Astronomers at Arizona State University and the Raman Research Institute have discovered that narrow plasma jets from supermassive black holes illuminate hydrogen gas along their paths across the circumgalactic medium, a finding published in The Astrophysical Journal Letters that helps explain how these compact centers regulate galaxy evolution and star formation.
The Enigma of Circumgalactic Gas and Star Formation
Galaxies are massive systems holding hundreds of billions of stars, yet astronomers have long puzzled over why they do not form even more stars given the vast fuel reserves surrounding them. Every large galaxy, including the Milky Way, is wrapped in a huge envelope of gas known as the circumgalactic medium, or CGM. This reservoir stretches 10 to 20 times the size of the visible galaxy and provides the raw material needed to fuel star formation.
If all the gas in the CGM cooled and collapsed inward, galaxies would contain far more stars than astronomers actually observe. For over a decade, researchers have proposed that energy released by supermassive black holes at galaxy centers could prevent that gas from cooling too quickly, though the exact physical mechanism remained under study.
Tracing Ionized Hydrogen Along Radio Jets
To investigate how a relatively tiny black hole can influence an enormous host galaxy, a team led by researchers from Arizona State University and the Raman Research Institute looked at active black holes that shoot out narrow streams of hot, fast-moving plasma. These active black holes are roughly the size of our solar system, while their host galaxies can hold about 100 billion such solar systems.
Because the ionized hydrogen glow in the CGM is extremely faint around any single galaxy, the research team combined observations from hundreds of galaxies. They pulled data from the Dark Energy Spectroscopic Instrument survey and radio jet measurements from the LOFAR Two-meter Sky Survey, searching for a telltale sign of ionized hydrogen known as H-alpha along the jet axes.
When averaged across all directions around the galaxies, the signal appeared weak. However, when the team examined the regions aligned directly with the radio jets, a clear and strong signal emerged.
Energy Distribution Across the Galaxy Envelope
The data revealed that the jet acts less like a lamp shining evenly in all directions and more like a powerful beam carving through the surrounding environment. The team found that the ionized hydrogen glow reaches peak intensity in two distinct zones: close to the stellar disk where the jet first hits the circumgalactic medium, and near the outer boundary of the CGM where the jet slows down and releases most of its energy.
This directional transfer of energy heats and disturbs the gas, suppressing its ability to cool, clump, and form new stars. The research demonstrates how energetic outbursts from a compact central engine can alter the evolutionary path of a galaxy, pushing active systems toward more quiescent states.