How did they detect such a faint signal?

The researchers hunted for H-alpha, a characteristic glow emitted by ionized hydrogen — hydrogen atoms stripped of their electrons by energy input. Around a single galaxy, this signal is far too weak to see clearly.

So the team stacked observations from hundreds of galaxies with active jets. They combined optical data from the Dark Energy Spectroscopic Instrument (DESI) survey with radio jet measurements from the LOFAR Two-meter Sky Survey (LoTSS), aligning the measurements along each jet's axis.

The result: when signals from all directions around the galaxies were averaged together, the H-alpha glow stayed weak. Along the radio jets, it became much stronger and clearly detectable. The jet doesn't act like a lamp illuminating everything equally; it works more like a concentrated beam that makes gas glow along its route.

The team found two especially bright regions:

• One close to the galaxy, where the jet first strikes the CGM.
• Another much farther out, near the CGM's outer edge, where the jet appears to dump much of its energy.

As a cross-check, the researchers examined a tracer of cooler gas using the absorption signature of magnesium. Unlike H-alpha, the magnesium signal was distributed evenly in all directions and showed no connection to jet direction. This suggests the cool gas already surrounds these galaxies uniformly, while the jet heats and ionizes gas only along its own trail.

Why does this matter for a galaxy's fate?

By heating, stirring, and disrupting gas throughout the CGM, the jets make it harder for that material to cool and fall back toward the galaxy. Without a steady inflow of cold gas, the galaxy has less fuel for new stars.

In this way, jets can act as a brake on galaxy growth. Over time, they may push a galaxy toward a quiet state with much lower star formation. "What excites me most is the scale of the connection," Roy said. "A black hole is incredibly small compared to a galaxy, but its impact can reach hundreds of thousands of light-years."

Why did earlier studies miss it?

Previous searches for this signal came up empty, and the new results suggest why. The H-alpha glow appears only when astronomers look specifically along the direction of the jets. If researchers assumed the CGM behaves the same in every direction — a common simplifying assumption — the effect would stay hidden.

The findings are preliminary in the sense that they describe a statistical pattern across many stacked galaxies rather than a single direct measurement, and the study does not prove that jets quench star formation on their own. Still, the work gives astronomers and theorists a concrete new way to test how black hole feedback shapes galaxy evolution.

Co-authors include Timothy Heckman of Johns Hopkins University and Tanmay Singh of Arizona State University. NASA, the Space Telescope Science Institute, and the NSF supported the work.