Finding A 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.

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

NGC 4945 is an active galaxy 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 material around it, which creates random fluctuations in the disk’s emitted light. As with most active galaxies, NGC 4945’s black hole and disk are shrouded by a dense cloud of dust called a torus, which blocks some of that light.

The cores of active galaxies can also drive jets of high-speed particles and generate strong winds containing gas and dust.

NGC 4945 is also a starburst galaxy, which means it forms stars at a much higher rate than our own. Scientists estimate it produces the equivalent mass of 18 stars like our Sun every year, or nearly three times the rate of the Milky Way. Almost all the star formation is concentrated in the galaxy’s center. A starburst event lasts between 10 and 100 million years, ending only when the raw material to make new stars is depleted.

Scientists looked at NGC 4945 with XMM-Newton (X-ray Multi-Mirror Mission). In their data, they saw what scientists call the iron K-alpha line. This feature occurs when very energetic X-ray light from the black hole’s disk meets cold gas elsewhere. (The gas measures around 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 black hole.

Because NGC 4945 is tilted nearly edge-on from our point of view, XMM-Newton was able to map the extent of its iron line both along and above the galaxy’s plane, tracing it out to 32,000 and 16,000 light-years, respectively – an order of magnitude farther than previously observed iron lines.

The science team thinks the cold gas highlighted by the line is a relic of a particle jet erupting from the galaxy’s central black hole about 5 million years ago. The jet was likely angled into the galaxy rather than pointing into space, driving a superpowered wind that’s still pushing cold gas through the galaxy. It may even have triggered the current starburst event.

See https://science.nasa.gov/centers-and-facilities/goddard/nasa-scientists-discover-a-novel-galactic-fossil/