Supermassive black holes (SMBHs) occupy the center of galaxies, with masses ranging from one million to 10 billion solar masses. Some SMBHs are in a bright phase called active galactic nuclei (AGN).
AGNs will eventually die out because there is a maximum mass limit for SMBHs; scientists have long wondered when this will be the case.
Kohei Ichikawa of Tohoku University and his research group may have discovered an AGN near the end of its lifespan by accident after picking up an AGN signal from the Arp 187 galaxy.
By observing the radio images in the galaxy using two astronomical observatories – the Atacama Large Millimeter / submillimeter Array (ALMA) and the Very Large Array (VLA) – they found a jet lobe, a sign distinctive of AGN.
However, they did not notice any signal from the nucleus, indicating that AGN activity might already be silent.
After further analysis of the multi-wavelength data, they found that all of the small-scale AGN indicators were silent, while the large-scale ones were bright. Indeed, AGN has recently been extinguished over the past 3,000 years.
Once an AGN goes out, the smaller scale AGN characteristics become weak because other sources of photons also shut down. But the large-scale ionized gas region is still visible as it takes around 3,000 years for photons to reach the edge of the region. Observing past AGN activity is known as a light echo.
“We used NASA’s NuSTAR X-ray satellite, the best tool to observe current AGN activity,” Ichikawa said. “This allows non-detection, so we were able to find out that the nucleus is completely dead.”
The results indicate that AGN deactivation occurs over a 3,000-year time scale and that the nucleus becomes more than 1,000 times weaker over the past 3,000 years.
Ichikawa, who co-wrote an article for the 238th meeting of the American Astronomical Society, said they would continue to investigate dying AGNs in the future. “We will be looking for more dying AGNs using a method similar to that in this study. We will also obtain the high spatial resolution tracking observations to study gas in and out, which could clarify how the AGN activity shutdown has occurred.
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