A Cosmic Murder Mystery

Image courtesy of the Space Telescope Science Institute.

“…would you believe it? … I stepped right into the middle of a baffling murder mystery and they put me to work…”

–Nick Charles (William Powell), The Thin Man

They say there are a million ways to die in the big city. Sometimes a body goes out quietly. Sometimes, it goes out with a bang.

Either way, it’s our jurisdiction, and we’re on the case.

I can tell you about one we just had. Actually, it’s more like a serial. About once a day, Earth-bound telescopes catch a glimpse of bright flashes coming from distant regions of the universe. Sometimes, if they got enough in them, we can even see the flash with our bare eyes. The Hubble Space Telescope caught one of these things going off (top image).

This bright flash – one last cry before a star shoves off this coil – tells us how it lived.

In this artist’s rendering, jets of high-energy radiation shoot out from a Gamma-ray burst, signaling the death of a massive star. (Image courtesy of NASA/Swift/Mary Pat Hrybyk-Keith and John Jones, via Wikipedia)

Before they die, stars teeter on a delicate balance between gravity and nuclear fusion. Gravity pulls the matter of the star inward – close enough for the atoms to fuse together. The fusion produces the light we see, and it pushes outward on the matter of the star.

But this detente can only last so long. Once a star runs out of nuclear “fuel,” gravity takes over, and fusion slows down. Then, in a few short cosmic moments – as long as you can hold your breath – the dying star gives up more energy than our sun will produce in all its 10 billion years of burning. One more blast, a final scream to let us know it once lived.

These momentary flares are Gamma-Ray Bursts (GRBs), the death throes of some massive stars. At the top of this case file, the series of images shows the afterglow of the GRB (lower middle) next to its host galaxy (center).

Why do these things depart so violently?

Well, it ain’t the colonel with the candlestick.

The biggest clue is the blast. Since we see GRBs from all the way across the universe, the energy released by a GRB must be enormous, more powerful than a supernova. We deduce that much of the star’s mass and energy are converted to light and other particles, like neutrinos, during its final moments. A sketch artist’s rendering shows these jets of energy shooting out from the center of the burst. The only way this could happen is if a whole star’s worth of matter gets pulled close to a black hole in just a few seconds.  

Now we know the perp (a black hole), and that its weapon is gravity. 

We’ve seen these types before. Now, we’re ready, we’re watching for it.

Our detectives developed the DESAlert system as a way to put out an APB to fellow astronomers. When a GRB is detected – for example, by the Swift satellite – DESAlert uses data automatically generated by Swift to find the GRB’s location on the sky.  DESAlert then looks within Dark Energy Survey data for observations in the same region of the sky.  

Massive stars that end with a burst rarely die alone: usually, they’re near or inside a galaxy. Our fellow detectives use information about its location to look for a GRB’s galactic accomplice amongst the line-up of nearby galaxies.

Adriano Poci (left) and Raymond Lam (right), students at Macquarie University in Sydney, Australia, were among the first Dark Energy Detectives to work on the case of the dying stars. They are two of the creators of the computer program that controls the DESAlert “galactic APB” system.

For the suspect galaxies in DES data, we have entire astrophysical profiles – shape, size, brightness, distance from Earth – ready for comparison: we share this data with other GRB detectives, who continue the search, trying to catch the bursters.

Dark Energy Detectives and their astronomer colleagues can learn even more about how stars form, how they gather together into galaxies, and how they change during their lifetime leading up to their spectacular fiery death.

They say there are a million ways to die in the big city. How many could there be in the dark reaches of the cosmos?

–Det. Kyler Kuehn @kwkuehn  [Australian Astronomical Observatory]