“Galactic fossil” might yield clues about black holes

Astronomers have found new X-ray evidence that can help to explain how supermassive black holes help form the galaxies swirling around them.

The US team has found X-rays around cold gas clouds in a nearby galaxy that appear to be the by-product of a black hole eruption.

“There’s ongoing debate in the scientific community about how galaxies evolve,” says research lead Dr Kimberly Weaver, an astrophysicist at NASA’s Goddard Space Flight Center.

“We find supermassive black holes in the centres of nearly all Milky Way-sized galaxies, and an open question is how much influence they have compared to the effects of star formation.”

Weaver presented the team’s research on spiral galaxy NGC 4945, which is 13 million light-years away, at the 243rd meeting of the American Astronomical Society.

“Studying nearby galaxies like NGC 4945, which we think we’re seeing in a transition period, helps us build better models of how stars and black holes produce galactic changes,” says Weaver.

NGC 4945 is an “active galaxy”: the behaviour of the supermassive black hole at its centre gives it bright and variable behaviour.

The researchers used the European Space Agency’s XMM-Newton satellite to examine NGC 4945’s X-ray activity, along with NASA’s Chandra X-ray observatory.

Coloured x-ray image
This image shows spiral galaxy NGC 4945 in X-rays. The contours represent the iron K-alpha line observed by the XMM-Newton’s EPIC instrument. Brighter colors indicate greater concentrations of X-rays. Credit: Weaver et al. 2024, ESA/XMM-Newton

They found a signal called the “iron K-alpha line”, which is caused by X-rays from a black hole running into very cold gas (-200°C).

These lines have been seen in active galaxies before, but never so distant from the black hole that made them.

“Chandra has mapped iron K-alpha in other galaxies. In this one, it helped us study individual bright X-ray sources in the cloud to help us rule out other potential origins besides the black hole,” says Dr Jenna Cann, a postdoctoral researcher at Goddard.

“But NGC 4945’s line extends so far from its centre that we needed XMM-Newton’s wide field of view to see all of it.”

The researchers think that the cold gas is the remnants of a jet ejected by the black hole 5 million years ago. This jet could have triggered the galaxy’s current “starburst” era, with a very high rate of star forming.

“There are a number of lines of evidence that indicate black holes play important roles in some galaxies in determining their star formation histories and their destinies,” says Dr Edmund Hodges-Kluck, an astrophysicist at Goddard.

“We study a lot of galaxies, like NGC 4945, because while the physics is pretty much the same from black hole to black hole, the impact they have on their galaxies varies widely.

“XMM-Newton helped us discover a galactic fossil we didn’t know to look for – but it’s likely just the first of many.”

Sign up to our weekly newsletter

Please login to favourite this article.