Chandra Views Milky Way Center

Milky Way middle
This 2°-wide mosaic is the most detailed X-ray view of the Milky Way’s central 900 light-years ever seen. Several bright sources are labeled. Click on image for higher resolution view.
Courtesy NASA, UMass, and Q. Daniel Wang.
Astronomers using the Chandra X-ray Observatory have peered deep into the heart of the Milky Way to create the sharpest-ever X-ray view of our galaxy’s center. The image was published in the January 10th Nature.

Q. Daniel Wang and Cornelia C. Lang (University of Massachusetts, Amherst) and Eric V. Gotthelf (Columbia University) took 30 Chandra images, totaling some 94 hours of exposure time, to create a 0.8°-by-2° (about 400- by-900-light-year) panorama of the Milky Way’s middle. They found nearly 1,000 discrete X-ray sources presumed to be white dwarfs, neutron stars, and black holes, as well as active galactic nuclei far in the background. Also apparent are clouds of hot gas.

At visible wavelengths, the core of the Milky Way is hidden by thousands of light-years of opaque dust. Previous studies had found a large amount of X-ray radiation emanating from behind the dusty curtain — in particular, emission lines from extremely hot, highly ionized iron atoms. Without knowing the source, astronomers were left to assume that hot gas was the culprit, which meant that the region’s hottest thin clouds should reach temperatures of some 100 million degrees Kelvin.

The problem was that astronomers didn’t have an instrument with enough resolving power to separate discrete sources from the gas. Before Chandra, the state of the art could discern only vague X-ray blobs. The few individual things astronomers did see weren’t nearly enough to account for the observed radiation. “Only a dozen sources had been identified before Chandra,” says Wang. Thus it was unclear whether the high-energy X-ray emission was coming from diffuse clouds of ultrahot gas or many unresolved discrete point sources.

Astronomers now have their answer. While some of the radiation does come from thin, hot gas (the hazy clouds in the image), the bulk of the high-energy X-rays are from individual objects. After accounting for these sources, Wang and his team found the gas to be about 10 times cooler than previously assumed — a balmy 10 million degrees.

The middle is bustling with activity. There is far more star formation in the center than in the galactic outskirts. The more massive of these new stars are constantly throwing off their outer layers, producing tremendous stellar winds. Shock waves from supernovae are apparent throughout the region too.

One remaining mystery surrounds an unexplained fluorescence of iron atoms a few hundred light-years from the Milky Way’s central, 3-million-solar-mass black hole in the source known as Sagittarius A. While the total X-ray output from matter falling into the black hole is somewhat low (though a small flare was seen during the team’s observations), the more distant fluorescence may be a residual glow from an era when the black hole was several hundred times brighter in X-rays than today. Or maybe cosmic rays from the multiple supernovae are the cause.

The Chandra panorama has helped astronomers gain a “new perspective of the interplay of the various components of our galaxy,” says Wang. Because the centers of galaxies directly influence the outer regions, understanding middles is a crucial step toward understanding galactic evolution and structure as a whole. For example, the Milky Way’s core gas appears to be fountaining above and below the disk and cooling as it falls back onto the surrounding regions. By tracing the history of that gas, astronomers hope to better understand the inner workings of our galaxy and countless others.