Galaxy survey fails to add up
Aug 15, 2006
Astronomers are struggling to explain new data from the Sloan Digital Sky Survey and various ground-based telescopes that reveal there are four times more galaxies in the direction of gamma-ray bursts than there are in the direction of quasars. This unexpected finding was made by Jason Prochaska and Gabriel Prochter of the University of California at Santa Cruz, who insist it is not a stastical fluke. Although there is no reason why galaxies should be more numerous in the direction of either of these two types of object, it could be that the light from them is being absorbed by material in intervening galaxies (Astrophysical J. Letters to be published).
Gamma-ray bursts (GRBs) are the most energetic explosions in the universe and happen when a massive star undergoes a supernova explosion. Quasars -- bright sources of electromagnetic radiation -- are thought to be powered by the accretion of material onto supermassive black holes in the centres of distant galaxies.
Prochaska and Prochter obtained their surprising result by simply counting the number of galaxies towards gamma-ray bursts and quasars. They did this by studying more than 50,000 spectra released by the Sloan Digital Sky Survey for galaxies in the direction of quasars and by using data from various ground-based telescopes for galaxies in the direction of GRBs. The data set for these galaxies was much smaller because only 15 GRBs were analysed.
The two astronomers found strong absorption signatures indicating the presence of galaxies along 14 out of the 15 GRB sightlines. When the data were "normalised" for the total number of GRBs and quasars searched, the team found that there were about four times as many galaxies in the direction of GRBs than towards quasars.
While the researchers admit that they don't why this, they have already ruled out several possible explanations. These include: some quasars being hidden by galactic dust, which would skew the results; the absorption lines in the GRB spectra being from gas ejected by the bursts themselves and not from gas in intervening galaxies; and intervening galaxies possibly acting as "gravitational lenses", so that galaxies towards GRBs look brighter than those towards quasars.
The result has also puzzled the wider astrophysics community. Charles Steidel of Caltech, for example, still suspects that it might have something to do with material that is shot out at very high velocity in the immediate vicinity of GRBs themselves.
Prochaska and Prochter would now like to increase the number of GRBs they have studied. Luckily, this will be possible with an ongoing NASA mission called Swift that is detecting about 100 GRBs a year, of which 10 to 20 will be useful for this study.
About the author
Belle Dumé is science writer at PhysicsWeb