Some of the universe’s largest galaxies have gone strangely quiet.
They stopped forming new stars billions of years ago and have remained dormant ever since—even though many still contain gas that could, in theory, spark new stellar life.
Astronomers have long struggled to understand how these galaxies manage to stay so inactive for so long.
A new study focusing on a rare type of galaxy called “red geysers” offers an important clue.
The research suggests that slow, steady streams of cool gas may be feeding the galaxies’ central supermassive black holes, helping them gently regulate their surroundings and keep star formation switched off.
Red geysers make up only about 6–8% of nearby quiescent galaxies, but they stand out because of their unusual gas behavior.
First identified using data from the Sloan Digital Sky Survey’s Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) program, these galaxies show faint, extended streams of ionized gas flowing outward across tens of thousands of light-years.
These subtle outflows resemble gentle fountains rather than violent eruptions, earning them the nickname “red geysers.”
The new study was led by Arian Moghni, an undergraduate astrophysics researcher at the University of California, Santa Cruz.
The work is currently under review at The Astrophysical Journal and was presented at the annual meeting of the American Astronomical Society.
While previous research linked red geysers to low-level activity from supermassive black holes, a key question remained unanswered: where does the black hole’s fuel come from? To investigate, the team studied the movement of cool, neutral gas inside 140 red geyser galaxies observed by the MaNGA survey.
The researchers focused on a specific spectral signal known as the sodium D absorption line, which traces cool gas at temperatures of roughly 100 to 1,000 degrees above absolute zero.
By mapping this signal across entire galaxies, the team could measure how the gas was moving.
They found that, rather than swirling chaotically or falling rapidly inward, most of the cool gas drifts slowly toward the galaxy’s center at around 47 kilometers per second. That is only about 10% of the speed expected if the gas were freely falling under gravity. Even more striking, the gas moves in an unusually orderly way, with far less random motion than the surrounding stars.
“This suggests the gas is flowing smoothly and coherently, not being violently stirred,” Moghni explained.
The study also uncovered a strong link between gas inflow and black hole activity. About 30% of the galaxies emit radio waves, a sign that their central black holes are active. In these galaxies, the amount of inflowing cool gas is significantly larger—covering about one-third more area than in red geysers without radio emission.
Galaxy interactions appear to amplify this effect. Red geysers that show signs of nearby companions or past minor mergers contain much larger reservoirs of infalling gas—on average about 2.5 times more than isolated systems. These interactions likely deliver fresh gas that slowly funnels toward the center, feeding the black hole.
Together, the findings support a self-regulating cycle. Interactions supply cool gas, which drifts inward and fuels gentle black hole activity. That activity then produces subtle feedback that prevents new stars from forming, allowing massive galaxies to remain quiet for billions of years.
By directly tracing cool gas across whole galaxies, the study provides rare, detailed evidence of how some of the universe’s biggest galaxies manage to stay silent—and why, sometimes, doing nothing requires just the right balance.
Source: UC Santa Cruz.
