How massive galaxies and black holes evolve together

Astronomers at Melbourne’s Swinburne University of Technology have developed a new look at how galaxies and their central supermassive black holes co-evolve.

Researchers have previously assumed that central black holes in elliptical-shaped galaxies – the largest galaxies in the universe – increase in mass linearly with the size of the galaxy. For example, it was believed that an elliptical galaxy with two times the stellar mass of a smaller galaxy should also have a central black hole twice as massive.

Overall, supermassive black holes in the centres of elliptical galaxies have been assumed to make up about 0.2% of the galaxy’s total mass.

For a quarter century, computer simulations aimed at illuminating how these central black holes and elliptical galaxies evolve together have used this model.

But the Swinburne researchers’ new observations revealed not a linear growth of central black holes with elliptical galaxy size, but “quadratic” growth. This means that an elliptical galaxy with twice as much stellar matter will have a central black hole four times bigger. A galaxy with 10 times greater stellar mass will see a 100-fold increase in the size of the supermassive black hole.

The findings are published in the Monthly Notices of the Royal Astronomical Society.

“The black holes are off their leash,” says author Professor Alister Graham.

The four largest galaxies in the known universe are elliptical. The largest, IC 1101, is located a billion light-years from Earth. Home to 100 trillion stars, IC 1101 has a radius of about 2 million light-years (compared to the Milky Way, which has a radius of just 53,000 light years). This means that, if IC 1101 were located where the Milky Way is, it would engulf our nearest large neighbour galaxy Andromeda.

Because elliptical galaxies can be so large, their central black holes are also massive.

This is seen in the nearby elliptical galaxy Messier 87 (M87). In 2019, M87’s central black hole became the first black hole to be directly imaged; it’s about 500 million times the mass of our Sun. Elliptical galaxy central black holes are regularly billions of times larger than the Sun.

Graham discovered quadratic growth among supermassive black holes in the elliptical-shaped swarm of stars that make up the centres of spiral galaxies (like the Milky Way). However, it was assumed that quadratic growth wasn’t a feature of central black holes of elliptical galaxies.

The astronomers found this quadratic relationship between elliptical galaxy size and central black hole size by analysing 100 galaxies imaged at infrared wavelengths by NASA’s Spitzer Space Telescope. The discovery changes our understanding of how galaxies and black holes co-evolve.

Many of the galaxies previously thought to be purely elliptical actually possess a huge disc of stars. The quadratic mass scaling became evident when these discs were taken into account. The research suggests two types of galaxy: elliptical and “spiral-less” disc galaxies.

The researchers believe that colliding galaxies throw stars into chaotic orbits above and below the neat circular orbits seen in spiral galaxies. This builds a “bulge”. Initially, these will develop into galaxies with prominent bulges and a dusty spiral-less disc. After some time, the bulge dominates and a pure elliptical galaxy is born.

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