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Novel Galactic ‘Fossil’: Revealing the Dynamics of Galaxy Evolution

Scientists studying at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, have found X-Ray activity that provides insight into how galaxies evolve.

Novel Galactic ‘Fossil’: Revealing the Dynamics of Galaxy Evolution
Sites of active star formation appear bright pink in this visible light image captured by the European Southern Observatory’s 2.2-meter telescope in Chile. The galaxy’s active core is mostly concealed by a cloud of dust. Image Credit: ESO

The neighboring spiral galaxy NGC 4945 is outlined in X-Rays as massive clouds of frigid gas. About 5 million years ago, the galaxy’s center supermassive black hole exploded, causing the gas to shoot through the galaxy.

There is an ongoing debate in the scientific community about how galaxies evolve. “We find supermassive black holes in the centers of nearly all Milky Way-sized galaxies, and an open question is how much influence they have compared to the effects of star formation. 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.

Kimberly Weaver, Program Scientist, Goddard Space Flight Center, NASA

On January 11th, 2023, during the 243rd meeting of the American Astronomical Society in New Orleans, Weaver gave a presentation of the findings on behalf of her team. The Astrophysical Journal is now reviewing a manuscript regarding this discovery. Data gathered by NASA’s Chandra X-Ray Observatory and the European Space Agency’s XMM-Newton (X-Ray Multi-Mirror Mission) satellite made the study feasible.

An active galaxy in the southern constellation of Centaurus, NGC 4945 is located around 13 million light-years distant.

An active galaxy features a supermassive black hole at its core, which is extraordinarily bright and variable and uses friction and gravity to heat a disk of gas and dust around it. The material around the black hole is gradually consumed by it, causing unpredictable oscillations in the light that is released by the disk.

Some of the light is obscured by a thick cloud of dust termed a torus, which covers the black hole and disk of NGC 4945, as is the case with most active galaxies.

Moreover, the cores of active galaxies are capable of producing powerful winds that carry gas and dust as well as jets of fast particles.

As a starburst galaxy, NGC 4945 produces stars far more quickly than the Milky Way does. It is estimated by scientists to create annually the mass equivalent of eighteen Sun-like stars, or approximately three times the pace of the Milky Way. The core of the galaxy is where almost all of the star creation is centered. The duration of a starburst event ranges from 10 to 100 million years, and it ends only when the resources needed to create new stars run out.

NASA's XMM-Newton project scientist Weaver and her group used the satellite to examine NGC 4945. They saw what is known as the iron K-alpha line in their data. This characteristic arises from the interaction of cold gas somewhere with extremely intense X-Ray light from the black hole’s disk. (The gas has a temperature of around –200 Celsius, or minus 400 Fahrenheit.)

Although the iron line is frequently seen in active galaxies, scientists had previously believed that it happened on scales far closer to the black hole.

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

Jenna Cann, Study Co-Author and Postdoctoral Researcher, Goddard Space Flight Center, NASA

XMM-Newton was able to map the breadth of NGC 4945's iron line both along and above the galaxy’s plane since it is inclined almost edge-on from our point of view. The iron line was traced out to 32,000 and 16,000 light-years, respectively, which is an order of magnitude further than previously reported iron lines.

According to the scientific team, the cold gas indicated by the line is believed to be a remnant of a particle jet that erupted from the core black hole of the galaxy some 5 million years ago. It is possible that the jet was pointed toward the galaxy rather than into space, creating a supercharged wind that continues to force cold gas across the galaxy. It could be the reason behind the present starburst phenomenon.

Weaver and her colleagues will continue to examine NGC 4945 to see if they can find out how the black hole affects the galaxy’s evolution in other ways. The same disk X-Rays that are presently emphasizing the cold gas may also begin to disperse it. Scientists may be able to assess how activity near a galaxy’s black hole can quench its starburst phase since stars require that gas to develop.

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. 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.

Edmund Hodges-Kluck, Study Co-Author and Astrophysicist, Goddard Space Flight Center, NASA

Launched from Kourou, French Guiana, in December 1999, the XMM-Newton observatory is operated by ESA. NASA provided funding for several components of the XMM-Newton equipment set and the NASA Guest Observer Facility at Goddard, which facilitates American astronomers’ use of the telescope.

Finding a New Galactic "Fossil"

Some 5 million years ago, a black hole eruption in the galaxy NGC 4945 set off a star-formation frenzy and shot a vast cloud of gas into intergalactic space. Watch and learn how two X-ray telescopes revealed the story. Video Credit: NASA’s Goddard Space Flight Center


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