Astronomers using the Cerro Tololo Inter-American Observatory (CTIO) have made groundbreaking discoveries regarding the dwarf spiral galaxy NGC 300. A team of international researchers has reported the detection of a stellar stream, shells, and a globular cluster in the galaxy’s halo. These findings were published on January 7, 2025, on the pre-print server arXiv, shedding light on the complex processes that contribute to the formation and evolution of dwarf galaxies.
NGC 300: A Dwarf Galaxy in the Sculptor Constellation
NGC 300, also known as Caldwell 70 or the Sculptor Pinwheel galaxy, is located about six million light-years away from Earth in the constellation Sculptor. It is a dwarf spiral galaxy with an approximate size of 110,000 light-years and a mass of about 2.6 billion solar masses. NGC 300 has long been a subject of interest for astronomers due to its relatively isolated position in space, making it an ideal target for studying stellar halos and other galactic structures without the influence of nearby massive galaxies.
The galaxy’s disk is nearly face-on, which gives it an unobstructed view of its features. It also has a nearly perfect exponential profile, making it a valuable target for astronomical surveys. These attributes make NGC 300 a particularly well-suited object for studying the processes of galaxy formation and evolution, including the accretion of smaller galaxies and the dynamics of stellar populations.
The DECam Local Volume Exploration Survey (DELVE)
One of the key programs responsible for uncovering these new findings is the DECam Local Volume Exploration survey (DELVE). DELVE is a long-term observational project that aims to capture deep optical images of galaxies in the nearby universe, particularly those that are isolated or relatively undisturbed. This allows astronomers to study the formation of stellar substructures, such as stellar streams and globular clusters, which provide insight into the hierarchical assembly of dwarf galaxies.
The specific sub-component of DELVE responsible for the NGC 300 observations is DELVE-DEEP. This part of the survey focuses on imaging analogs of the Large and Small Magellanic Clouds, which are dwarf galaxies known for their stellar substructures. DELVE-DEEP aims to identify stellar features in these galaxies, thereby revealing the accretion history and assembly mechanisms of dwarf galaxies.
Observations of NGC 300 using the Dark Energy Camera (DECam) on the Blanco 4-meter telescope at CTIO were conducted over 21 nights between July 2021 and July 2023. The team acquired a total of 14 distinct fields, which included exposures in both g-band and i-band filters. These observations were supplemented by data from the Dark Energy Survey (DES) to reach the desired depth and resolution. The resulting data provided a high-quality view of the galaxy’s outer regions, revealing new structures in the galaxy’s halo.
Discovery of a Stellar Stream: Stream N
One of the most significant discoveries from this study is the detection of a large, low surface brightness stellar stream extending over 130,000 light-years from the center of NGC 300. This feature, named Stream N, is composed of red giant branch (RGB) stars, which are older stars that have evolved from the main sequence. Notably, the stars in Stream N are more metal-poor than those in the galaxy’s inner regions and disk.
The metal-poor nature of the stars in Stream N suggests that this stream may have originated from an accretion event, possibly the merger or interaction of NGC 300 with a smaller galaxy. Such stellar streams are often the result of tidal disruption, where the gravitational forces of a more massive galaxy pull apart a smaller, infalling galaxy, scattering its stars into a long, thin structure. Stream N provides valuable evidence of such processes in the formation of dwarf galaxies.
The Smaller Stream S and its Connection to Stream N
In addition to Stream N, the astronomers detected another radial protrusion on the opposite side of the galaxy, which they have named Stream S. Like Stream N, Stream S is also metal-poor, and the researchers propose that it may be connected to Stream N as a potential wraparound of the larger stream. The hypothesis is that the two streams are related to the same accretion event, but further observations and detailed modeling will be required to confirm this theory. The discovery of Stream S provides additional clues about the dynamics of galaxy mergers and the formation of stellar substructures in dwarf galaxies.
Shell-like Structures: Evidence of Accretion or In-Situ Formation?
Another intriguing aspect of the study is the detection of two shell-like structures along the western and southern sides of NGC 300. These shells appear to be metal-poor, similar to the stellar streams, and the researchers suggest that they may be connected to an accretion event, possibly the remnant signatures of an ancient merger or interaction with a smaller galaxy.
However, the possibility that these shells formed in situ, from the galaxy’s own internal processes, cannot be ruled out. In situ formation refers to the idea that the shells could have originated from star formation processes within NGC 300 itself, rather than from external accretion. This remains an open question, and further studies will be needed to determine the precise origins of these features.
Discovery of a Globular Cluster: NGC 300-GCF25
The observations also led to the discovery of a globular cluster in the halo of NGC 300. This cluster, designated NGC 300-GCF25, is notable for being metal-poor, consistent with the other stellar features discovered in the galaxy’s halo. The globular cluster is estimated to be around 10 billion years old, suggesting it formed early in the history of NGC 300. It is located between the two shell-like structures in radial projection, which further supports the idea that the galaxy’s halo contains remnants of ancient accretion events.
Globular clusters are dense collections of stars that are typically very old and reside in the halos of galaxies. Their presence in NGC 300 provides additional evidence for the galaxy’s complex formation history, which likely involved multiple accretion events over the course of billions of years. The discovery of NGC 300-GCF25 adds another layer of understanding to the galaxy’s stellar population and its evolutionary history.
Implications for Galaxy Evolution
The findings from this study offer valuable insights into the processes that shape dwarf galaxies like NGC 300. The detection of stellar streams, shell-like structures, and a globular cluster suggests that NGC 300 has experienced multiple accretion events, which have contributed to its current stellar population and structure. These discoveries provide further evidence for the hierarchical assembly of galaxies, in which smaller galaxies merge and interact over time to form larger systems.
Dwarf galaxies like NGC 300 are considered important laboratories for studying galaxy formation, as they are relatively simple systems that can provide clues about the early stages of galaxy evolution. The isolated nature of NGC 300 makes it an ideal object for studying the effects of accretion without the influence of a nearby massive companion, such as the Milky Way. The stellar streams and other features observed in NGC 300 offer a glimpse into the complex processes that govern the formation and evolution of these galaxies.
Conclusion
The deep optical observations of NGC 300 have provided astronomers with a wealth of new information about the galaxy’s stellar halo. The detection of Stream N, Stream S, shell-like structures, and the globular cluster NGC 300-GCF25 adds to our understanding of how dwarf galaxies evolve and grow over time. These findings highlight the importance of surveys like DELVE in unraveling the complex history of galaxy formation and the processes that shape the universe’s smallest galaxies.
As astronomers continue to study NGC 300 and other galaxies in the nearby universe, we can expect further discoveries that will enhance our knowledge of how galaxies form, interact, and evolve across cosmic timescales. The study of stellar streams, shells, and globular clusters provides crucial insights into the assembly history of galaxies and the dynamic processes that govern their evolution.
Reference: Catherine E. Fielder et al, Streams, Shells, and Substructures in the Accretion-Built Stellar Halo of NGC 300, arXiv (2025). DOI: 10.48550/arxiv.2501.04089