Galaxies such as the Milky Way expand by combining with smaller galaxies over vast timescales, a process not typically associated with dwarf galaxies. These smaller galaxies were once believed to be too lightweight to attract additional mass and grow similarly. However, fresh observations are challenging this assumption, indicating that dwarf galaxies are capable of gaining mass from other small galaxies.
Researchers led by Catherine Fielder from the University of Arizona have captured the most detailed images of a small galaxy and its surrounding area, uncovering characteristics usually linked to much larger galaxies. These observations offer an exceptional insight into the formation and evolution of small galaxies, suggesting that the processes driving their growth may be more widespread than previously considered.
Fielder shared these findings at the 245th meeting of the American Astronomical Society in National Harbor, Maryland, during a press briefing on January 16.
Galaxies, including our own Milky Way, evolve over billions of years through a method called hierarchical assembly, where they merge with smaller galaxies. This “building block” approach has been well documented in bigger galaxies, where streams of ancient stars—leftovers from consumed galaxies—trace their tumultuous histories. These streams, along with other faint traces of old, scattered stars, contribute to what is known as a stellar halo. This halo is a vast, low-density cloud of stars surrounding a galaxy’s luminous central disk and reflects its evolutionary past.
Traditionally, it was believed that smaller galaxies like the nearby Large Magellanic Cloud have fewer chances to gain mass and merge with other smaller systems, such as dwarf galaxies, due to their lower gravitational strength. Clarifying how such galaxies accrue mass in line with hierarchical assembly continues to be a puzzle.
Using the Dark Energy Camera (DECam) attached to the 4-meter Blanco Telescope at Chile’s Cerro Tololo Inter-American Observatory, the researchers conducted an in-depth imaging survey of 11 dwarf galaxies. This included the spiral galaxy NGC 300, which has a mass comparable to that of the Large Magellanic Cloud. Through the DECam Local Volume Survey, or DELVE, they captured unprecedented details of NGC 300’s features. Spanning approximately 94,000 light-years, NGC 300’s disk is slightly smaller than the Milky Way and contains merely around 2% of its stellar mass.
“NGC 300 is an excellent candidate for this study because it’s in an isolated position,” stated Fielder, a research associate at the U of A Steward Observatory. “This isolation allows it to avoid the influences of massive companions like the Milky Way, which affect nearby dwarf galaxies like the Large Magellanic Cloud. It’s almost like examining a cosmic ‘fossil record.'”
The team crafted stellar maps around the small galaxy and identified an extensive stellar stream stretching more than 100,000 light-years from its center.
“We view a stellar stream as strong evidence that a galaxy has drawn mass from its environment, as these formations do not easily form through internal processes,” remarked Fielder, whose results will be published in The Astrophysical Journal.
Additionally, the researchers observed star arrangements in shell-like formations resembling concentric waves spreading outward from the core of the galaxy, alongside signs of a stream wrap. This indicates that whatever formed the stream may have altered its course in orbiting NGC 300.
“Initially, we were uncertain if we would uncover anything in these small galaxies,” she noted. “However, the features surrounding NGC 300 provide us with compelling evidence that it has indeed accreted something.”
The team also discovered a previously unidentified, metal-poor globular star cluster in the galaxy’s halo, serving as another piece of evidence for past accretion events.
Astronomers often assess the ages of stellar populations by examining a characteristic known as “metallicity,” which pertains to the chemical elements found in stars. Heavier elements are primarily created in larger stars nearing the ends of their life cycles, necessitating several generations of star formation for their accumulation. Consequently, star groups that lack heavier elements—characterized by low metallicity—are deemed older, as explained by Fielder.
“The stars we detected around NGC 300 are ancient and metal-poor, suggesting a clear narrative,” said Fielder. “These structures likely originated from a small galaxy that was disrupted and absorbed into NGC 300.”
Collectively, these discoveries illustrate that even dwarf galaxies can form stellar halos through the accretion of smaller galaxies, mirroring the growth patterns seen in larger galaxies, according to Fielder.
“NGC 300 now represents one of the most remarkable examples of stellar halo assembly driven by accretion in a dwarf galaxy, providing insight into how galaxies develop and evolve throughout the universe.”
