When our powerful infrared telescopes allowed astronomers to look further into the future, they made some amazing discoveries. One of them involves supermassive black holes (SMBH), the physics-defying behemoths at the center of massive galaxies such as the Milky Way. In fact, the SMBH grew much faster in the jewelry world than it does in the modern Universe.
A period of the Universe’s evolution called the Cosmic Noon saw the most rapid growth of the SMBH. Since then, SMBH’s growth has slowed. The Cosmic Noon, which lasted from about 2 billion years after the Big Bang to about 4 billion years after, was also the time of peak star formation activity.
In a new study in The Astrophysical Journal, astronomers have found that the modern SMBH is unable to gather information as quickly as in the past. It is mainly due to the unavailability of cold gas, the preferred food of SMBH. Astrophysicists sometimes call this phenomenon “AGN downsizing.”
The research is called “The Drivers of the Decline in Supermassive Black Hole Growth at z < 2." Lead author is Zhibo Yu of Pennsylvania State University. Yu is a grad student in the Department of Astronomy and Astrophysics at Penn State.
“Understanding the growth history of supermassive black holes (SMBHs) is one of the most important topics for extragalactic studies.” writers write. There are strong correlations between the mass of the SMBH and the mass of the galaxy wave and its dispersion. There is also a correlation between the long-term expansion rate of the SMBH and the total stellar mass of the galaxy and the star formation rate.
“These relationships show that SMBHs and host galaxies evolve in a coordinated way,” the researchers explain. “Therefore, tracing the growth history of SMBHs can shed light on the mechanism driving stellar-SMBH coevolution.”
Scientists know that the growth of the SMBH peaked at about z ≈ 1.5–2 and has declined until today. z ≈ 1.5–2 corresponds to about 9.5 to 10.5 billion years ago, when the Universe was only about 3 to 4 million years old. There were brighter AGN at higher redshifts. The question is why the decline?
“A long-standing mystery has been the cause of this dramatic decline,” said lead author Yu in a press release. “With these X-ray data and observations that support other wavelengths, we can test different ideas and narrow down the answer.”
X-ray observations from Chandra played an important role in this work. As the SMBH accumulates material, it is called an active galactic nucleus (AGN). As the AGN consumes more matter, this matter begins to form a circular ring around the SMBH. Objects inside the ring are hot and emit x-rays, which Chandra can see. A comparison of the x-ray emissions from different AGN at different distances shows how fast they are growing. As the AGN emits x-rays, the SMBH grows rapidly.
* Artist’s drawing shows a black hole surrounded by a ring of accretion. When the material inside these rings heats up they emit x-rays. Observing these x-rays is an important part of measuring SMBH and AGN. In this simulation, SMBH also launches a jet. Image credit: NASA/JPL-Caltech*
In this work, Yu and his co-researchers analyzed the observations of galaxies at about 1.3 million and 8,000 SMBH. The observations come from Chandra, along with other space telescopes, the XMM-Newton Observatory and eROSITA.
The researchers used what is known as the “wedding cake design” in their study. This means that the data they used came from different sources. They used shallow studies of large areas of the sky—called “pencil-beam studies)—to very detailed studies of small areas of the sky.
XMM-Newton and eROSITA produced intermediate and low-level data with broad, shallow observations. Chandra inked the top of the cake with deep details that covered a small area. This allowed the x-ray telescope to detect the growing fainter and more distant SMBH.
“By combining this data from different X-ray telescopes, we can build a better picture of how these black holes grow than any single telescope can,” said co-author Fan Zou of the University of Michigan. “We can find out why over ten billion years the growth of supermassive black holes has gone from busy to relaxed to frozen.”
There are three different reasons that could explain the decline in SMBH growth.
The first is that the accretion rate is slowing down now for some reason. The second is that the average masses of SMBH are low now. The third is that there are few who accept SMBH now.
* This diagram shows three possible explanations for the decline in SMBH growth. The top panel shows low consumption rates, the middle panel shows supermassive black holes, and the bottom shows an actively growing SMBH. Image credit: Penn State/Z.Yu*
Part of the difficulty in distinguishing between the three is that two different groups of SMBHs can produce large amounts of x-rays. Very massive SMBHs can produce a lot of x-rays, but so do rapidly accreting SMBHs. Wedding cake design helped get around this by incorporating information from a variety of research. The researchers also combined observations from different wavelengths, including optical and infrared, which helped them measure the abundance of black holes and accretion rates.
Yu and his colleagues found that the first explanation best explained the observations. SMBH is adding content more slowly than in the past. They usually feed on cold gas, which is also what stars form. So the stars may have stolen some of their food.
“It appears that the consumption of black holes has decreased dramatically as the universe ages,” said co-author Niel Brandt, of Penn State University. “Perhaps this is because the cold gas they can consume has decreased since the afternoon.”
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