This dying supermassive black hole stretched out 3,000 light-years "light echo", astronomers observed

Key Takeaways:

These black holes, at the center of galaxies, can be super active and emit large amounts of radiation, fueling the galaxy.
They typically emit radio waves and X-rays, and often have jets of ionized gas shooting out.

Like everything else, even these powerful galactic cores eventually turn off.
Scientists observed a galaxy with jets (a sign of a past AGN) but no radio waves or X-rays from the core, suggesting the central engine is no longer active.
Studying more galaxies like Arp 187 could reveal how and why AGNs shut down.

This radio band composite image of Arp 187, obtained by the VLA and ALMA telescopes (blue: VLA 4.86 GHz, green: VLA 8.44 GHz, red: ALMA 133 GHz), shows clear bimodal jet lobes. But the central nucleus (center of the image) is dark. (Image credit: ALMA (ESO/NAOJ/NRAO), Ichikawa et al.)

Supermassive black holes, which have masses millions or even billions of times that of the sun, are located at the center of galaxies like the Milky Way.

Scientists indicate to some of those supermassive black holes as active galactic nuclei (AGN), and they are known to emit large amounts of radio waves and X-rays. AGN are responsible for the twin jets of ionized gas you see shooting away in pictures of many galaxies.

Every AGN must eventually shut off, just as everything else must. However, scientists are still unsure of exactly how and when that occurs. At Tohoku University in Sendai, Japan, astronomer Kohei Ichikawa is leading a team of researchers who may have discovered a clue. Observing the far-off galaxy Arp 187, those scientists have detected what they believe to be an AGN in its final stages.

Ichikawa et al. used radio telescopes at the Very Large Array (VLA) in New Mexico and the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile to observe Arp 187. Twin jet lobes, an obvious indicator of an AGN, were observed. But they couldn’t detect radio waves, which also should have been coming from an active nucleus.

The observational difference between a standard AGN (left) and a dying AGN (right) discovered by Ichikawa et al. In the dying AGN, the nucleus is very faint in all wavelength bands because AGN activity is already dead, while the extended ionized region is still visible for about 3,000 light-years since it takes about 3,000 years for the light to cross the extended region. (Image credit: Ichikawa et al.)

Thus, the scientists used NASA’s NuSTAR (“Nuclear Spectroscopic Telescope Array”) X-ray satellite to get a closer look at Arp 187’s core. The team reported in a study earlier this month at the virtual 238th meeting of the American Astronomical Society that while AGN typically produce an abundance of X-rays, no such signal appears in the NuSTAR data.

Thus, the researchers conclude that the AGN of Arp 187 has darkened at some point during the last few thousand years (as seen from Earth).

The massive jets of an AGN allow for this observation. The 3,000 light-year distribution of Arp 187s means that their matter stream continues for millennia after the AGN core “dies.” This time of mourning is known to astronomers as a “light echo.” It resembles the smoke from a freshly put out fire.

According to the researchers, their finding was “serendipitous.” Arp 187 could be a stepping stone to learning more about what happens at the end of an AGN’s life, study team members said.

Ichikawa stated in a statement, “We will use a similar method as this study to search for more dying AGN.” “We will also obtain the high spatial resolution follow-up observations to investigate the gas inflows and outflows, which might clarify how the shutdown of AGN activity has occurred.”