Observations of gamma-ray burst reveal surprising ingredients of early galaxies
The new results indicate that some galaxies were already rich in heavy elements less than two billion years after the Big Bang.
November 2, 2011
An international team of astronomers has used the brief but brilliant light of a distant gamma-ray burst (GRB) as a probe to study the makeup of distant galaxies. Surprisingly, the new observations made with the European Southern Observatory’s (ESO) Very Large Telescope (VLT) have revealed two galaxies in the young universe that are richer in the heavier chemical elements than the Sun. The two galaxies may be in the process of merging. Such events in the early universe will drive the formation of many new stars and may be the trigger for GRBs.
This artist’s impression shows two galaxies in the early universe. The brilliant explosion on the left is a gamma-ray burst. The light from the burst travels through both galaxies on its way to Earth (outside the frame to the right). Analysis of observations of the light from this gamma-ray burst made using ESO’s Very Large Telescope have shown that these two galaxies are remarkably rich in heavier chemical elements. Credit: ESO/L. Calçada
GRBs are the brightest explosions in the universe. Scientists first spot them using orbiting observatories that detect the initial short burst of gamma rays. After pinning down their positions, astronomers immediately study them using large ground-based telescopes that can detect the visible-light and infrared afterglows that the bursts emit over the succeeding hours and days. The NASA Fermi Gamma-ray Space Telescope first spotted one such burst, called GRB 090323. Soon afterward, it was picked up by the X-ray detector on NASA’s Swift satellite and with the GROND system at the MPG/ESO 2.2-meter telescope in Chile, and then studied in great detail using ESO’s (VLT) just one day after it exploded.
The VLT observations show that the brilliant light from the GRB had passed through its own host galaxy and another galaxy nearby. These galaxies are seen as they were about 12 billion years ago. Such distant galaxies are rarely caught in the glare of a GRB.
“When we studied the light from this gamma-ray burst, we didn’t know what we might find. It was a surprise that the cool gas in these two galaxies in the early universe proved to have such an unexpected chemical makeup,” said Sandra Savaglio from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. “These galaxies have more heavy elements than have ever been seen in a galaxy so early in the evolution of the universe. We didn’t expect the universe to be so mature, so chemically evolved, early on.”
As light from the GRB passed through the galaxies, the gas there acted like a filter and absorbed some of the light from the GRB at certain wavelengths. Without the GRB, these faint galaxies would be invisible. By carefully analyzing the telltale fingerprints from different chemical elements, the team was able to work out the composition of the cool gas in these distant galaxies and, in particular, how rich they are in heavy elements.
Scientists expected that galaxies in the young universe to contain smaller amounts of heavier elements than galaxies at the present day, such as the Milky Way. The heavier elements are produced during the lives and deaths of generations of stars, gradually enriching the gas in the galaxies. Astronomers can use the chemical enrichment in galaxies to indicate how far they are through their lives. But the new observations, surprisingly, revealed that some galaxies were already rich in heavy elements less than two billion years after the Big Bang, something unthinkable until recently.
The newly discovered pair of young galaxies must be forming new stars at a tremendous rate to enrich the cool gas so strongly and quickly.
As the two galaxies are close to each other, they may be in the process of merging, which would also provoke star formation when the gas clouds collide. The new results also support the idea that GRBs may be associated with vigorous massive star formation.
Energetic star formation in galaxies like these might have ceased early in the history of the universe. Twelve billion years later, at the present time, the remains of such galaxies would contain a large number of stellar remnants such as black holes and cool dwarf stars, forming a hard-to-detect population of “dead galaxies,” just faint shadows of how they were in their brilliant youths. Finding such corpses in the present day would be a challenge.
“We were very lucky to observe GRB 090323 when it was still sufficiently bright so that it was possible to obtain spectacularly detailed observations with the VLT,” said Savaglio. “Gamma-ray bursts only stay bright for a short time, and getting good quality data is very hard. We hope to observe these galaxies again in the future when we have much more sensitive instruments; they would make perfect targets for the E-ELT.”