Born to Be Starless: Why Many Mini-Galaxies Never Shine
Simulations of the universe based on our best theory of dark matter—called Lambda Cold Dark Matter (ΛCDM)—predict that galaxies like the Milky Way should be surrounded by many small “subhalos” that could host tiny galaxies. But for years, astronomers have noticed something strange: we don’t see nearly as many of these satellite galaxies as the simulations suggest should be out there. This mismatch is known as the “missing satellite problem.” In this paper, Seyoung Jeon and collaborators use advanced computer simulations to investigate why so many of these small structures remain invisible, or “starless.”
How the Simulations Work
The authors use two state-of-the-art simulations, NewHorizon and NewHorizon2, which model both dark matter and the normal matter (called baryons) that forms stars and galaxies. These simulations focus on a small region of the universe with high detail—down to tens of parsecs (a tiny fraction of a light-year). The simulations also include realistic physics, like the effects of ultraviolet (UV) radiation and explosions from supernovae. By identifying Milky Way-like galaxies in these simulations, the researchers examine the subhalos around them to determine which ones form stars and which remain dark.
Starless vs. Starred: The Two Types of Subhalos
The team finds that most subhalos around galaxies like the Milky Way don’t contain stars at all—they’re “starless.” To understand why, they classify subhalos into two categories: those that contain galaxies (“starred”) and those that don’t. Interestingly, even subhalos with similar masses can fall into either group. This rules out simple explanations like “only big subhalos form stars.” Instead, the researchers dig deeper into the life stories of these objects to find out what’s really going on.
What Doesn’t Explain the Starlessness
Two ideas often used to explain why subhalos don’t form stars are supernova feedback (where star explosions blow gas away) and environmental effects (like being stripped of gas while orbiting a bigger galaxy). However, the paper shows that these effects are not enough to turn a once-star-forming subhalo into a dark one. Most starless subhalos never had any stars to begin with, meaning these processes didn’t even have a chance to act. Even when some gas is removed, the existing stars remain behind, which means these processes cannot fully erase a galaxy’s signature.
Reionization: The Turning Point
The real game-changer is something that happened in the early universe: reionization. About 13 billion years ago, the first stars and galaxies emitted powerful UV radiation that heated up the gas in the universe. Jeon and colleagues find that whether or not a subhalo forms stars depends heavily on whether it could gather enough gas to “self-shield” against this heating. Starless subhalos tend to be born in less dense regions and grow more slowly. When reionization hits, their gas gets heated and can’t cool down to form stars. In contrast, starred subhalos managed to pull in enough dense gas early on to resist the UV onslaught and form stars.
Born This Way
In the end, the paper argues that starless subhalos aren’t failed galaxies—they were never going to be galaxies in the first place. Their location in the cosmic web and slower growth meant they couldn’t form dense, cool gas in time. By the time reionization occurred, it was too late for them to catch up. They were, quite literally, born to be starless.
Final Thoughts
This study offers a powerful new perspective on the missing satellite problem by showing that many subhalos never had the right conditions to shine. It confirms that the combination of cosmic reionization and initial environmental conditions—not just mass—is key to understanding which subhalos end up visible. The work highlights the need for even higher-resolution simulations and more detailed models of early universe physics to fully capture the fate of these tiny structures.
Source: Jeon
