Scientists Have Finally Solved a 20-Year-Old Mystery on Supermassive Black Holes

'These monster objects force their host galaxies into a kind of semi-retirement from star formation.'

Scientists Have Finally Solved a 20-Year-Old Mystery on Supermassive Black Holes
A supermassive black hole simulation. 4X-image / iStock

Sometimes big things are held back by even bigger ones.

Supermassive black holes at the center of galaxies can slow the birth of new stars, according to a Tuesday presentation at the virtual National Astronomy Meeting (NAM 2021) and shared under embargo with IE.

And, in addition to enhancing our grasp of how galaxies evolve, it could also lead to shifts in our understanding of how or when intelligent life may form in the local universe.

Supermassive black holes sway the evolution of galaxies

The research team of astronomers used machine learning and three novel simulations to verify conclusions from a sky survey, and resolved a 20-year-old debate in the formation of stars. The process of star formation in galaxies has long remained a center of intense study for research among astronomers. And, after decades of work to grasp how gases collapse and condense into new stars both within and without the Milky Way, new all-sky observational projects have come to know that not all galaxies in the local region of the cosmos are actively forming stars. It turns out there is a great abundance of "quiescent" objects from which stars form at substantially lower frequencies.

For 20 years, the question of what could interfere in galaxies' star formation baffled scientists. But the new study from a team of astronomers, including a doctoral student of the University of Cambridge named Joana Piotrowska, have engineered an experiment capable of determining the cause for this phenomenon. The simulations used to verify the observations included Illustris, IllustrisTNG, and EAGLE. The astronomers then applied a machine-learning algorithm to classify the galaxies into different types, namely: star-forming and quiescent, and then analyze the data according to three discursive parameters. One noted the total mass of stars in the galaxy, another took into account the mass of supermassive black holes churning at the center of galaxies (which are millions to billions of times the sun's mass, and finally the mass of the dark matter halo surrounding galaxies. With these factors taken together, the scientists could best predict how the galaxies might evolve.


A slowed star-forming phase could affect the formation of life in the local universe

Employing these parameters, the scientists were able to identify which of three physical processes was causing galaxies into semi-retirement from star formation: supernova explosions, energy injection from supermassive black holes, or the shock heating of gas in colossal halos. And the simulations pointed to supermassive black hole mass as the most crucial factor in applying the brakes on star formation. Critically, the simulation results supported observations taken in the local universe, adding credibility to the findings. "It's really exciting to see how the simulations predict exactly what we see in the real universe," said Piotrowska in an embargoed release shared with IE. "Supermassive black holes — objects with masses equivalent to millions or even billions of Suns — really do have a big effect on their surroundings. These monster objects force their host galaxies into a kind of semi-retirement from star formation."


With this latest discovery, another layer of complexity is added not only to what we know about supermassive black holes and galactic evolution, but also how life might evolve in the local universe. If an equation (like the Drake equation) predicts the statistical frequency of intelligent life forming on planets in the universe, one of those terms must include when in the universe's history we're looking for life (the present), and how long it may take for most of the stars in a galaxy to form. If galaxies like our Milky Way experienced semi-retirement at some point in the distant past, it could drastically delay the emergence of star systems capable of supporting life. Or, at least, it could lower the frequency with which life-supporting star systems could form, until the supermassive black hole allowed its host galaxy to resume active star-forming periods.

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