically house only a few hundred to a thousand stars, in stark contrast to the hundreds of billions found in the Milky Way. These small, spread-out formations often remain hidden among their brighter cosmic counterparts, making them easier to discover in the immediate vicinity of our own galaxy.
This proximity to the Milky Way, however, complicates efforts to understand these galaxies fully, as the Milky Way’s gravitational pull and heated outer atmosphere can strip the dwarf galaxies of their gas and disrupt their natural development. Furthermore, as astronomers move beyond the Milky Way, these ultra-faint dwarf galaxies become increasingly indistinct and challenging to identify using conventional techniques and algorithms.
To address this challenge, University of Arizona astronomer David Sand conducted a manual search for faint and ultra-faint dwarf galaxies towards the spiral galaxy NGC 300 and the Sculptor constellation. “During the pandemic,” Sand recalls, “I was watching TV and browsing the DESI Legacy Survey viewer, concentrating on areas of the sky that hadn’t been previously explored. After a few hours of casual searching, I discovered them—boom! They stood out.”
The images that Sand analyzed were obtained from the DECam Legacy Survey (DECaLS), one of three public surveys that make up the DESI Legacy Imaging Surveys [1], covering 14,000 square degrees of the sky in order to provide targets for the ongoing Dark Energy Spectroscopic Instrument (DESI) Survey. DECaLS was conducted with the 570-megapixel Dark Energy Camera (DECam), situated on the National Science Foundation’s (NSF) VÃctor M. Blanco 4-meter Telescope located at Cerro Tololo Inter-American Observatory (CTIO) in Chile, part of the NSF NOIRLab initiative.
The Sculptor galaxies, as mentioned in the research, are among the initial ultra-faint dwarf galaxies identified in an untouched, isolated setting devoid of the influence from the Milky Way or other significant structures. To further study these galaxies, Sand and his team utilized the Gemini South telescope, part of the International Gemini Observatory, which is partly backed by the NSF and managed by NSF NOIRLab. The findings of their research are published in The Astrophysical Journal Letters and were presented at a press conference during the AAS 245 meeting in National Harbor, Maryland.
Using the Gemini Multi-Object Spectrograph (GMOS), Gemini South captured remarkable details of all three galaxies. The analysis revealed that they lack gas and only harbor very old stars, indicating that star formation effectively ceased long ago. This supports existing theories that ultra-faint dwarf galaxies function as stellar ‘ghost towns’ where star creation was halted in the early Universe.
This trend aligns perfectly with what astronomers anticipate for such minuscule galaxies. Gas is crucial for forming new stars, but ultra-faint dwarf galaxies have insufficient gravitational force to retain this vital material, making it easy to lose when faced with the dynamic structure of the cosmos.
However, the Sculptor galaxies are distant enough from larger galaxies that suggests their gas was not removed by more substantial neighbors. One possible explanation is an event known as the Epoch of Reionization—shortly after the Big Bang, high-energy ultraviolet light surged throughout the universe, potentially boiling away the gas in the tiniest galaxies. Another theory posits that some of the first stars in these dwarf galaxies exploded as supernovae, launching gas away from these galaxies at velocities reaching 35 million kilometers per hour (around 20 million miles per hour).
If reionization played a role, these galaxies could provide valuable insights into the early Universe. “We don’t fully understand the intensity or uniformity of this reionization effect,” Sand explains. “It could be patchy, meaning it wasn’t uniform across the entire universe. We’ve found three such galaxies, but that’s not sufficient. It would be advantageous to discover hundreds, as understanding the portion affected by reionization could reveal essential details about the early Universe, which is otherwise hard to investigate.”
“The Epoch of Reionization uniquely ties the current structure of galaxies to their earliest formation on a cosmic level,” comments Martin Still, NSF program director for the International Gemini Observatory. “The combination of the DESI Legacy Surveys and thorough follow-up observations from Gemini allows scientists to conduct forensic analysis to glean insights into the Universe and its evolution to the state we observe today.”
To expedite the search for additional ultra-faint dwarf galaxies, Sand and his team are utilizing the Sculptor galaxies as a basis to train an artificial intelligence tool called a neural network for identifying more such galaxies. This approach aims to automate and speed up the discovery process, enabling astronomers to analyze an even larger dataset and draw more significant conclusions.
Notes
[1] The data from the DESI Legacy Imaging Surveys is accessible to the astronomical community via the Astro Data Lab at NSF NOIRLab’s Community Science and Data Center (CSDC).
