Photo credits: NASA, ESA, K. Kuntz (JHU), F. Bresolin (University of Hawaii), J. Trauger (Jet Propulsion Lab), J. Mold (NOAO), Y.-H. Chu (University of Illinois, Urbana) and STScI

Black holes with masses of millions of suns inhibit the birth of new stars, astronomers say. Using machine learning and three state-of-the-art simulations to secure the results of a large sky survey, the researchers are resolving a 20-year-old debate about the formation of stars. Joanna Piotrowska, PhD student at the University of Cambridge, will present the new work today (Tuesday, July 20) at the virtual National Astronomy Meeting (NAM 2021).

Star formation in galaxies has long been a focus of astronomy research. Decades of successful observations and theoretical modeling led to our good understanding of how gas collapses to form new stars both in and outside of our own Milky Way galaxy. However, thanks to all-sky observation programs like the Sloan Digital Sky Survey (SDSS), astronomers realized that not all galaxies in the local universe are active star formation – there is an ample population of “dormant” objects forming stars at significantly lower rates.

The question of what stops star formation in galaxies remains the greatest unknown in our understanding of galaxy evolution that has been debated for the past 20 years. Piotrowska and her team conducted an experiment to find out what could be responsible for it.

Using three state-of-the-art cosmological simulations – EAGLE, Illustris and IllustrisTNG – the astronomers investigated what we would expect in the real universe, as observed by the SDSS, if various physical processes stopped star formation in massive galaxies.

Using a machine learning algorithm to classify galaxies into star formation and rest, the astronomers asked which of the three parameters: the mass of the supermassive black holes at the center of galaxies (these monster objects typically have millions or even billions of times the mass of our sun ), the total mass of stars in the galaxy or the mass of the dark matter halo around galaxies, best predicts how galaxies will evolve.

Using these parameters, the team was then able to find out which physical process: energy injection through supermassive black holes, supernova explosions or shock heating of gas in massive halos is responsible for forcing galaxies into semi-retirement.

The new simulations predict the mass of the supermassive black hole as the most important factor in slowing down star formation. It is crucial that the simulation results match observations of the local universe, which gives weight to the researchers’ results.

Piotrowska says, “It’s really exciting to see how the simulations predict exactly what we will see in the real universe. Supermassive black holes – objects with masses of millions or even billions of suns – really have a huge impact on their surroundings. These monster objects force their host galaxies into a kind of half-board from star formation. ”

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