Universe Today
As our powerful infrared telescopes allow astronomers to peer further and further back in time, they've discovered some puzzling things. One of them concerns supermassive black holes (SMBH), the physics-challenging behemoths at the center of large galaxies like the Milky Way. As it turns out, SMBH grew much more rapidly at high redshifts than they do in the contemporary Universe.
The period of time in the Universe's evolution called the Cosmic Noon saw very rapid SMBH growth. Since then, SMBH growth has slown. The Cosmic Noon, which spanned from about 2 billion years after the Big Bang to about 4 billion years after, was also a period of peak star formation activity.
In new research in The Astrophysical Journal, astrophysicists found that modern SMBH simply can't accrete material as rapidly in the past. It's largely due to the unavailability of cold gas, the SMBH's preffered diet. Astrophysicists sometimes call the phenomenon "AGN downsizing."
The research is titled "The Drivers of the Decline in Supermassive Black Hole Growth at z < 2." The 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." the authors write. There are tight correlations between a SMBH's mass and the mass of the host galaxy's bulge and its velocity dispersion. There is also a correlation between a SMBH's long-term average accretion rate and the host galaxy's total stellar mass and the star formation rate in its bulge.
"These relations indicate that SMBHs and host galaxies evolve in a coordinated manner," the researchers explain. "Therefore, tracing the growth history of SMBHs can provide insights into the mechanism that drives galaxy–SMBH coevolution."
Scientists know that SMBH growth peaked at about z ≈ 1.5–2 and then declined toward the present day. z ≈ 1.5–2 corresponds to around 9.5 to 10.5 billion years ago, when the Universe was only about 3 to 4 billion years old. There were more high-luminosity AGN at higher redshifts. The question is, why the big slowdown?
“A longstanding mystery has been the cause of this big slowdown,” said lead author Yu in a press release. “With these X-ray data and supporting observations at other wavelengths, we can test different ideas and narrow down the answer.”
X-ray observations from Chandra played an important role in this work. When a SMBH is actively accreting material, it's called an active galactic nuclei (AGN). When an AGN is consuming matter, the matter first forms a rotating ring around the SMBH. The material in the ring heats up and emits x-rays, which Chandra can observe. Comparisons of x-ray emissions from different AGN at different redshifts indicate how rapidly they're accreting material and growing. The more x-rays an AGN emits, the more rapidly the SMBH is growing.
*This artist's illustration shows a black hole surrounded by an accretion ring of material. As the material in these rings heat up they emit x-rays. Observing these x-rays is an important part of measuring SMBH and AGN. In this illustration, the SMBH is also launching a jet. Image Credit: NASA/JPL-Caltech*
In this work, Yu and his co-researchers analyzed the observations of about 1.3 million galaxies and 8,000 SMBH. The observations are from Chandra, as well as from other space telescopes, the XMM-Newton Observatory and eROSITA.
The researchers employed what's known as a "wedding-cake design" in their study. This means that the data they used came from different sources. They used shallow surveys of large areas of the sky—called "pencil-beam studies)— all the way down to extremely deep studies of much smaller sky regions. These tiered surveys and studies are kind of like the different layers in a wedding cake.
XMM-Newton and eROSITA generated data for the middle and bottom tiers with wide, shallow observations. Chandra contributed the top of the cake with deep observations that covered a smaller area. This allowed the x-ray telescope to detect fainter and more distant growing SMBH.
“By combining these data from different X-ray telescopes, we can construct a better picture of how these black holes are growing than any one telescope could do alone,” 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 hectic to leisurely to glacial.”
There are three different reasons that could potentially explain the slowdown in SMBH growth.
The first is that the rate of accretion is simply slower now for some reason. The second is that typical SMBH masses are lower now. The third is that there are simply fewer actively accreting SMBH now.
*This illustration shows the three explanations being considered for the decline in SMBH growth. The top panel shows slower rates of consumption, the middle panel shows smaller typical black hole masses, and the bottom shows fewer actively growing SMBH. Image Credit: Penn State/Z.Yu*
Part of the difficulty in differentiating between the three is that two different categories of SMBH can produce more x-ray emissions. SMBH that are more massive can produce lots of x-rays, but so can lower-mass SMBH that happen to be accreting more rapidly. The wedding-cake layer design helped get around this by including data from different types of surveys. The researchers also combined observations from different wavelengths, including optical and infrared, that helped them measure black hole masses and accretion rates.
Yu and his colleagues found that the first explanation explains the observations best. SMBH are simply accreting matter more slowly than in the past. They mostly feed on cold gas, which is also what stars form from. So it may be that stars have stolen some of their food.
"It appears that black holes’ consumption of material has greatly slowed down as the universe has aged,” said co-author Niel Brandt, also of Penn State University. “This is probably because the amount of cold gas available for them to ingest has decreased since cosmic noon."
