Stacking the spectra of 324 pairs of radio galaxies and background quasars, a team led from Arizona State University has found ionized hydrogen glowing along the path of supermassive black hole jets, in gas that sits far outside the visible edge of each galaxy. The release describing the work says that by keeping this gas hot and preventing it from falling inward, the jets “may help determine whether a galaxy keeps forming stars or becomes quiet,” and that is the only claim about star formation it makes. The rest is measurement.
The wording is conditional because the measurement is statistical. The glow is the evidence; whether it shuts off star formation for good is the open part.
324 radio galaxy and quasar pairs stacked for H-alpha
The work comes from Sanchayeeta Borthakur of Arizona State University and Namrata Roy, now at the Raman Research Institute in India, with Timothy Heckman of Johns Hopkins University and Tanmay Singh of ASU. The paper, “Lighting Up the CGM: Strong, Jet-Aligned Hα Emission around Radio Galaxies,” appears in The Astrophysical Journal Letters, volume 1009, issue 2, L34, and used optical spectra from the Dark Energy Spectroscopic Instrument together with radio maps from the LOFAR Two-metre Sky Survey.
No single galaxy shows the signal. It is too faint, so the team lined up spectra of background quasars whose sight lines pass near radio galaxies and averaged them. According to the arXiv version, the final catalog holds 324 radio galaxy and quasar pairs, with projected separations of roughly 20 to 800 kiloparsecs, which puts many sight lines well beyond the galaxy itself and deep into the circumgalactic medium, the diffuse gas halo around a galaxy.
Jet-aligned Hα glow and its reach
The central result is directional. Along the jet axis, within about 20 degrees, the paper reports a mean integrated Hα flux of 1.19 × 10-17 erg per square centimetre per second, a signal it describes as roughly 100 times brighter than in normal halos. Averaged over all directions around the galaxies, the detection vanishes. The emission peaks twice: near the host galaxy, and again near the projected radio lobes where the jets meet the outer halo.
The jets “can disturb star-forming gas far beyond a galaxy’s visible edge, leaving a glowing trail hundreds of thousands of light-years long,” the release says. A statement from India’s Department of Science and Technology, whose Raman Research Institute co-led the work, quotes Roy: “The jet was illuminating only the gas in its path, rather than affecting the gas equally in all directions.”
Magnesium II absorption, which traces cool gas whether or not it is lit up, shows no directional preference. That is a useful limit: The Brighter Side of News reads it as meaning the jets do not create extra cool gas but “compress, heat or ionize clouds already distributed through the circumgalactic medium.”
From hot halo gas to a quiet galaxy: the measured link and the missing one
The proposed chain runs in steps, and each one is a place where the argument could break. Jets heat, stir and disrupt halo gas, so it cools less readily and falls back toward the galaxy more slowly. With less incoming cold gas, the galaxy has less fuel for new stars. The release states that “without that incoming supply of cold gas, the galaxy has less fuel available for creating new stars.”
Only the first link in that chain, the jets lighting up and disturbing the gas, is measured here. The Brighter Side of News quotes the paper as offering “statistical evidence that jets physically influence cool halo gas” while it “does not prove that the observed jets permanently shut down star formation.” Star formation rates in the sample galaxies are not the measured quantity.
Scale matters for the word “whole” in any description of the effect. The circumgalactic medium extends 10 to 20 times a galaxy’s size, according to the Department of Science and Technology statement, so a jet that disturbs gas there is acting on the reservoir that feeds the entire galaxy and not only on its centre. That reach is why the authors frame the result as relevant to galaxy growth in general, even though the sample covers radio galaxies with strong jets and not galaxies at large.
Borthakur is less restrained in the release, calling the result “pathbreaking” and saying it solves the long-standing mystery of how black holes influence galaxies. The same coverage that carries her quote also carries the paper’s limit, and the conditional wording is the one the paper and release use for the star formation claim itself.
What would settle the question is a direct tie between jet-aligned halo gas and the star formation histories of the same galaxies, which the 324-pair sample was not built to provide. Until that link is measured, the jets are a candidate regulator of galaxy growth with an observed footprint of hundreds of thousands of light-years, and not a proven switch.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
More from Morning Overview
- Comcast is shutting down its email service, and Xfinity customers must act to keep it
- Invisible volcanic gas at a California ski resort has killed three ski patrollers
- Verizon just moved to cut landline phone service in 9 states within weeks
- Tropical Storm Rachel is dumping up to 12 inches on four Mexican states on its way to major hurricane strength
