Galaxies are made up of a variety of astronomical objects, including black holes, planets, and stars. At the core of a galaxy is a supermassive black hole (SMBH), one of the most powerful and dangerous entities in the universe.
A puzzling question for scientists is whether SMBHs gave birth to galaxies or galaxies formed SMBHs. Events in the early universe may hold the key to this mystery.
The James Webb Space Telescope (JWST), launched by NASA in 2021, may be able to provide answers to this question. Using infrared technology, it captures data and images that the Hubble Space Telescope cannot.
A recent study published in The Astrophysical Journal Letters used data from JWST to investigate how active galactic nuclei (AGN) in the early Universe contributed to the formation of stars and black holes.
Interesting Engineering (IE) spoke to Prof. Joseph Silk, the study’s first author, from Johns Hopkins University and the Institute of Astrophysics in Paris.
Regarding their work, Prof. Silk told IE: “The puzzling JWST results on distant galaxies and SMBHs were a surprise, unpredicted by previous simulations of galaxy formation.”
First, let’s understand what JWST is looking for – active galactic nuclei.
SMBHs and AGNs
The central region around the galaxy is compact and emits a large amount of radiation at different wavelengths in the electromagnetic spectrum. This region, known as an AGN, has a high luminosity, much brighter than anything a star can produce.
Not all galaxies have AGNs, but most large galaxies have an SMBH at their center. SMBHs are much more massive than ordinary black holes, which may be scattered throughout a galaxy.
The relationship between AGN and SMBH is important and may answer the question of which came first: the SMBH or the galaxies. AGN are powered by accumulation of material on SMBH.
Accretion is a phenomenon where the gravitational attraction of an SMBH causes particles of matter (such as dust or gases) to accumulate around it, forming an AGN.
AGNs are responsible for shaping the environment of their host galaxy, which ultimately shapes star and planet formation. AGNs are called “active” because they are constantly spewing jets, outflows, and intense brightness.
As this is one of the most violent and dynamic phenomena in galaxies, it can help us understand the evolution of SMBHs and how they contribute to galaxy formation.
As Prof. Silk explained, “SMBHs were at least ten times more common in the early universe than in our present vicinity. They are also much more dominant relative to the host galaxy’s stellar mass than what we see today. All this suggests that massive black holes formed in the earliest stages of galaxy formation.
Redshifted light and the early universe
To study the early formation of galaxies and black holes, we need to understand the data collected by JWST.
Light traveling towards us carries important information about the universe. The more distant the light’s origin, the further back in time we observe, as it takes time for light to travel from distant objects to reach us.
Let’s dig into it. As the universe expands, light emitted in the early universe must travel a greater distance to reach us, causing the light to be stretched or redshifted.
Red-shifted light is light whose wavelength has shifted toward the red part of the electromagnetic spectrum, which is indicative of the light’s age.
JWST’s focus is on collecting data on AGN in high-redshift galaxies, which are some of the oldest structures in the Universe. These early structures contain information about the early universe and the processes surrounding the formation of black holes and galaxies.
The researchers focused on ultra-compact galaxies reddened by dust, often called “little red dots”. Professor Silk explained the reasoning behind this nickname.
“Most high-redshift galaxies observed by JWST are called small red dots, red because they are dusty and dots because they are so compact. They often contain SMBH,” he said.
Because of the presence of SMBHs, these galaxies are of particular interest in determining the evolution of galaxies in the early Universe.
Using simulations and observational data from JWST, the researchers proposed a close link between the evolution of galaxies and the SMBH in the early Universe. This led them to define three separate epochs based on the redshift of the galaxies, using the “z” parameter to explain the formation of both.
Defining epochs
The redshift parameter “z” tells us how much the light from a celestial object is stretched. In simple terms, it tells us how far away a given celestial object is, effectively allowing us to look back in time.
The First Epoch: Early Universe (z > 15)
During this time, the universe was young, with galaxies just beginning to form. These high-redshift galaxies had dense star clusters at their centers called nuclear star clusters.
These dense stars formed a compact region near the center of the galaxy (hence the name high-redshift ultracompact galaxies), where they eventually died to form black holes.
“The black holes rapidly merged with each other in this extremely dense region to form an IMBH (intermediate mass black hole) or even an SMBH. Thus, the SMBH is rapidly formed. Its growth was enhanced by the really high central density,” said Prof. Silk.
This idea is supported by the large number of such galaxies observed at high redshifts by JWST, more than predicted by models. Furthermore, these galaxies are one-tenth or one-hundredth the size of a similar galaxy Silk mentioned today.
When black holes formed, the accretion led to the formation of AGN.
Second epoch: Bursts of star formation (5 < z < 15)
AGNs are now visible and turbulent, causing gases to escape, which will lead to star formation. The bigger the black hole gets, the more stars begin to form.
Prof. Silk explained how the gas clouds that fall into the SMBH are heated by the strong gravitational pull of the SMBH, resulting in an intense ball of energy.
He further said, “Thanks to the rapid rotation (and magnetic field) of the SMBH, most of the mass falls inward to disappear into the black hole, but some becomes a very energetic jet and energy outflow.”
“It is this jet that slams into nearby orbital gas clouds, engulfing them, and its enormous pressure compresses them. The clouds shrink and shatter into stars.
The Third Age: The Hardening (z < 5)
As the Universe moves to lower redshift, it expands further. Winds near AGN cause the gases necessary for star formation to be dispersed.
If the gas reservoir is depleted, star formation will also be quenched, leading to lower star formation rates over time in the galaxy.
Synergy, co-evolution and the future of JWST
There appears to be a tight link between the evolution of SMBHs and their host galaxies that relies on the synergistic relationship between AGN activity and stellar activity.
This means that AGN activity driven by the accretion of matter onto the SMBH affects star formation by releasing large amounts of energy. Conversely, stellar growth can affect the SMBH by causing stellar mass loss, which can contribute to the accretion disc.
This bimodal or double synergy tells us that the joint evolution of SMBHs and their galactic hosts is complex. Studying AGN can provide more insight into these complex processes, which is why the data collected by JWST is so important.
Regarding future measurements from JWST, Prof. Silk said: “New observations will be available from JWST next year. These will provide improved spectroscopy. This will allow us to more precisely measure the masses of SMBHs and stars, especially in the centers of galaxies that host SMBHs.
However, it also highlighted the lack of high-resolution simulations needed to fully understand the phenomena of cloud collapse (gas clouds) and star formation.
Therefore, the question of whether SMBHs or galaxies formed first remains unresolved.
FOR THE EDITOR
Tejasri Gururaj Tejasree is a multifaceted Science Writer & Communicator, using his experience from a Master’s degree in Physics to make science accessible to all. In her free time, she enjoys spending quality time with her cats, binge watching TV shows and rejuvenating by taking naps.
