Galaxies Manage Their Own Growth to Preserve Star-Forming Gas

When observing spiral or barred spiral galaxies, you'll notice multiple regions where stars are actively forming. These areas are rich in hydrogen gas, essential for star formation.

Initially, the universe's first galaxies had abundant star-forming gas. If left unchecked, these galaxies would have rapidly consumed their gas, leading to a burst of star formation followed by a period of inactivity with only dying stars remaining. However, galaxies seem to have a way to regulate their star formation, thanks to the supermassive black holes at their centers.

Approximately 400 to 700 million years after the Big Bang, the first galaxies emerged during the Epoch of Reionization. These early, small, and faint galaxies consisted primarily of hydrogen and helium, hosting dense clusters of massive, short-lived Population III stars (the first generation of stars). The intense radiation from these stars ionized the surrounding gas, clearing the cosmic fog and making the universe transparent. These primordial galaxies began merging and interacting, setting the stage for the variety of galaxies we see today.

A recent study published in the Monthly Notices of the Royal Astronomical Society investigates why galaxies are not as large as expected. The research suggests that galaxies, including the earliest ones, avoid early demise through mechanisms resembling a "heart and lungs" system that regulates their "breathing." Without these regulatory processes, galaxies would age quickly, becoming filled with dead and dying stars and lacking new star formation.

Observations show that galaxies manage to prevent excessive growth and star death. Astrophysicists at the University of Kent propose that galaxies regulate their growth rate in a manner similar to breathing. They liken the supermassive black hole at a galaxy's center to a heart and the supersonic jets it emits to airways feeding a pair of lungs.

Supermassive black holes appear to pulse like a heart. These pulses generate shock fronts that oscillate along the jets, similar to a diaphragm inflating and deflating lungs. This process transmits energy along the jets, counteracting gravity and slowing gas accretion and star formation. This concept was developed by PhD student Carl Richards, whose simulations depicted a black hole pulsing like a heart.

Richards explains, “We realized that there must be a mechanism for the jets to support the galaxy's ambient gas, and that’s what we discovered in our computer simulations.” He continued, “The unexpected behavior emerged when we analyzed the simulations under high pressure and allowed the heart to pulse.”

Evidence of ripples, similar to those in Richards’ simulations, has been found in galaxy clusters like Perseus. These ripples are believed to sustain a galaxy’s environment, although their generation mechanism was previously unclear. Conventional simulations have struggled to explain gas flows into galaxies, but the work from the University of Kent team may have provided the answer.