Weighing a Black Hole in the early universe

With the upgraded GRAVITY-instrument at the ESO VLTI, a team of astronomers led by the Max Planck Institute for Extraterrestrial Physics has determined the mass of a Black Hole in a galaxy only 2 billion years after the Big Bang. With 300 million solar masses, the black hole is actually under-massive compared to the mass of its host galaxy, indicating that at least for some systems there might be a delay between the growth of the galaxy and its central black hole.

In the more local universe, astronomers have observed tight relationships between the properties of galaxies and the mass of the supermassive black holes residing at their centers, suggesting that galaxies and black holes co-evolve. A crucial test would be to probe this relationship at early cosmic times, but for these far-away galaxies traditional direct methods of measuring the black hole mass are either impossible or extremely difficult. Even though these galaxies often shine very brightly (they were dubbed “quasars” or “quasi-stellar objects” when they were first discovered in the 1950s), they are so far away that they cannot be resolved with most telescopes.

“In 2018, we did the first breakthrough measurements of a quasar’s black hole mass with GRAVITY,” says Taro Shimizu, staff scientist at MPE and the corresponding author of the new study now published in Nature. “This quasar was very nearby, however. Now, we have pushed all the way out to a redshift of 2.3, corresponding to a lookback time of 11 billion years.” GRAVITY+ now opens a new and precise way to study black hole growth at this critical epoch, often called “cosmic noon”, when both black holes and galaxies were rapidly growing.

“This is really the next revolution in astronomy – we can now get images of black holes in the early universe, 40 times sharper than possible with the James Webb telescope,” points out Frank Eisenhauer, the MPE director who leads the group developing the GRAVITY instrument and the GRAVITY+ improvements.

GRAVITY combines all four 8-metre-telescopes of the ESO Very Large Telescope interferometrically, essentially creating one giant virtual telescope with a diameter of 130 metres. With the latest upgrades using a new wide-field, off-axis fringe-tracking mode, GRAVITY-Wide was now able to observe the central region of the galaxy SDSS J092034.17+065718.0, one of the most luminous quasars in the early universe.

The team was able to spatially resolve the so-called ‘broad line region’, observing the motion of gas clouds around the central black hole as they rotate in a thick disk. This allows a direct, dynamical measurement of the mass of the black hole. With 320 million solar masses, the black hole mass turns out to be actually underweight compared to its host galaxy which has a mass of about 60 billion solar masses. This suggests that the host galaxy grew faster than the supermassive black hole, indicating a delay between galaxy and black hole growth for some systems.

“The likely scenario for the evolution of this galaxy seems to be strong supernova feedback, where these stellar explosions expel gas from the central regions before it can reach the black hole at the galactic center,” says Jinyi Shangguan, postdoc in the MPE IR group. “The black hole can only start to grow rapidly – and to catch up to the galaxy’s growth overall – once the galaxy has become massive enough to retain a gas reservoir in its central regions even against supernova feedback.” To determine whether this scenario is also the dominant mode of the co-evolution for other galaxies and their central black holes, the team will follow-up with more high-precision mass measurements of black holes in the early universe are needed. Stay tuned for more quasar observations with GRAVITY+!