Black Hole Jets Light Up Gas Far Beyond a Galaxy’s Stars, Study Finds

Jets blasted out by supermassive black holes appear to warm the huge halo of gas around their galaxies, and that may help explain why less of that gas falls inward to make new stars. The finding comes from a new study in The Astrophysical Journal Letters, as described by Universe Today.

The result adds to a long-running question in astronomy: how can a black hole, tiny compared with its home galaxy, shape how that galaxy grows? Here is what the study found and what it does and does not show.

What did the researchers look at?

Most research on black holes and galaxies focuses on the crowded central region, where the black hole sits along with most of the stars, gas and dust. This study, with Namrata Roy listed as first author, looked at the outer region instead.

Galaxies are surrounded by a large region of gas called the circumgalactic medium, or CGM. Over time, this gas cools and drops inward, supplying raw material for new stars. Yet surveys of the CGM find that much of it stays at the galaxy's outer edge, according to Universe Today. The new work asks whether the central black hole is the reason.

How did they test it?

The team examined regions of H-alpha emission around hundreds of galaxies with active jets. H-alpha light comes from ionized hydrogen, so seeing it tells astronomers the gas is warm enough to be ionized. The researchers used data from two sky surveys: the Dark Energy Spectroscopic Instrument (DESI) survey and the LOFAR Two-meter Sky Survey (LoTSS).

What did they find?

As expected, they found H-alpha emission close to the jets. They also found that light from the jets ionizes gas farther away. Universe Today reports that this means the jets heat the CGM more than scientists had thought.

The team also looked at an absorption line of magnesium, which pointed to a lower overall temperature within the CGM. According to Universe Today, the authors conclude from this that the heating is driven by the jets themselves.

The finding runs against an earlier assumption. Jets carry material to the outer edge of a galaxy only along the black hole's polar axis, so it had been thought that much of the surrounding gas would be untouched. The study describes a subtler interaction that reaches beyond the jets' direct path.

Why does it matter?

This is basic research on how galaxies evolve, with no direct effect on daily life. Its importance is in what it says about how galaxies, including large ones, regulate their own growth.

Universe Today summarizes the idea as jets stirring the pot. Heat passes into the circumgalactic gas, which reduces the amount that falls toward the galaxy. With less material available, fewer stars can form. In that sense a black hole can discourage star formation far from itself, even though black holes are also dangerous to stars that wander too close.

The scale difference is what makes this notable. Supermassive black holes can range from millions to billions of solar masses, but they are confined to a region about the size of a stellar system. Their host galaxies can span more than 100,000 light-years and can exceed 100 billion solar masses.

How far does the result go?

The study does not claim jets are the only influence on star formation. Universe Today notes that other factors also drive star production, and the new work shows that some of the interactions between black holes and their surroundings are subtle rather than dramatic.

The research is observational, based on survey data from many galaxies rather than a close-up look at any single system. It was published in a peer-reviewed journal, as a letter, a format typically used for shorter reports of new results.

Around Prague, based on reporting by Universe Today