At the center of many large galaxies lies a supermassive black hole, whose mass can reach millions or billions of times that of the Sun. To grow, it absorbs surrounding gas. But where exactly does this matter come from? A team traced its journey using 30 detailed simulations of massive galaxies.
The researchers identified several possible origins. Gas can arrive from the space between galaxies, come from another galaxy, or have been present since the earliest stages of formation. Another source lies directly within the galaxy: the stars themselves release matter as they evolve.
Artist’s view of the environment of a supermassive black hole at the center of a galaxy, surrounded by accreting matter.
Credit: ESO/L. Calçada — CC BY 4.0
Some stars gradually lose their outer layers. Others suddenly expel part of their matter when they explode as supernovae. This gas then returns to the galaxy, where it can circulate for a long time before moving closer to the center. Researchers refer to it as “recycled” gas.
In the simulations, this recycled gas represents the main source of matter absorbed by the black holes studied. It is also rich in elements heavier than hydrogen and helium.
This composition provides a clue to the gas’s history. The first gases in the Universe contained almost exclusively hydrogen and helium. Stars then produced heavier elements and released some of them. Matter rich in these elements therefore bears the trace of having passed through one or more generations of stars.
Even gas from other origins tends to change before reaching the black hole. According to the simulations, it generally remains in the galaxy and becomes progressively enriched. The black hole thus receives matter whose composition depends strongly on the evolution of the stars surrounding it, rather than a simple direct supply from outside.
The results are consistent with the high concentrations of heavy elements inferred from quasar observations. A quasar appears when a supermassive black hole absorbs a large amount of matter and its surroundings become extremely bright. The composition of this matter can be studied through the characteristics of the light it emits.
The study remains based on simulations of 30 massive galaxies, with particularly massive black holes. The authors can now compare more precisely the predicted chemical proportions with those measured around black holes observed at different periods in the history of the Universe.