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 grew in the early universe.
This particular supermassive black hole is feeding on surrounding matter, powering a bright quasar that is so far away 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.
Not only did the team determine that the black hole has a mass equal to two billion suns, they also found that both the quasar and the galaxies involved in this merger are highly evolved. This is surprising, given that they existed when the 13.8-year-old universe was just a baby.
The merger of these three galaxies will likely provide the supermassive black hole with enormous amounts of gas and dust, which will facilitate its growth and ensure it can continue to supply PJ308-21 with energy.
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“Our research shows that both the black holes at the center of the high redshift [early and distant] “Quasars and the galaxies they host experience extremely efficient and rapid growth already in the first billion years of cosmic history, aided by the rich galactic environment in which these sources are formed,” team leader Roberto Decarli, a researcher at the Italian National Institute for Astrophysics (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 of the galaxy hosting PJ308-21 and two of its satellite galaxies.
Decarli added that the work was a real “emotional rollercoaster” for the team, which developed innovative solutions to overcome the initial difficulties in reducing the data and produce images with an uncertainty of less than 1% per pixel.
A very metallic quasar
Quasars are born when supermassive black holes with masses millions or billions of times that of the Sun, located at the centers of galaxies, are surrounded by an abundance of gas and dust. This material forms a flattened cloud, called an accretion disk, that swirls around the black hole, gradually feeding it.
The black hole’s immense gravitational forces generate powerful tidal forces in this accretion disk, heating this gas and dust to temperatures as high as 120,000 degrees Fahrenheit (67,000 degrees Celsius). This causes the accretion disk to emit light across the electromagnetic spectrum. This emission can often be brighter than the combined light from every star in the surrounding galaxy, making quasars like PJ308-21 some of the brightest objects in the cosmos.
While black holes do not have any characteristics that can be used to determine how evolved they are, 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 a little helium. This formed the basis of the first stars and galaxies, but during the lives of these stellar bodies they forged elements heavier than hydrogen and helium, which astronomers call “metals.”
As these stars ended their lives in massive supernova explosions, these metals were spread throughout their galaxies and became the building blocks of the next generation of stars. This process saw stars, and through them galaxies, become increasingly “metal-rich.”
The team found that, like most AGNs, the active core of PJ308-21 is rich in metals, and the gas and dust around it is ‘photoionized’. This is the process by which particles of light, called photons, provide the energy needed for electrons to escape from atoms, creating electrically charged ions.
One of the galaxies merging with the galaxy PJ308-21 is also rich in metals. The matter in that galaxy is also partially photoionized by the electromagnetic radiation from the quasar.
Photoionization also occurs in the second satellite galaxy, but in that case it is caused by a period of rapid star formation. This second galaxy also differs from the first and the AGN in that it appears to be metal-poor.
“NIRSpec allows us to study for the first time in the PJ308-21 system the optical band that is rich in valuable diagnostics on the properties of the gas near the black hole in the galaxy hosting the quasar and in the surrounding galaxies,” says team member and INAF astrophysicist Federica Loiacono. “For example, we can see the emission of hydrogen atoms and compare it with that of the chemical elements produced by the stars to determine how rich the gas is in metals.”
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Although light leaves this quasar from the early universe across the broad spectrum of the electromagnetic spectrum, including optical light and X-rays, the only way to observe it is in the infrared.
That’s because, while the light was traveling for over 12 billion years to reach JWST, the expansion of the universe has “stretched” its wavelengths significantly. That “shifts” the light toward the “red end” of the electromagnetic spectrum, a phenomenon understandably called “redshift,” which astronomers refer to as “z.”
JWST is good at observing “high redshift” or “high z” objects and events like PJ308-21 because of its sensitivity to infrared light.
“Thanks to the sensitivity of the JWST in the near and mid-infrared, it was possible to study the spectrum of the quasar and its companion galaxies with unprecedented precision in the distant Universe,” Loiacono concluded. “Only the excellent ‘vision’ provided by the JWST can guarantee these observations.”
The team’s research was accepted for publication in the journal Astronomy & Astrophysics in June 2024.