Galaxies are cosmic islands, bustling with stars, planets, and black holes, representing the hubs of cosmic activity. However, the universe is not solely composed of galaxies. Vast stretches of interstellar space, often described as galactic deserts, lie between them, sparsely populated with hydrogen atoms or solitary rogue planets. This stark contrast naturally leads to a fundamental question in astronomy: where exactly does a galaxy end, and interstellar space begin?
The demarcation line isn’t as clear-cut as a national border. Astronomers have proposed various plausible methods to Define Where a galaxy’s ‘edge’ resides. These definitions range from the point where gas and dust concentrations diminish below a critical level to the extent of a galaxy’s gravitational influence. Understanding these boundaries is crucial for comprehending the structure and evolution of the cosmos.
How Brightness Helps Define Galaxy Edges
Nushkia Chamba, a postdoctoral fellow at NASA’s Ames Research Center, specializes in the study of galactic outskirts, as well as galaxy formation and evolution. She offers insights into how astronomers have traditionally defined where a galaxy’s edge is located and the factors determining this boundary.
“Traditionally, the ‘boundary’ of a galaxy has been defined using fixed levels of brightness, technically known as surface brightness isophotes,” Chamba explains. This method relies on the idea that as you move further from the galactic center, the light emitted by stars and gas gradually fades. The edge was often considered to be where this brightness dips below a certain threshold.
However, Chamba has been at the forefront of efforts to refine this definition. “Our work in 2020 and 2022 introduced an alternative concept of the ‘edge of a galaxy’ as a definition of size,” she states, advocating for a more physically grounded approach to define where a galaxy truly ends.
Star Formation: A New Way to Define Galactic Boundaries
Chamba and her team have pioneered a definition based on the physical processes within galaxies, specifically star formation. They propose that the boundary of a galaxy should be defined where the gas density is sufficient for star formation to occur. In essence, the outermost regions where stars are still being born, or are likely to form, mark the galaxy’s edge according to this new criterion.
This concept reframes the galactic edge as a dynamic and evolving entity, much like the expanding periphery of a city where new developments are continuously taking place. This analogy highlights that defining where a galaxy ends is not about drawing a static line, but rather understanding an active, ongoing process.
Read More: How Many Galaxies Are There? Astronomers Are Revealing the Enormity of the Universe
Environmental Factors in Defining Galaxy Size
Since star formation is key to define where a galaxy ends, astronomers are keenly interested in how a galaxy’s environment influences its size and its capacity for star formation in its outer reaches. The presence of neighboring galaxies can significantly impact these boundaries.
“We are working to understand how galaxies regulate their size throughout their existence and how their environment affects their growth. This depends on whether they are situated in a dense cluster with thousands of galaxies or are relatively isolated,” Chamba elaborates.
This year, Chamba and her team discovered that galaxies in crowded environments can be up to 50 percent smaller than their isolated counterparts. This finding supports the current understanding of how environmental interactions strip gas from galaxies in clusters, hindering star formation and growth compared to isolated galaxies that retain their gas for longer durations. The environment, therefore, plays a crucial role in defining where a galaxy’s star-forming edge lies.
“Numerous factors influence a galaxy’s edge: its environment, morphology (shape and size), and stellar mass. We are still investigating the impact of dark matter on star formation, which remains an open question in astrophysics. Furthermore, the epoch when most of a galaxy’s star formation occurred is also important, as the edge of a galaxy evolves not only due to its structure and environment but also over time,” Chamba clarifies.
Read More: Spiral Arms in Space Explain How Star Systems Form
Dwarf Galaxies: Environmentally Vulnerable
Dwarf galaxies are particularly susceptible to environmental influences. As they traverse crowded galactic neighborhoods, they are more prone to losing gas and dust, materials essential for star formation. This loss directly impacts their ability to sustain star formation and, consequently, define where their edges are located.
Recent research by Chamba and her colleagues has also highlighted the role of supernovae in gas dispersion within galaxies. ‘Stellar feedback’ from these powerful explosions can regulate gas inflows and outflows, either promoting or inhibiting star formation. Understanding these feedback mechanisms is vital for accurately defining where star formation ceases and the galaxy truly ends.
The advancement of deep imaging techniques, utilizing telescopes like the James Webb Space Telescope, has revolutionized astronomers’ ability to observe faint gas concentrations in distant galaxies. This capability is crucial for mapping star formation and precisely defining where the boundaries of these galaxies exist.
Read More: How Galaxies Live, Breathe and Die
Future Directions in Galaxy Edge Research
Chamba indicates that ongoing research is exploring the influence of enigmatic components like dark matter on star formation and, consequently, on defining where galaxy edges are situated. The prevailing cosmological model suggests that the size of galaxies – their total mass – is determined by the properties of dark matter halos. These halos exert gravitational influence on baryonic matter, the ordinary matter that constitutes stars and planets.
The factors driving galaxy evolution over cosmic time remain largely mysterious, and Chamba’s work is at the forefront of unraveling these complexities. Precisely defining where a galaxy ends is not merely a matter of spatial measurement, but a key to understanding the intricate processes shaping the universe.
Read More: Black Hole Theory Finally Explains How Galaxies Form
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Conor Feehly is a science writer based in New Zealand covering diverse topics, including astronomy and neuroscience, with a focus on the intersection of science and philosophy. He holds a master’s degree in science communication from the University of Otago and regularly contributes to Discover Magazine, as well as New Scientist, Nautilus Magazine, Live Science, and New Humanist, among others.
