It was recently in the news that astronomers spotted the oldest, most distant black hole since the Big Bang. How do they know it is the oldest and most distant?
Heather
Mazatlán, Sinaloa, Mexico
A research team led by Akos Bogdan of the Center for Astrophysics | Harvard & Smithsonian discovered a supermassive black hole (SMBH) in UHZ1, a galaxy whose light has taken 13.2 billion light-years to reach us. This means astronomers are observing it as it was 13.2 billion years ago, when the universe was only 3 percent its current age, making it the oldest black hole yet detected.
The concept of distance is tricky to define when dealing with vast stretches of both space and time. Thus, it’s simplest to say that UHZ1’s light-travel distance is 13.2 billion light-years, though that does not account for the universe’s expansion during the time the light has been traveling toward us. And a galaxy’s estimated distance and age will vary depending on which cosmological model you use. But under any model, because UHZ1 has a longer light-travel time than any other known black hole, it is also the most distant.
The physical characteristic of a galaxy that astronomers measure — from which they calculate its distance and age — is redshift: the elongation of light waves that results from a celestial body’s movement away from us. This is similar to the Doppler effect we experience with sound. When a police siren moves away from us, the frequency at which sound waves reach us is reduced, which we perceive as a lower pitch. The same principle applies to light.
The fact that all distant galaxies are receding from us is due to the expansion of the universe. The farther away a galaxy is, the more space there exists between us to expand, meaning more distant objects appear to be moving away from us faster than closer objects. This principle, known as the Hubble flow, enables scientists to estimate the distances of galaxies: The higher a galaxy’s redshift, the faster it is receding and the farther it is from us. Astronomers denote redshift as a value called z. The z value of UHZ1 is 10.1, corresponding to its light-travel time of 13.2 billion years in standard cosmological models.
An additional stroke of luck enabled Bogdan’s research team to detect this SMBH: The galaxy cluster Abell 2744 happens to lie between us and UHZ1, at a distance of nearly 4 billion light-years, along the same line of sight. The gravity of Abell 2744’s substantial mass is bending light from UHZ1 around it, focusing it like a magnifying glass. As a result of this gravitational lensing, UHZ1 appears four times brighter than it normally would. This allowed the James Webb Space Telescope and the Chandra X-Ray Observatory to collect data that led the team to identify a source of X-rays as UHZ1’s central black hole.
Edward Herrick-Gleason
Planetarium Director, Southworth Planetarium, University of Southern Maine, Portland, Maine
