Using the James Webb Space Telescope (JWST), astronomers have observed the dramatic “dance” between a supermassive black hole and two satellite galaxies. The observations could help scientists better understand how galaxies and supermassive black holes evolved in the early universe.
This particular supermassive black hole feeds on the surrounding matter and powers a bright quasar that is so distant that JWST sees it as it was less than a billion years after the Big Bang. The quasar, designated PJ308-21, is located in an active galactic nucleus (AGN) in a galaxy that is in the process of merging with two massive satellite galaxies.
The team not only found that the black hole has a mass equivalent to two billion suns, but also found that both the quasar and the galaxies involved in this merger were highly evolved, a surprise given that they existed, when the 13.8-year-old Cosmos was just a baby.
The merger of these three galaxies would likely supply the supermassive black hole with enormous amounts of gas and dust, facilitating its growth and allowing it to continue to power PJ308-21.
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“Our study reveals that both black holes at the center are at high redshift [early and distant] quasars and the galaxies that host them have undergone extremely efficient and turbulent growth since the first billion years of cosmic history, aided by the rich galactic environment in which these sources form,” team leader Roberto DeCarli, a researcher at the National Institute for Astrophysics of Italy (INAF), said in a statement.
The data were collected in September 2022 by JWST’s Near InfraRed Spectrograph (NIRSpec) instrument as part of the 1554 program, which aims to observe the merger between the galaxy that hosts PJ308-21 and two of its satellite galaxies.
DeCarli added that the work represented a real “emotional rollercoaster” for the team, who developed innovative solutions to overcome the initial difficulties of data reduction and produce images with uncertainty of less than 1% per pixel.
Quasars are born when supermassive black holes, with masses millions or billions of times that of the sun, found at the heart of galaxies, are surrounded by an abundance of gas and dust. This matter forms a flattened cloud called an accretion disk, which orbits the black hole and gradually feeds it.
The enormous gravitational forces of the black hole generate powerful tidal forces in this accretion disk, which heat this gas and dust to temperatures of up to 120,000 degrees Fahrenheit (67,000 degrees Celsius). This causes the accretion disk to emit light in the electromagnetic spectrum. This emission can often be brighter than the combined light of every star in the surrounding galaxy, making quasars like PJ308-21 some of the brightest objects in space.
While black holes have no characteristics that can be used to determine how much they have evolved, their accretion disks (and therefore quasars) do. In fact, galaxies can be “aged” in the same way.
The early universe was filled with hydrogen, the lightest and simplest element, and some helium. This formed the basis of the first stars and galaxies, but during the life of these stellar bodies they forged elements heavier than hydrogen and helium, which astronomers call “metals”.
When these stars ended their lives in massive supernova explosions, these metals were scattered throughout their galaxies and went on to be the building blocks of the next generation of stars. This process saw stars, and through them galaxies, become progressively “metal-rich”.
The team found that, like most AGN, PJ308-21’s active core is rich in metals, and the gas and dust around it is being “photoionized”. This is the process by which particles of light, called photons, provide the energy that electrons need to escape from atoms, creating electrically charged ions.
One of the galaxies that is merging with the host galaxy PJ308-21 is also metal-rich, and its matter has also been partially photoionized by electromagnetic radiation from the quasar.
Photoionization also occurs in the second satellite galaxy, but in this case it is caused by a burst of rapid star formation. This second galaxy also differs from the first and the AGN in that it appears metal-poor.
“Thanks to NIRSpec, for the first time we can probe in the PJ308-21 system the optical band, rich in valuable diagnostic data about the properties of the gas near the black hole in the galaxy hosting the quasar and in the surrounding galaxies,” said the member of team and INAF astrophysicist Federica Loiacono. “We can see, for example, the emission of hydrogen atoms and compare it to that of the chemical elements produced by the stars to determine how rich the gas is in metals.”
Although light leaves this quasar from the early universe in the broad range of the electromagnetic spectrum, including optical light and X-rays, the only way to observe it is in the infrared spectrum.
That’s because as the light traveled more than 12 billion years to reach JWST, the expansion of the universe has “stretched” its wavelengths significantly. This “shifts” the light in the direction of the “red end” of the electromagnetic spectrum, a phenomenon understandably called “redshift”, which is denoted as “z” by astronomers.
JWST is able to see “high redshift” or “high z” objects and events like PJ308-21 due to its sensitivity to infrared light.
“Thanks to JWST’s near- and mid-infrared sensitivity, it was possible to probe the spectrum of the quasar and its companion galaxies with unprecedented precision in the distant universe,” concluded Loiacono. “Only the excellent ‘view’ offered by JWST is able to provide these observations.”
The team’s research was accepted for publication in June 2024 in the journal Astronomy & Astrophysics.
