NASA scientists have observed a stunning cosmic fossil that was created when a supermassive black hole erupted over 5 million years ago.
The team at the space agency's Goddard Space Flight Center used the X-ray Multi-Mirror Mission (XMM-Newton) space telescope with help from NASA's Chandra X-ray Observatory to spot the X-rays that outline cold gas clouds in the relatively close spiral galaxy NGC 4945, 13 million light-years away in the constellation of Centaurus (the Centaur).
The gas appears to have ripped through the galaxy when it blasted from around the central black hole, which has a mass equivalent to around 1.6 million suns and currently feeds on matter surrounding it, powering the bright galactic engine called an active galactic nucleus (AGN).
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This gas that has been arranged by this supermassive black hole is the raw material that collapses to create stars. Thus, these findings could help scientists better understand how supermassive black holes, which can grow to masses billions of times that of the sun, influence their surroundings and guide galactic evolution.
"There's 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," Goddard astrophysicist and team leader Kimberly Weaver said in a statement. 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."
An active star birthing galaxy
While most galaxies have a supermassive black hole at their hearts, not all of these cosmic titans are accreting matter, nor is their gravitational influence churning surrounding matter, generating the violent conditions needed for AGN.
Galaxies like NGC 4945 that possess AGNs, which can often outshine every star in the entire galaxy combined are called "active galaxies." But sometimes the thick dust in the shape of a donut or "torus" in the center of these active galaxies is dense enough to shroud at least some of the light from the AGN from outside observers.
Light isn't the only emission from the AGNs at the heart of active galaxies, though; matter that isn't fed to the black hole to be treated upon, or "accreted," can be channeled by magnetic fields to the poles of the black hole. From here, these particles are accelerated to speeds approaching that of light and blasted out as jets that can stretch for thousands of light years. Additionally, particles can escape the AGNs in active galaxies via powerful winds of gas and dust.
Possessing a supermassive black hole-powered AGN isn't the only thing that separates NGC 4945 from other more sedate galaxies like the Milky Way. This nearby spiral galaxy is also classified as a "starburst galaxy," indicating it is undergoing an intense burst of star formation.
It is estimated that NGC 4945 is forming stellar bodies at a rate of 18 sun-like stars each year, three times faster than the star formation rate of the Milky Way. This star birth is mainly centered in the heart of the galaxy and will only end when the raw material of stellar birth, cool and dense gas clouds, is exhausted. This process is expected to take between 10 million and 100 million years.
Spotting a misplaced cosmic fossil in NGC 4945
Looking at NGC 4945 with the European Space Agency (ESA) telescope XMM-Newton, Weaver and team spotted a feature called the iron K-alpha line, which occurs when high energy X-ray light from the matter around the central supermassive black hole's disk hits cold gas elsewhere in the galaxy.
The iron line is commonly seen in galaxies with an AGN, but this observation took the team by surprise because it wasn't as close to the central black hole as they expected, and as existing theories suggest it should be.
The fact that NGC 4945 is seen edge-on from the vantage point of Earth meant that NASA scientists were able to track the iron line out 32,000 light years over the galactic plane and 16,000 years across the tilted galaxy.
"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," team member and postdoctoral researcher at Goddard Jenna Cann said. "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."
The team theorizes that the cause of this extraordinary feature is jets of particles erupting from the supermassive black hole of NGC 4945, 5 million years ago. They think this jet was probably orientated so it blasted through the galaxy rather than escaping from the galactic plane.
This eruption caused a super-strong wind that still blows in NGC 4945 as we see it today, and it could also have triggered the galaxy's intense starburst period.
The team will now continue to observe NGC 4945 and search for other clues showing how the central black hole is influencing the galaxy's evolution. They will also investigate if the X-rays from the central region could dissipate cold gas, eventually drawing a close to intense star formation and quenching NGC 4945's starburst phase.
"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," team member and Goddard astrophysicist Edmund Hodges-Kluck said. "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." Hodges-Kluck continued.
Weaver presented the team's findings at the 243rd meeting of the American Astronomical Society in New Orleans, Louisiana, on Wednesday, Jan. 11.
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