Credit: arXiv (2024). DOI: 10.48550/arxiv.2409.11468
Among its other notable achievements and puzzles, the James Webb Space Telescope (JWST) discovered a greater number of bright galaxies in the distant universe than expected. While scientists are still debating the excess, one research group has proposed that gamma-ray bursts could be used as a powerful probe to examine the excess, which could also observe the formation of stars and galaxies in the universe distant and primitive.
JWST recently observed the most distant galaxy ever observed, which formed just 300 million years after the Big Bang. We now find ourselves at the edge of the observable universe, where objects are receding at almost the speed of light.
Traveling so fast relative to Earth, the light from these objects has a significant redshift when it arrives here. Its wavelengths are stretched and are therefore significantly longer than when they were emitted, the so-called Doppler effect (familiar to us due to the shift in sound frequencies when a vehicle moves away of our ears).
The ratio of observed wavelength to emitted wavelength is called z-factor, a parameter widely used by astronomers and astrophysicists.
The most distant observed galaxy had a z-factor of 14.32. JWST also found brighter galaxies with az of 10 or more than expected from extrapolations of their numbers at lower z. (Here, “bright” means ultraviolet light; z=10 corresponds to a recession speed equal to 98.4% of the speed of light.)
The excessive formation rate of bright stars revealed by observations from the James Webb Space Telescope (JWST). The vertical axis shows the rate per unit volume, with the redshift z parameter on the horizontal axis, a measure of distance. The black curve dates from the pre-JWST era. The red triangles show the lower limit of the star formation rate obtained via JWST. The solid red line indicates the excess star formation rate as determined by JWST. Credit: Tatsuya Matsumoto
Various explanations for this excess have been proposed, such as active star formation and a very heavy initial mass function that produces more ultraviolet radiation, but the cause remains unclear. This is an important question, since the ultraviolet brightness spectrum contains key information about the assembly history of these galaxies, their star-forming activity, and the stellar population of the distant universe.
With colleagues, Tatsuya Matsumoto of Kyoto University in Japan explored the potential of high-z gamma-ray bursts to explain the origin of excess bright star and galaxy formation. Their work, published on the arXiv preprint server, has been submitted to Letters from astrophysical journals.
They used data taken by the Einstein Probe, a space-based X-ray telescope launched primarily by China in January 2024. The Einstein Probe is equipped with the Wide X-ray Telescope, which is advantageous for observing gamma-ray bursts. at high z; he recently observed a gamma-ray burst with an az value of 4.859.
Matsumoto said that if one potential reason for the JWST excess “is that stars form more efficiently in these galaxies, gamma-ray bursts should occur more frequently and could be detected by the Einstein probe.” In particular, they found that “the formation rate of gamma-ray bursts may have different behaviors at z = 10 or higher, and detection of this rate by the Einstein probe or future gamma-ray burst missions will clarify the cause of the excess JWST”. “.
Gamma-ray bursts are explosive events in the universe – in fact, the brightest and most extreme events observed in the universe. Lasting from about ten milliseconds to several hours, their intense radiation would be released when a star goes supernova and then implodes. (Others appear to be created by the merger of two neutron stars.) They are extremely rare—only a few per galaxy per million years—and most observed gamma-ray bursts are billions of years away. -light of us.
A typical gamma-ray burst (GRB) releases as much energy in just a few seconds as the sun will in its ten-billion-year lifespan. If such an explosion were to occur in the Milky Way and its jet was aimed directly at Earth, it would end most life forms on the planet. GRBs are of two types: “short”, with durations less than approximately 2 seconds, and “long” with durations greater than 2 seconds. Short GRBs make up about 30% of all GRBs and long GRBs make up 70%.
The number of GRBs in the early universe has been poorly constrained due to limitations of previously available detectors. To overcome this problem, Matsumoto and his team developed a complex analytical relationship between the variance of GRB formation rates as z varies, that is, as time goes back to the beginning of the universe. “Since long gamma-ray bursts are produced by the collapse of massive stars,” they wrote, “they probe star formation activities in the high z of the universe, directly tracing the formation history of stars.”
Importantly, they find that the distribution of GRBs with redshift depends on the potential cause of the JWST excess. If the excess is caused by an increase in the inherent rate of star formation, the redshift distribution will show an excess at z of about 10 or more. If a transition from the initial mass function to a higher mass function creates the JWST excess, the redshift distribution will also show an excess but to a different degree. If other effects cause the excess JWST, such as a contribution from active galactic nuclei, the distribution will gradually extrapolate beyond az of 10.
Additional GRB data should clarify the reason for this excess. “In addition to the Einstein probe,” Matsumoto said, “future missions such as HiZ-GUNDAM will detect gamma-ray bursts and deepen our understanding of the early universe.”
More information:
Tatsuya Matsumoto et al, Probing the origin of excess star formation discovered by JWST thanks to gamma-ray bursts, arXiv (2024). DOI: 10.48550/arxiv.2409.11468
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