Key Takeaways:
- Supermassive black holes, like the one in galaxy Arp 187, can host active galactic nuclei (AGN) that emit powerful radiation and jets of ionized gas.
- Researchers discovered a “dying” AGN in Arp 187, showing signs of a core that recently went dark.
- Observations from telescopes like ALMA, VLA, and NuSTAR reveal radio jets but no active core.
- Arp 187’s AGN still displays 3,000-light-year jets, marking a “light echo” of its past activity.
- This rare discovery could unlock new insights into the process of AGN shutdown.
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Astronomers captured a glimpse of Arp 187’s fading AGN, which “died” thousands of years ago.
The End of a Supermassive Black Hole’s Active Phase
At the heart of galaxies like the Milky Way, supermassive black holes lie dormant or active, with some powering what astronomers call active galactic nuclei (AGN). These AGN emit powerful jets of radiation, sometimes stretching thousands of light-years from the galaxy’s center, and can produce intense X-rays and radio waves. Despite the significance of AGN in the cosmos, scientists have never fully understood how and when these powerful cores shut down. A new discovery may provide key insights, thanks to research led by astronomer Kohei Ichikawa of Tohoku University, who identified what could be a rare glimpse of a dying AGN.
Catching a Glimpse of Arp 187’s Dying Core
The galaxy Arp 187, which lies far beyond our Milky Way, has presented astronomers with a unique opportunity to study a supermassive black hole entering its last phase. Ichikawa’s team used powerful radio telescopes at the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the Very Large Array (VLA) in New Mexico to study Arp 187’s nucleus. They detected the signature twin jet lobes of an AGN but surprisingly couldn’t find radio signals from the galaxy’s core, indicating a lack of recent AGN activity.
Intrigued by this discrepancy, the researchers further analyzed the galaxy with NASA’s NuSTAR X-ray satellite, a telescope known for its capability to observe high-energy phenomena like AGN. Their results showed that Arp 187’s core no longer emits X-rays, a key sign of AGN activity. This led the researchers to conclude that the AGN had “died” within the last several thousand years, meaning its core no longer actively fuels the jets or emits X-rays, leaving only the traces of its past activity.
Observing the “Light Echo” of a Dying AGN
What makes this discovery possible, even with the AGN core no longer active, are the massive jets that stretch over 3,000 light-years from the galaxy’s center. These jets, visible due to the “light echo” they create, act like smoke from an extinguished fire, allowing astronomers to observe the remnants of AGN activity even after the core has gone dark. This period, sometimes called the “mourning phase,” could last for millennia as the jets slowly dissipate, allowing scientists a unique insight into the fading phase of supermassive black holes.
Ichikawa and his colleagues consider the discovery of Arp 187’s dying AGN “serendipitous.” With further research, they hope to find similar cases to deepen our understanding of how AGN power down and what signals we can detect as they transition into dormancy. In the future, Ichikawa’s team plans to conduct high-resolution follow-up studies to explore how gas inflows and outflows influence the shutdown process, potentially revealing more about the lifecycle of these cosmic giants.
The findings, presented at the 238th meeting of the American Astronomical Society, open up new avenues to understand the lifespans of AGN and the impact they have on their surrounding galaxies, showing that even the end of such an entity’s active life offers valuable clues to the evolution of galaxies.
