RRI Study Shows Black Hole Jets Choke Star Birth in Galaxies

Astronomers from the Raman Research Institute (RRI), an autonomous institute of the Department of Science and Technology, working with colleagues at Arizona State University, have traced one way in which the black hole at a galaxy's core can decide whether that galaxy keeps making stars. Their paper has appeared in the journal The Astrophysical Journal Letters.

The gas beyond the galaxy

Past the visible disc of stars and dust, every galaxy is wrapped in a thin, faint envelope of gas that stretches 10 to 20 times the width of the galaxy itself. Astronomers call it the circumgalactic medium, or CGM, and it is the fuel store from which new stars are eventually built. If all of that gas collapsed into stars, the sky would be full of blindingly bright galaxies, and it is not. For over a decade the suspicion has been that the supermassive black hole sitting at the centre keeps the gas too hot to cool and collapse, but nobody could say how.

A jet that punches

A black hole is a swallower of matter, but it also throws out jets of searing plasma, a soup of charged particles. The team asked whether such a jet reaches all the way into the outer gas. Ionised gas glows in a particular way, so they looked for that glow. Averaged over every direction around the galaxies they found nothing at all. Along the line of the jet, the signal was strong. The jet, in other words, lights up only the gas lying in its path and leaves the rest alone.

Two places where the punch lands

The signal turned out to be strongest at two spots, at the rim of the stellar disc where the jet first strikes the outer gas, and again at the far boundary of that envelope, where the slowed jet shoves against the gas a second time. Energy is dumped at both, the gas is stirred and warmed, and cool gas that might otherwise have clumped into stars never gets the chance. The galaxy grows quiet and passive.

Who did the work

The lead author is Namrata Roy, assistant professor in the astronomy and astrophysics division at RRI. A co-author, Sanchayeeta Borthakur, associate professor at Arizona State University, summed up the finding by saying black holes can shape the lives of galaxies far beyond the small central region they occupy. To see the scale of it, an average supermassive black hole may be about as wide as the solar system, while the galaxy around it can hold 100 billion such systems.