Black hole jets light up gas hundreds of thousands of light-years from home
Key takeaways
- Jets from supermassive black holes make gas glow along their path deep into the circumgalactic medium, hundreds of thousands of light-years out
- The signal only appeared when astronomers stacked hundreds of galaxies and measured along the jet direction
- Heated, stirred-up gas struggles to cool and fall inward, which can slow a galaxy's star formation
A supermassive black hole's jet can leave a trail of glowing gas hundreds of thousands of light-years long, well past the visible edge of its galaxy. The finding comes from a study led by Namrata Roy of the Raman Research Institute and Sanchayeeta Borthakur of Arizona State University, published in The Astrophysical Journal Letters.
The result adds a strong clue to how black hole jets shape galaxy evolution, and to why galaxies surrounded by plenty of gas often stop making stars.
The gas reservoir around every big galaxy
Large galaxies, the Milky Way included, sit inside a vast halo of gas called the circumgalactic medium, or CGM. It reaches roughly 10 to 20 times farther than the visible galaxy, Arizona State University explains. Some of that gas cools, falls inward and eventually forms stars. The long-running puzzle is that galaxies make far fewer stars than all that fuel suggests they should.
How black hole jets change galaxy evolution
The team combined optical data from the Dark Energy Spectroscopic Instrument (DESI) with radio jet measurements from the LOFAR Two-metre Sky Survey. The glow they wanted, H-alpha light from ionised hydrogen, is too faint to pick out around a single galaxy, so they stacked hundreds of jet-hosting galaxies together.
Averaged in every direction, the signal stayed weak. Measured along the jets, it became much stronger. Very roughly, the jet behaves less like a lamp and more like a torch beam, lighting up gas along its path. The researchers found two bright zones, one near the galaxy where the jet first meets the CGM, and one near the outer edge where it appears to dump much of its energy.
Gas that gets heated and stirred like this struggles to cool and sink inward, which acts as a brake on star formation. As a check, cooler gas traced by magnesium absorption showed no link to the jet direction and sat around the galaxies more evenly.
Why earlier searches missed it
Previous studies looked for this signal and came up empty. Direction seems to be the reason: if you assume the CGM behaves the same on every side, the jet's effect averages away into noise.
The scale is the striking part. A supermassive black hole is roughly comparable in size to our solar system, while its host galaxy may hold around 100 billion solar systems. Stacking surveys like this one will only get richer as wide-field instruments such as NASA's Roman Space Telescope add more galaxies to the pile. Roman's launch this year was built around exactly that kind of wide, statistical sky survey.
The next test sits with theorists. Simulations of galaxy growth already include black hole feedback, and now they have to reproduce a glow that only shows up when you look straight down the jet.
