NASA scientists discover new galactic ‘fossil’

Researchers at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, have discovered X-ray activity that sheds light on the evolution of galaxies.

X-rays paint giant clouds of cold gas in the nearby spiral galaxy NGC 4945. The gas appears to have passed through the galaxy after its central supermassive black hole erupted about 5 million years ago.

“There is an ongoing debate in the scientific community about how galaxies evolve,” said Kimberly Weaver, an astrophysicist at Goddard who led the work.

“We find supermassive black holes at the centers of almost all Milky Way-sized galaxies, and an open question is how much influence they have in relation to the effects of star formation. By studying nearby galaxies like NGC 4945, which we think we see in a transition period, helps us build better models of how stars and black holes produce galactic changes.”

Weaver presented the results on behalf of his team at the 243rd meeting of the American Astronomical Society in New Orleans on January 11. An article on this discovery is currently under review by The Astrophysics Journal.

The work was made possible thanks to data collected by the ESA (European Space Agency) XMM-Newton (X-ray Multi-Mirror Mission) satellite with the help of NASA’s Chandra X-ray Observatory.

NGC 4945 is an active galaxy located about 13 million light years away in the southern constellation Centaurus.

An active galaxy has an unusually bright and variable center, powered by a supermassive black hole that heats a surrounding disk of gas and dust through gravitational and frictional forces. The black hole slowly consumes the matter around it, which creates random fluctuations in the light emitted by the disk. As with most active galaxies, the black hole and disk of NGC 4945 are enveloped by a dense cloud of dust called a torus, which blocks some of this light.

The cores of active galaxies can also propel jets of particles at high speeds and generate strong winds containing gas and dust.

NGC 4945 is also a star galaxy, which means it forms stars much faster than ours. Scientists estimate that it produces the mass equivalent to 18 stars like our sun each year, almost three times the speed of the Milky Way. Almost all star formation is concentrated in the center of the galaxy. A star explosion lasts between 10 and 100 million years and only ends when the raw material needed to make new stars is exhausted.

Weaver, an XMM-Newton project scientist at NASA, and his team observed NGC 4945 with the satellite. In their data, they saw what scientists call the K-alpha line of iron. This feature occurs when highly energetic X-rays from the black hole’s disk encounter cold gas elsewhere. (The gas measures about minus 400 degrees Fahrenheit or minus 200 Celsius.) The iron line is common in active galaxies, but until these observations, scientists previously thought it occurred on scales much closer to the hole black.

“Chandra has mapped K-alpha iron in other galaxies. In this one, it helped us study individual sources of bright X-rays in the cloud to help us rule out other potential origins besides the black hole ” said Jenna Cann, co-author and postdoctoral researcher at Goddard. “But NGC 4945’s line extends so far from its center that we needed XMM-Newton’s wide field of view to see everything.”

Because NGC 4945 is almost edge-on tilted from our perspective, XMM-Newton was able to map the extent of its iron line along and above the plane of the galaxy, tracing it back to 32,000 and 16,000 light years, respectively, an order of magnitude further than the previously observed iron lines.

The science team believes the cold gas highlighted by the line is a relic of a jet of particles bursting from the galaxy’s central black hole about 5 million years ago. The jet was likely oriented toward the galaxy rather than pointed toward space, resulting in a super-powerful wind that still pushed cold gas through the galaxy. It may have even triggered the current starburst event.

Weaver and his colleagues will continue to observe NGC 4945 to see if they can discover other ways in which the black hole affects the galaxy’s evolution. The same X-rays from the disk that currently highlight the cold gas could also begin to dissipate it. Since stars would need this gas to form, scientists might be able to measure how activity around a galaxy’s black hole can extinguish its stellar explosion phase.

“There is a number of evidence indicating that black holes play an important role in some galaxies in determining their star formation history and fate,” said co-author Edmund Hodges-Kluck, an astrophysicist at Goddard.

“We study a lot of galaxies, like NGC 4945, because even though the physics is roughly the same across black holes, the impact they have on their galaxies varies greatly. XMM-Newton told us helped discover a galactic fossil that we haven’t discovered.” I don’t know how to search, but this is probably just the first of many.