Mapping gas composition, velocity

The team recognised that if QSO1’s black hole is as massive as it looks, they should be able to use the integral field unit (IFU) on Webb’s NIRSpec (Near Infrared Spectrograph) to trace the effects of its gravity on the gas swirling around it, while also mapping the distribution of various elements in the gas.

Cambridge graduate student Ignas Juodžbalis and Cosimo Marconcini of the University of Florence in Italy, lead authors on one of the studies, used the IFU observations to map motions of hydrogen gas surrounding the black hole. When they plotted the rotation velocity as a function of distance from the centre, they found that the gas has Keplerian motion: it orbits a central point in the same way that planets in our Solar System orbit the Sun.

“This is important because it tells us that most of the mass of QSO1 is concentrated in the black hole at the centre,” said Ignas. “If the mass were more distributed, as it would be if there were a lot of stars, the gas would not have this perfect Keplerian rotation.

Since Keplerian motion is governed by simple laws of gravity, the team was able to use the gas velocity measurements to calculate the black hole mass directly, a feat that had not previously been possible. They found that not only is the black hole immense – roughly 50 million solar masses – it makes up an astonishing two-thirds of QSO1’s total mass. This proportion is thousands of times greater than in nearby galaxies, where supermassive black holes make up only a tiny fraction of the host galaxy’s total mass.

The IFU composition maps supported these results, showing that the gas throughout QSO1 is almost entirely hydrogen and helium, with very little of the heavier elements like oxygen that would be expected in a galaxy rich with stars and stellar debris. With a metallicity less than 0.5% of the Sun, QSO1 is one of the most pristine galactic environments ever measured.

“This is a phenomenal result,” said Cosimo. “It is the first direct measurement of a black hole mass within the first billion years after the Big Bang, and it is consistent with the previous measurements.” The team thinks this is a good sign that the assumptions used for indirect mass measurements are valid and the masses of other black holes in the early Universe have not been overestimated.