Astronomers are Getting Really Good at Weighing Baby Supermassive Black Holes

In the 1970s, astronomers deduced that the persistent radio source coming from the center of our galaxy was actually a supermassive black hole (SMBH). This black hole, known today as Sagittarius A*, is over 4 million solar masses and is detectable by the radiation it emits in multiple wavelengths. Since then, astronomers have found that SMBHs reside at the center of most massive galaxies, some of which are far more massive than our own! Over time, astronomers observed relationships between the properties of galaxies and the mass of their SMBHs, suggesting that the two co-evolve.

Using the GRAVITY+ instrument at the Very Large Telescope Interferometer (VLTI), a team from the Max Planck Institute for Extraterrestrial Physics (MPE) recently measured the mass of an SMBH in SDSS J092034.17+065718.0. At a distance of about 11 billion light-years from our Solar System, this galaxy existed when the Universe was just two billion years old. To their surprise, they found that the SMBH weighs in at a modest 320 million solar masses, which is significantly under-massive compared to the mass of its host galaxy. These findings could revolutionize our understanding of the relationship between galaxies and the black holes residing at their centers.

The relationship between a galaxy’s properties and its SMBH has been observed many times in the local Universe. To determine if this has always been the norm, astronomers have been eagerly waiting to get a look at galaxies that existed during Cosmic Dawn, the period shortly after the Big Bang when the first galaxies formed. However, it remains extremely difficult (or even impossible) to measure black hole masses for these far-away galaxies using traditional direct methods, even where quasars (“quasi-stellar objects”) are involved.

Illustration of the GRAVITY+ observations of a quasar in the early Universe. © T. Shimizu; background image: NASA/WMAP; quasar illustration: ESO/M. Kornmesser; VLT array: ESO/G. Hüdepohl

This particularly bright class of galaxies is a subset of galaxies with very Active Galactic Nuclei (AGNs), where the centers will temporarily outshine all the stars in the disk. Fortunately, next-generation telescopes and instruments are allowing astronomers to get a look at these early galaxies for the first time. This includes the GRAVITY interferometric instrument aboard the VLTI, which combines light from all four 8-meter (26.25 ft) telescopes of the ESO Very Large Telescope interferometrically, creating a single virtual telescope with a diameter of 130 meters (426.5 ft).

Thanks to recent upgrades, the GRAVITY instrument’s successor (GRAVITY+) is allowing astronomers to precisely study black hole growth at another critical epoch called “Cosmic Noon,” when both black holes and galaxies were rapidly growing. “In 2018, we did the first breakthrough measurements of a quasar’s black hole mass with GRAVITY. This quasar was very nearby, however.” said Taro Shimizu, a staff scientist at the Max Planck Institute for Extraterrestrial Physics, in an MPE press release: “Now, we have pushed all the way out to a redshift of 2.3, corresponding to a lookback time of 11 billion years.”

Thanks to the improved performance enabled by GRAVITY+, astronomers can push the envelope and take images of black holes in the early Universe 40 times sharper than what is possible even with the James Webb Space Telescope (JWST). With the help of the GRAVITY+, the team was able to build on their previous observations and spatially resolve the motion of the gas and dust that make up the accretion disk around the central black hole of SDSS J092034.17+065718.0. This allowed them to obtain a direct measurement of the mass of the central black hole.

This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. Credit: ESO, ESA/Hubble, M. Kornmesser

At 320 million solar masses, the black hole is actually underweight compared to its host galaxy, about 60 billion solar masses. This suggests that the host galaxy grew faster than the SMBH at its center, which could mean there is a delay between galactic and black hole growth for some galaxies. Said Jinyi Shangguan, an MPE scientist with the research group:

“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. 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.”

Moving forward, the team plans to conduct follow-up observations of other galaxies at Cosmic Noon and make high-precision measurements of their central black holes. These observations will determine if this mass imbalance is the dominant mode of co-evolution for early galaxies and their SMBHs.

Further Reading: Max Planck Institute for Extraterrestrial Physics

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