How supermassive black holes halt star formation

It’s the jets that’ll get you, not the gravity. Supermassive black holes may be known for their ability to pull in and consume tremendous amounts of matter, but often their biggest effects are a result of the matter they don’t consume. A lot of material is lost in the course of tearing apart entire stars and devouring their remains. This material is gathered up by immense magnetic fields and blasted out into the galaxy. The resulting jets tear through the galaxy, halting the formation of new stars and eventually making it to intergalactic space.

Almost all galaxies host a supermassive black hole in their core. The black hole is dormant for most of the galaxy’s life, but occasionally it awakens. An interaction with another galaxy – a merger or a near miss when another galaxy gets too close – can trigger a wave of star formation. The galaxy lights up with the intense blue light of young stars, becoming a starburst galaxy. The gravitational to-and-fro also sends gas, dust, and stars crashing towards the supermassive black hole. Not long after that, the jets will come.

The infalling material forms a flat disk, called an accretion disk, as it spirals towards the black hole. The disk is a swirling vortex of highly energetic electrically charged particles, pulled in and whipped up by the black hole’s gravity. The rotating electrical charges create powerful magnetic fields. These fields are twisted by the rotation and drawn away from the disk, carrying matter in a pair of tightly packed jets of material, moving at close to the speed of light, out into the galaxy.

We mainly think of galaxies as made up of stars, which make up most of the mass, but the stars themselves only take up a small amount of a galaxy’s volume. The space between the stars is filled with gas. You have probably seen stunning photos of colourful nebulae. These nebulae are part of the interstellar medium, which includes the molecular clouds from which stars form. To create stars, the molecular clouds have to be cold enough to allow them to collapse. The black hole’s jets deposit enough energy into the interstellar gas to heat it. That prevents the gas from collapsing, robbing the galaxy of the stars yet to be formed. We say star formation has been quenched.

The starburst phase is over. The jets, meanwhile, continue into intergalactic space.

Now free of the galaxy, the jets are greatly diminished from their glory days in the galactic centre. They’re no longer as focused and no longer as energetic. Their material meets a new, sparser gas. The intergalactic medium is even thinner than the interstellar gas and barely affects the jets, but eventually they will dissipate completely. By that time, they will have travelled hundreds of thousands of light years.

As the jets sweep into the intergalactic medium, it is excited to higher energy levels from which it will take billions of years to come down. The excited medium crackles with radio emissions as it radiates the excess energy. These radio emissions trace out the jets’ path and the expanding shock front they create. This eventually forms two giant lobes of radio emission, vastly bigger than the black hole which created them. Bigger, in fact, than the entire galaxy.

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Solar System

• NASA has published this year’s edition of Spinoff, their annual magazine showcasing all the ways NASA technology has been reused for new things. Refer to this next time someone questions if it’s worth spending money on space research. [NASA]

• A meteorite discovered in Africa in 2023 may be the remains of a long-dead planet, because of the unique mix of minerals it contains. [New Scientist]

• The Trump administration’s all-out assault on science has come for NASA, with mass firings and the potential for large budget cuts. [Nature, Space.com, Ars Technica]

• A new map of the asteroid belt has been produced, by tracking the paths of meteorites as they fall to Earth. By recording their entry from multiple angles, astronomers are able to triangulate where they originated in the asteroid belt. [Universe Today]

Galaxy

• New evidence has been found that two of Earth’s five mass extinctions were caused by nearby supernovae. It’s been speculated about before, but this new research shows the nearby supernova rate matches the timing of these unexplained mass extinctions. [EarthSky, EurekaAlert]

• The presence of positively charged hydrogen in the centre of the Milky Way might be a clue about the nature of dark matter. [KCL]

• A study of 90 neutron stars has narrowed down the distribution of their birth masses, finding it peaks slightly lower than previously thought but also finding that it’s skewed with a long tail at higher masses. [Universe Today]

• JWST has been hunting for brown dwarfs in the Flame Nebula. Observing in the infrared means JWST can see through dust that would obscure these observations in visible wavelengths. [Space.com]

Universe

• JWST has found that most galaxies appear to be rotating in the same direction, rather than being randomly distributed as you’d expect. While it is possible that there is really a preferred direction in the Universe, it’s likely that this is an observational bias. [Universe Today]

• New results from the Dark Energy Spectroscopic Instrument (DESI) hint that dark energy, which drives the accelerating expansion of the universe, may have changed over time. These results are intriguing, but it’s too early to get really excited. [NOIRLab, BBC, Ars Technica]

• The Large Hadron Collider has found evidence of two new charge-parity (CP) violations (ways that matter can be created in greater quantities than anti-matter). Such CP violations hint at how the Universe ended up with mostly matter and very little anti-matter. [New Scientist]

• A new study from the European Southern Observatory shows that a planned hydrogen plant near the Paranal Observatory in Chile will harm observations because of light pollution and ground vibrations. [ESO, Nature]

Finally

Every lunar eclipse is also a solar eclipse, if you happen to be on the Moon:

What is Three Alpha? Other than being the name of the newsletter you’re reading now, the name “three alpha” comes from the triple-alpha process, a nuclear chain reaction in stars which turns helium into carbon. Read more here.

Who writes this? My name is Dr. Adam McMaster. I’m an astronomer in the UK, where I mainly work on finding black holes. You can find me on BlueSky, @adammc.space.

Let me know what you think! You can send comments and feedback by hitting reply or by emailing [email protected].