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Astronomers discover dying echoes of a supermassive black hole

Astronomers discover dying ech...
An artist's illustration of an active galactic nucleus (AGN), complete with large radio jets. Galaxy Arp 187 has the remains of these jets but the center has fallen silent
An artist's illustration of an active galactic nucleus (AGN), complete with large radio jets. Galaxy Arp 187 has the remains of these jets but the center has fallen silent
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An artist's illustration of an active galactic nucleus (AGN), complete with large radio jets. Galaxy Arp 187 has the remains of these jets but the center has fallen silent
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An artist's illustration of an active galactic nucleus (AGN), complete with large radio jets. Galaxy Arp 187 has the remains of these jets but the center has fallen silent
A diagram comparing a normal active galactic nucleus (AGN) (left) with the anomalous Arp 187 (right), which seems to be dying
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A diagram comparing a normal active galactic nucleus (AGN) (left) with the anomalous Arp 187 (right), which seems to be dying
The location of the black hole at the center of the galaxy Arp 187 is circled. No signal was detected, indicating it's gone quiet
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The location of the black hole at the center of the galaxy Arp 187 is circled. No signal was detected, indicating it's gone quiet
The radio lobes of the galaxy Arp 187 are clear in this composite radio image from ALMA and VLA data. The now-silent black hole lies in the center
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The radio lobes of the galaxy Arp 187 are clear in this composite radio image from ALMA and VLA data. The now-silent black hole lies in the center
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Japanese astronomers have discovered the echoes of a “dying” supermassive black hole. While the object is quiet now, the team spotted the signatures of two huge radio jets that indicate it only recently fell silent after a bright, active phase.

It’s thought that supermassive black holes lurk at the center of most galaxies. Some of them are more outgoing than others – the one at the heart of our own Milky Way, for instance, is pretty calm. But others work overtime, throwing off huge amounts of light and radiation as they chow down on matter. These are known as active galactic nuclei (AGN), or quasars if they’re particularly bright.

The galaxy Arp 187 looks fairly quiet nowadays, but apparently that wasn’t always the case. In fact, rather recently it seems to have been an AGN, according to researchers at Tohoku University.

The team observed Arp 187 with two radio telescopes, ALMA and the Very Large Array (VLA), and found a strange sight that didn’t belong – two huge lobes of radio emissions. They resemble those thrown out by AGN, but unlike a garden variety AGN the black hole at the center was silent.

The radio lobes of the galaxy Arp 187 are clear in this composite radio image from ALMA and VLA data. The now-silent black hole lies in the center
The radio lobes of the galaxy Arp 187 are clear in this composite radio image from ALMA and VLA data. The now-silent black hole lies in the center

Next the team looked closer at data that captured multiple wavelengths of radiation, including radio, mid-infrared and X-rays. That confirmed that all the usual small-scale signs of AGN activity had ceased, but the large-scale ones, the lobes, were still visible. That suggests the activity had shut down sometime in the last few thousand years – very recently, in cosmic terms.

That time delay is because the lobes themselves span about 3,000 light-years from end to end, meaning that it would take that long for them to fade away entirely after the source stops.

A diagram comparing a normal active galactic nucleus (AGN) (left) with the anomalous Arp 187 (right), which seems to be dying
A diagram comparing a normal active galactic nucleus (AGN) (left) with the anomalous Arp 187 (right), which seems to be dying

"We used the NASA NuSTAR X-ray satellite, the best tool to observe current AGN activity," says Kohei Ichikawa, lead reseacher on the study. "It enables non-detection, so we were able to discover that the nucleus is completely dead.”

The study can help astronomers better understand the life and death cycles of these AGN, and the timeframes that these transitions occur on. Previous research has seen calm galaxies suddenly fire up into raging quasars far quicker than thought possible. And even the Milky Way seems to have gone through these extremely active phases during its lifespan.

The research was originally published in the Astrophysical Journal, and recently presented at the 238th Meeting of the American Astronomical Society.

Source: Tohoku University

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