This radio band composite image of Arp 187 was acquired by VLA and ALMA telescopes (blue: VLA 4.86 GHz, green: VLA 8.44 GHz, red: ALMA 133 GHz), showing clear double-peak jet lobes. But the central core (the center of the image) is dark. (Image source: ALMA (ESO / NAOJ / NRAO), Ichikawa, etc.)
supermassive black holes are located in the dark centers of galaxies such as the Milky Way, with masses millions or even billions of times the sun.
Some of these supermassive black holes are what scientists call active galactic nuclei (AGN), and they emit large amounts of radiation, such as X-rays and radio waves. AGN is responsible for the double ionized gas jets seen in images of many galaxies.
Everything goes well, and every AGN will be closed one day. But scientists have never fully understood how or when this happens. Now, researchers led by astronomer Kohei Ichikawa from Tohoku University in Sendai, Japan may have found a clue. By observing the distant galaxy Arp 187, these researchers have seen what they think is the last few days of AGN.
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Ichikawa and his colleagues observed Arp 187 using radio telescopes in the Atacama Large Millimeter/Submillimeter Array (ALMA) in northern Chile and the Very Large Array (VLA) in New Mexico. They found double jet lobes, which is a clear sign of AGN. But they cannot detect radio waves and the radio waves should come from an active nucleus.
Observational difference between standard AGN (left) and dying AGN (right) found by Ichikawa et al. In a dying AGN, the nucleus is very weak in all wavebands because the AGN activity has died and the extended ionization region is still visible within about 3,000 light years, because it takes about 3,000 years for light to pass through the region. extended.
Observational difference between standard AGN (left) and dying AGN (right) found by Ichikawa et al. In the dying AGN, the nucleus is very weak in all wavebands because AGN activity has died, and the extended ionization region is still visible within about 3,000 light-years, because it takes about 3,000 years for light to pass through the extended region. (Image source: Ichikawa et al.)
Later, the researchers used NASA’s NuSTAR (“Nuclear Spectroscopic Telescope Array”) X-ray satellite to observe the core of Arp 187 again. A research report published by the team earlier this month at the 238th meeting of the American Astronomical Society stated that AGN usually produces a large amount of X-rays, but no such signals appear in the NuSTAR data.
Therefore, researchers believe that at some point in the past few thousand years (observed from Earth), the AGN of Arp 187 has been obscured.
This observation is possible because the jet of AGN is huge. Arp 187 stretches for 3,000 light years, which means that after the AGN nuclei “dead”, you can see their material flow for thousands of years. Astronomers call this period of mourning the “echo of light.” It’s like seeing the smoke from a fire that has just been extinguished.
Researchers called their discovery “accidental.” Members of the research team say that Arp 187 may be a stepping stone to learn more about what happens at the end of AGN’s life.
“We will use methods similar to this study to find more dying AGNs,” Ichikawa said in a statement. “We will also obtain follow-up observations with high spatial resolution to study the inflow and outflow of gas, which can reveal how the closure of AGN activities occurred.”