Supermassive black holes (SMBH) occupy the center of galaxies with masses ranging from one million to 10 billion solar masses. Some SMBHs are in a light phase called active galactic nuclei (AGNs).
AGNs will eventually burn out as there is a maximum mass limit for SMBHs; researchers have long since considered when it will be.
By observing the radio images in the galaxy using two astronomy observatories – Atacama Large Millimeter / submillimeter Array (ALMA) and Very Large Array (VLA) – they found a jetlobe, a hallmark of AGN.
However, they noticed no signal from the core, indicating that the AGN activity may already be quiet.
After further analysis of multi-wavelength data, they found that all small AGN indicators were quiet while the large ones were bright. This is because AGN has recently been turned off within the last 3,000 years.
When an AGN dies, smaller AGN functions become weak because additional photon supplies are also shut down. But the large-scale ionized gas region is still visible, as it takes about 3,000 years for photons to reach the edge of the region. Observation of previous AGN activity is known as light echo.
“We used NASA NuSTAR X-ray satellite, the best tool for observing current AGN activity, ”said Ichikawa. “It enables non-detection, so we were able to detect that the core is completely dead.”
The results indicate that AGN extinction occurs within a 3000-year time scale, and the core becomes more than 1000 times weaker over the last 3000 years.
Ichikawa, who co-authored a paper for the 238th meeting of the American Astronomical Society, says they will continue to investigate dying AGNs going forward. “We will search for more dying AGNs using a similar method as this study. We also obtain follow-up observations with high spatial resolution to examine the gas inflow and outflow, which can clarify how the shutdown of the AGN activity has taken place.
Meeting: AAS 238
Funding: Program for the establishment of a consortium for the development of human resources in science and technology, JST, JSPS KAKENHI