Galaxies Regulate their Own Growth so they Don’t Run Out of Star Forming Gas

Look at most spiral or barred spiral galaxies and you will see multiple regions where stars are forming. These star forming regions are comprised of mostly hydrogen gas with a few other elements for good measure. The first galaxies in the Universe had huge supplies of this star forming gas. Left unchecked they could have burned through the gas quickly, generating enormous amounts of star formation. Life fast though and die young for such an energetic burst of star formation would soon fizzle out leaving behind dead and dying stars. In some way it seems, galaxies seem to regulate their star formation thanks to supermassive black holes at their centre. 

The first galaxies formed about 400 to 700 million years after the Big Bang, during the Epoch known as Reionization. These early galaxies were small and faint, mostly composed of hydrogen and helium, and contained dense clusters of massive, short-lived Population III stars (the first generation of stars.) The intense radiation from these stars ionised the surrounding gas, clearing the fog that permeated space making the universe transparent for the first time. These primordial galaxies began merging and interacting, laying the foundation for the galaxy types seen today.

A new study published in the Monthly Notices of the Royal Astronomical Society explores why galaxies are not as large as astronomers would expect. The research suggests that galaxies, even those that formed first, avoid an early death because they have mechanisms similar to “heart and lungs,” which regulate their “breathing”. Without these regulatory processes our bodies, and galaxies would have aged much faster, resulting in massive galaxies filled with dead and dying stars and devoid of new star formation.

Observations show that galaxies are not so big and full of dying stars having outgrown themselves. It seems something limits their ability to allow gas to form into stars. Astrophysicists at the University of Kent believe they may have the answer: galaxies could be controlling their growth rate through a process not too dissimilar to “breathing.” They compare the supermassive black hole at the centre of a galaxy to a heart and the supersonic jets emerging from the poles with the radiation and gas they emit to airways feeding a pair of lungs.

The supermassive black holes seem to pulse just like a heart. These pulses cause a shock front to oscillate along the jets like a diaphragm inflating and deflating the lungs. This process transmits energy along the jet slowly counteracting the pull of gravity and slowing gas accretion and star formation. The idea was developed by PhD student Carl Richards and his simulations showed a black hole pulsing like a heart. 

Assisted by magnetic fields, a spiraling wind helps the supermassive black hole in galaxy ESO320-G030 grow. In this illustration, the core of the galaxy is dominated by a rotating wind of dense gas leading outwards from the (hidden) supermassive black hole at the galaxy’s center. The motions of the gas, traced by light from molecules of hydrogen cyanide, have been measured with the Atacama Large Millimeter/submillimeter Array. Image credit: M. D. Gorski/Aaron M. Geller, Northwestern University, CIERA, the Center for Interdisciplinary Exploration and Research in Astrophysics.

Richards explains “We realised that there would have to be some means for the jets to support the body – the galaxy’s surrounding ambient gas – and that is what we discovered in our computer simulations,” He continued “The unexpected behaviour was revealed when we analysed the computer simulations of high pressure and allowed the heart to pulse.”

Evidence of ripples just like those in Richards’ simulations, in extra-galactic media have been found in galaxy clusters like the Perseus cluster. These ripples are thought to sustain a galaxy’s environment, though their generation mechanism was unclear. Conventional simulations fail to explain gas flows into galaxies, but the work of the team from the University of Kent may well have answered the question.

Source : How the ‘heart and lungs’ of a galaxy extend its life.

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