University of Arizona Steward Observatory astronomers have discovered “cosmic ghost towns” that were halted in their development by cosmic forces billions of years ago.
The “ghost towns,” named Sculptor A, B, and C, consist of three faint and ultra-faint galaxies about 6.5 million light-years from Earth. Despite their barren nature, these desolate regions of space may hold surprisingly important answers about the structure of our modern galaxy.
Faint Galaxies
As their name implies, due to their low star density, ultra-faint galaxies are dim regions of space. Such an ultra-faint galaxy will typically harbor only hundreds or thousands of stars compared to the teeming hundreds of billions in the Milky Way. These small galaxies quietly reside in the sky amongst the majority of much brighter galaxies, with only the closest examples being readily discoverable.
Unfortunately, the influence of the Milky Way severely erodes astronomers’ ability to study these ultra-faint galaxies. The Milky Way projects strong gravitational forces and hot gases, altering the evolutions of neighboring faint galaxies. Additionally, due to their dimness, ultra-faint galaxies are too complex for traditional computer algorithms to detect and make sense of.
Where algorithms struggle with visual noise, the human eye remains supreme. Sand manually searched visual images taken for the DECam Legacy Survey, part of a series of surveys covering about a third of the sky. The surveys aimed to identify targets for the Dark Energy Spectroscopic Instrument. In his review, Sand discovered the unusual faint and ultra-faint galaxies.
“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.”
Faint Galaxies Remain Untouched
Their remote and isolated locations have left the sculptor galaxies unadulterated by powerful exterior influences. Sand’s team peered through the International Gemini Observatory’s Gemini South Telescope to get a closer look. The telescope’s Gemini Multi-Object Spectrograph observed all three galaxies, recording detailed data.
When Sand’s team analyzed the Gemini data, they found gas-less regions of space with ancient stars as their sole inhabitants. The findings buoyed earlier theories that ultra-faint dwarf galaxies experienced a cutoff in star formation 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 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,” Sand said.
Investigating Hypotheses
Intriguingly, the Sculptor galaxies align with a different earlier theory explaining ultra-faint galaxies. In the Epoch of Reionization theory, scientists proposed that high-energy ultraviolet protons filled the cosmos and boiled away entire galaxies’ gases shortly after the Big Bang.
The isolation of the Sculptor galaxies fits this explanation, as they are not close enough for a larger galaxy to have siphoned off their gases. Alternatively, some astronomers suspect that early stars in those faint galaxies may have ejected all the gas out of their galaxies during supernova explosions.
“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,” Sand explained.
Continuing On The Trail Of Faint Galaxies
With many possibilities swirling around, faint galaxies represent a crucial frontier in early universe research. Particularly of interest is the Epoch of Reionization, which has significant ramifications for understanding the modern structure of the universe.
To push their work ahead, Sand’s team identifies that astronomers need to locate more objects like the Sculptor galaxies and refine machine learning algorithms to resolve the dim, distant objects without the slow down of human intervention.
The paper “Three Quenched, Faint Dwarf Galaxies in the Direction of NGC 300: New Probes of Reionization and Internal Feedback” appeared on December 2, 2024 in The Astrophysical Journal Letters.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.