The Oddball Hosts of Gamma-ray Bursts

Oct 22, 2008
The Universe is populated by a variety of galaxy-types; a sampling is shown in this Hubble image of Abell Cluster S0740. Gamma-ray bursts prefer some and avoid others.

There's a universal tendency to heed Dylan Thomas's exhortation and go out with a bang instead of a whimper. Nowhere is this more evident than deep in the cosmos.

When their time is up, stars make their exits in a number of flamboyant ways. The most massive stars leave with the greatest fanfare of all – blasting out gamma-ray bursts (GRBs), tremendous explosions that rock the Universe like nothing else. These spectacular blasts, second in power only to the Big Bang, occur when stars 50 to 100 times more massive than our sun use up all their fuel and collapse. Most astrophysicists believe that gamma-ray bursts herald the formation of a black hole.

All types of galaxies--spirals, ellipticals, dwarfs and irregulars--contain super-massive stars. Curiously, though, not all types of galaxies produce gamma-ray bursts. This is one of the puzzles being discussed today at the 2008 Gamma-Ray Burst Symposium held in Huntsville, Alabama.

Andrew Fruchter of the Space Telescope Science Institute is at the Symposium to share what he knows.

First of all, he clarifies, there are two types of gamma-ray bursts: long ones, produced by the explosion of supermassive stars as described above, and short ones, produced by some other still-unknown process. "Short GRBs aren't choosey about their hosts," says Fruchter. "They're found in all types of galaxies. But the host galaxies of long GRBs tend to be oddballs, small and irregular, instead of 'regular' spiral galaxies like our own Milky Way."

Fruchter believes he understands the discrepancy. Extreme supernova explosions of the type that produce GRBs require stars of both great mass and low metallicity. (In astronomy, "metals" are any elements heavier than hydrogen or helium.) "Larger galaxies tend to be more metal-rich than smaller ones," he says. "So GRBs avoid those larger galaxies."

The underlying mechanism works like this:

"Metals in a star produce strong stellar winds -- the metals' atoms reflect the star's light and act like a solar sail, getting an extra push that hydrogen and helium alone would not get," says Fruchter. "This activity causes some of the star's mass to flow out into space."

So, stars with high metallicity tend to lose a lot of their mass before they explode. "[Metals] can cause such great mass loss that instead of turning into black holes upon collapse, some stars may only turn into neutron stars. It's quite possible that a black hole may need to be present to create a gamma-ray burst."

In galaxies crowded with high-metallicity stars, gamma-ray bursts are thus suppressed. Oddball galaxies of lower-metallicity get all the best explosions!

Great mass. Low metallicity. "We should also add to that list rapid spin," says Symposium participant Chip Meegan of the NASA Marshall Space Flight Center.

It's becoming clear that a star must spin rapidly to produce an explosion with a GRB's power. "The general consensus is that GRBs emit most of their energy in a jet. Jets in astrophysics are usually formed by rotating objects," Fruchter says.

"If a slowly spinning star collapses into a black hole, most of the energy just disappears into the black hole," explains Meegan. Rapidly rotating stars have a trick for allowing some of that energy to escape: "The centrifugal force of rotation causes the infalling material to form a torus and makes a less dense region form along the axis of rotation. This provides a channel for the some of the matter and energy to blast out along the poles instead of being absorbed by the black hole."

Mystery solved? Maybe. Meegan thinks there are more surprises in the offing:

"Gamma-ray bursts have amazed us many times before, and I suspect that they aren't out of surprises yet. The unpredictability is what makes this such an interesting field."

The Sixth Huntsville Gamma-Ray Burst Symposium 2008 is sponsored by NASA's Fermi and Swift Projects and hosted by the Fermi GBM Team based at the Marshall Space Flight Center in Huntsville.

Source: Science@NASA, by Dauna Coulter

Explore further: New mass map of a distant galaxy cluster is the most precise yet

add to favorites email to friend print save as pdf

Related Stories

Recommended for you

Satellite galaxies put astronomers in a spin

10 hours ago

An international team of researchers, led by astronomers at the Observatoire Astronomique de Strasbourg (CNRS/Université de Strasbourg), has studied 380 galaxies and shown that their small satellite galaxies almost always ...

Video: The diversity of habitable zones and the planets

11 hours ago

The field of exoplanets has rapidly expanded from the exclusivity of exoplanet detection to include exoplanet characterization. A key step towards this characterization is the determination of which planets occupy the Habitable ...

Ultra-deep astrophoto of the Antenna Galaxies

11 hours ago

You might think the image above of the famous Antenna Galaxies was taken by a large ground-based or even a space telescope. Think again. Amateur astronomer Rolf Wahl Olsen from New Zealand compiled a total ...

The most precise measurement of an alien world's size

12 hours ago

Thanks to NASA's Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the radius of a planet outside our solar system. The size of the exoplanet, dubbed Kepler-93b, ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Oct 22, 2008
Might possibly the large spin rate of GRBs also contribute to greatly increasing the magnetic field of the collapsing object, producing a magnetar in its wake? Stars with just the right mass may not be massive enough to produce a black hole, so it seems reasonable that a rapidly spinning, collapsing objects may still produce collimated gamma ray jets & in the process form a hot, dense, highly magnetized neutron star (magnetar).
2.5 / 5 (2) Oct 22, 2008
Interesting if true, there would be fewer terrestrial planets in irregular galaxies then.