Galaxies are made up of various astronomical objects, including black holes, planets, and stars. At the core of a galaxy lies a supermassive black hole (SMBH), one of the most powerful and hazardous entities in the Universe.
A puzzling question for scientists has been whether SMBHs gave birth to galaxies or if galaxies formed SMBHs. The events in the early Universe might hold the key to this mystery.
The James Webb Space Telescope (JWST), launched by NASA in 2021, can perhaps provide answers to this question. Utilizing infrared technology, it captures data and images that the Hubble Space Telescope cannot.
A recent study published in The Astrophysical Journal Letters has leveraged data from the JWST to explore 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 John Hopkins University and the Institute of Astrophysics of Paris.
Regarding their work, Prof Silk told IE, “The puzzling results of JWST on distant galaxies and SMBHs were a surprise, not predicted by previous simulations of galaxy formation.”
First, let us understand what JWST seeks—active galactic nuclei.
SMBH and AGN
The central region around a galaxy is compact and emits a large amount of radiation of all different wavelengths across the electromagnetic spectrum. This region, known as AGN, has a high luminosity, much brighter than anything a star can produce.
Not all galaxies have AGN, but most large galaxies have SMBHs at the center. SMBHs are much more massive than regular black holes that may be scattered across a galaxy.
The relationship between AGN and SMBH is important and may answer the question of which came first: SMBH or galaxies. AGNs are powered by the accretion of material onto SMBH.
Accretion is a phenomenon in which the gravitational pull of the SMBH causes particles of matter (like dust or gases) to accumulate around it, forming the AGN.
AGNs are responsible for shaping the environment of their host galaxy, which eventually shapes the formation of stars and planets. AGNs are called “active” because they constantly spew jets, outflows, and intense luminosity.
Since it is one of the more turbulent and dynamic phenomena in galaxies, it can help us to understand the evolution of SMBH and how they contribute to the formation of galaxies.
As Prof. Silk explained, “SMBHs are at least ten times more frequent in the early universe than in our current vicinity. Moreover, they are much more dominant relative to the mass of stars in the host galaxy as compared to 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 galaxy and blackhole formation, we must understand the data collected by JWST.
Light traveling to us brings crucial information about the Universe. The farther the light’s origin, the further back in time we are observing, as it takes time for light to journey from distant objects to reach us.
Let’s delve into this further. As the Universe expands, the light emitted in the early Universe must travel a greater distance to reach us, resulting in the light being stretched or redshifted.
Redshifted light is light whose wavelength has shifted towards the red part of the electromagnetic spectrum, indicative of the age of the light.
JWST’s focus is on collecting data about AGN at high redshift galaxies, which are some of the oldest structures in the Universe. These early structures hold information about the early Universe and the processes surrounding the formation of black holes and galaxies.
The researchers are concentrating on ultracompact dust-reddened galaxies, often referred to as “little red dots.” Professor Silk explained the reasoning behind this nickname.
“Most of the high redshift galaxies observed by JWST have been called little red dots, red because they are dusty, and dots because they are so compact. They often contain SMBHs,” he said.
Due to the presence of SMBHs, these galaxies are of particular interest when determining the evolution of galaxies in the early Universe.
Using simulations and the observation data from JWST, the researchers proposed a close relationship between the evolution of galaxies and SMBHs in the early Universe. This led them to define three distinct epochs based on the redshift of galaxies using the parameter “z” to explain the formation of both.
Defining epochs
The redshift parameter “z” tells us how much the light from a celestial object has been stretched. In simple terms, it tells us how far away a 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 starting to form. These high redshifted galaxies had dense star clusters at their center, called nuclear star clusters.
These dense stars formed a compact region near the galaxy’s center (hence the name ultra-compact high redshift galaxies), where they eventually died—forming black holes.
“The black holes rapidly merged with each other in this exceptionally dense region to form an IMBH (intermediate-mass black hole) or even an SMBH. That’s how the SMBH formed rapidly. Its growth was boosted thanks to the really high central density,” said Prof. Silk.
This idea is supported by the large number of such galaxies seen at high redshifts by JWST, more than those predicted by models. Additionally, these galaxies are a tenth or hundredth the size of a similar galaxy today mentioned Silk.
As black holes formed, accretion led to the formation of AGN.
The second epoch: Star formation bursts (5 < z < 15)
AGN are now prominent and turbulent, leading to the outflow of gases that will lead to the formation of stars. The bigger the black hole grows, the more stars start forming.
Prof. Silk explained how gas clouds that fall into SMBHs heat up due to the strong gravitational pull of the SMBH, which results in an intense ball of energy.
He further said, “Thanks to the rapid spin (and magnetic field) of the SMBH, most mass falls inwards to disappear in the black hole, but some are converted into a very energetic jet and outflow of energy.”
“It is this jet that crashes into nearby orbiting gas clouds, overwhelming them, and its huge pressure compresses them. The clouds collapse and fragment into stars.”
The third epoch: The quenching (z < 5)
As the Universe transitions to lower redshifts, it has expanded further. The 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 within a galaxy.
Synergy, co-evolution, and the future of JWST
There appears to be a close relationship between the evolution of SMBHs and their host galaxies, which relies on the synergetic relationship between AGN activity and star activity.
This means that AGN activity, driven by the accretion of matter onto SMBHs, affects star formation by releasing large amounts of energy. Conversely, star growth can affect SMBHs by causing a loss of stellar mass, which can contribute to the accretion disk.
This bimodal or dual synergy tells us that the co-evolution of SMBHs and their galactic hosts is complex. The study of 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 in the next year. These will provide improved spectroscopy. This will enable us to more precisely measure the masses of the SMBH and the stars, especially in the centers of the galaxies that host the SMBHs.”
However, he also highlighted the absence of high-resolution simulations needed to fully understand the phenomena of cloud crushing (gas clouds) and star formation.
Therefore, the question of whether SMBHs or galaxies formed first remains unresolved.
ABOUT THE EDITOR
Tejasri Gururaj Tejasri is a versatile Science Writer & Communicator, leveraging her expertise from an MS in Physics to make science accessible to all. In her spare time, she enjoys spending quality time with her cats, indulging in TV shows, and rejuvenating through naps.
Daisy Hips is a science communicator who brings the wonders of the natural world to readers. Her articles explore breakthroughs in various scientific disciplines, from space exploration to environmental conservation. Daisy is also an advocate for science education and enjoys stargazing in her spare time.
