Chandra Explains Why Black Hole Growth Slowed Since Cosmic Noon
Zhibo Yu
We welcome Zhibo Yu as our guest blogger. He is the first author of a paper that is the subject of our latest Chandra press release and is a fourth-year PhD student in the Department of Astronomy and Astrophysics at Penn State University, where he works with Prof. Niel Brandt on surveys of active galactic nuclei. His research primarily focuses on the coevolution of supermassive black holes and their host galaxies. Zhibo joined Penn State in 2022. Before that, he earned his B.S. degree in physics from Fudan University, China, where he worked with Prof. Cosimo Bambi on black-hole X-ray binaries
In the late 1950s, astronomers began to discover powerful radio sources in the sky with very small angular sizes in visible light. These strange “star-like” objects were later known as “quasi-stellar objects” (QSOs), or quasars, which are very far away from our own Galaxy and are extremely luminous. Afterwards, in the early 1970s, Dutch-born American astronomer Maarten Schmidt was among the first to find that quasars appear to be more abundant when they are farther away from us.
Today, astronomers believe that these luminous quasars are powered by actively growing supermassive black holes (SMBHs) in the centers of galaxies. Many growing SMBHs are not as bright as quasars and are generally referred to as active galactic nuclei (AGNs). When astronomers observe distant growing SMBHs, they are also looking back in time because light takes time to travel from these black holes to us. The farther away a black hole is, the earlier in cosmic history we are seeing it.
Thus, what Schmidt found reflects the growth history of the SMBH population. Since then, astronomers have surveyed this growth history back to about 13 billion years ago, just a few hundred million years after the Big Bang. It has been firmly established that SMBH growth was most active about 10 billion years ago, at the so-called “cosmic noon”; since then, their growth has slowed dramatically and continuously to the present day.
A longstanding mystery has been the cause of this major slowdown in SMBH growth. By “SMBH growth,” we refer to the collective behavior of all growing black holes, rather than the growth rate of any individual one. This can be measured by adding up the growth power of black holes at a given cosmic epoch. When a black hole consumes gas to support its growth, the gas heats up and emits X-rays.
In most cases, this is a clear signature of a growing black hole. The brightness of these X-rays reflects the growth power of the black hole, which is generally determined by both how rapidly it accretes material (the accretion rate) and the mass of the black hole. Combined with the number of growing SMBHs detected at the centers of galaxies, we can estimate the total accretion power for a given cosmic epoch. This quantity depends on three factors: accretion rates, black hole masses, and the number of actively growing black holes. Repeating this measurement at different cosmic times allows us to reconstruct the total growth history of the SMBH population.
Previous studies using Chandra and other X-ray telescopes have measured the history of the total accretion power by surveying different parts of the sky, revealing a dramatic slowdown since cosmic noon. Based upon the three factors affecting the cosmic SMBH accretion power, astronomers have proposed three main scenarios for this slowdown: is it caused by decreasing accretion rates, smaller typical black hole masses, or fewer actively growing black holes?
However, it is not easy to distinguish between these scenarios. One key challenge is that both more massive black holes and faster-growing black holes produce brighter X-ray emission. To address this problem, we selected sky areas covered by Chandra, together with those observed by ESA’s XMM-Newton and the German-led eROSITA telescope, which also have data at other wavelengths, including optical and infrared.
The excellent X-ray coverage in these regions allows us to reliably measure black hole growth, while data at other wavelengths provide estimates of the stellar mass of the host galaxy, which, through a scaling relation, can be used to infer the mass of the central black hole.
Supermassive Black Hole Growth Scenarios.
Credit: Penn State/Z.Yu
Our combination of Chandra, XMM-Newton, and eROSITA data forms a “wedding-cake” survey design: Chandra provides the top tier with deep observations over small areas, detecting fainter and more distant growing black holes, while XMM-Newton and eROSITA contribute the middle and bottom tiers with wider but shallower observations. Together, these data, including 1.3 million galaxies and 8000 AGNs, allow us to robustly characterize the slowdown of cosmic SMBH growth and quantitatively show that decreasing accretion rates are the primary driver of this slowdown.
Much remains to be explored. For example, what causes the decrease in accretion rates? This may be due to the declining supply of cold gas available to feed black holes. Alternatively, could the reduced frequency of galaxy mergers in the present-day universe play an important role? Further work is underway, so please stay tuned!