SMBH: Astroboffins reveal how to stop pop(ping) stars in a supermassive galaxy

The mass of the supermassive black hole at the centre of galaxies controls star formation and determines how it evolves over time, according to new observations published in Nature.

Galaxies are made out of clumps of interstellar dust, gas, stars, black holes, and dark matter all gravitationally bound together. As they age, the light from old stars dwindles out and the rate of new star formation grinds to a halt – a process also known as "quenching".

A team of astrophysicists have found that that the time scale for quenching to kick in all comes down to how hefty the central supermassive black hole is. They are often more than a million times more massive than our Sun.

Ignacio Martín-Navarro, first author and a postdoc researcher at the University of California Observatories, said: "For galaxies with the same mass of stars but different black hole mass in the center, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes. So star formation lasted longer in those galaxies with smaller central black holes."

All large galaxies that stretch from hundreds and thousands to millions of light years across – the current evidence suggests – contain a supermassive black hole at their centre. The active galactic nucleus (AGN) at the centre of an active galaxy emits strong beams of electromagnetic radiation that can heat and scatter surrounding gas and dust, preventing it from collapsing into stars.

"We used black hole mass as a proxy for the energy put into the galaxy by the AGN, because accretion onto more massive black holes leads to more energetic feedback from active galactic nuclei, which would quench star formation faster," Martín-Navarro explained.

Supermassive black holes with more mass are more active and forceful, dispelling higher volumes of star-making material. They cause star quenching to happen at a faster rate compared to galaxies with smaller supermassive black holes, although the mechanism behind this has not yet been uncovered.

The idea of supermassive black holes driving galaxy evolution has been around for decades and explored through computer simulations. But no one had found any actual observed evidence until now.

Jean Brodie, co-author of the letter and a professor of astronomy and astrophysics at the University of California, Santa Cruz, said: "This is the first direct observational evidence where we can see the effect of the black hole on the star formation history of the galaxy."

The researchers measured the mass of supermassive black holes and analysed the light spectra emitted from galaxies in the Hobby-Eberly Telescope Massive Galaxy Survey to work out a galaxy’s star formation history.

Spectroscopy "tells you how much light is coming from stellar populations of different ages", Martín-Navarro said.

Next, they compared the masses of the different black holes and their star formation histories. Quenching rates were only affected by the mass of the supermassive black holes, and not the galaxy's other properties such as its type, size or shape.

The details of how a black hole dominates star formation are still a mystery. Aaron Romanowsky, co-author of the paper and an astronomer at San Jose University and UC Observatories, said there are different ways a black hole can release energy out into the galaxy.

"Theorists have all kinds of ideas about how quenching happens, but there's more work to be done to fit these new observations into the models," he said.

"We don't know the answer in detail, but there is probably a general physical principle at work here. A bigger black hole provides a bigger energy source to halt the star formation. It's like having a bigger bucket of water to put out a fire. However, black holes are even more potent than this. Although supermassive black holes are colossal in comparison to the Sun, they are dwarfed by their host galaxies, which are typically 200 times more massive. It is amazing that something so small could affect something so big – like a spoonful of water putting out a bonfire – which is a testament to the power of gravity unleashed by black holes.

"Our Milky Way galaxy is still actively forming stars, even though it hosts a supermassive black hole. This black hole probably plays some role in slowing down the formation of new stars, but it is undersized and plays a relatively small role. The Milky Way may not stop forming stars until several billions of years from now when it collides and merges with our neighbor galaxy, Andromeda."

"Black-hole-regulated star formation in massive galaxies" was published in Nature on 1 January, 2018. ®