ALMA probes the mystery of dark gamma-ray bursts
Sen—Astronomers have for the first time directly mapped out the molecular gas and dust in the host galaxies of gamma-ray bursts (GRBs) the biggest explosions in the Universe. Less gas was observed than expected, and correspondingly more dust, making some GRBs appear as “dark GRBs”.
Gamma-ray bursts that last more than a couple of seconds, known as long-duration gamma-ray bursts (LGRBs), are associated with supernova explosions.
Within seconds, a typical burst releases as much energy as the Sun will in its ten-billion-year lifetime. The explosion itself is often followed by a slowly fading emission, known as an afterglow, thought to be created by collisions between the ejected material and the surrounding gas.
Some gamma-ray bursts that have no afterglow are referred to as dark bursts. One possible explanation is that clouds of dust absorb the afterglow radiation.
An artist’s conception of the environment around gamma ray burst GRB 020819B based on ALMA observations. Image credit: NAOJ
Scientists have been working to understand how GRBs form by probing their host galaxies. Astronomers expected that the massive stars that were GRB progenitors would be found in active star-forming regions in these galaxies, which would be surrounded by a large amount of molecular gas, the fuel for star formation. However, there had been no observational result to back up this theory.
A Japanese team of astronomers led by Bunyo Hatsukade from the National Astronomical Observatory of Japan, used observations from the Atacama Large Millimeter/submillimeter Array (ALMA) to detect radio emission from molecular gas in two dark LGRB hosts ( GRB 020819B and GRB 051022 ) about 4.3 billion and 6.9 billion light-years away, respectively. Although such radio emission had never been detected in the GRB host galaxies, ALMA made it possible with its unprecedentedly high sensitivity.
ALMA was also able to uncover the distribution of molecular gas and dust in GRB host galaxies. Observations of the GRB 020819B revealed a dust-rich environment in the outskirts of the host galaxy, whereas molecular gas was found only around its centre. This is the first time that such a distribution among GRB host galaxies has been revealed.
GRB 020819B host galaxy. Radio intensity distributions of molecular gas (left) and dust (middle), both observed with ALMA. Optical image by the Frederick C. Gillett Gemini North telescope (right). Cross in the upper centre shows the GRB site. Image credit: Bunyo Hatsukade (NAOJ), ALMA (ESO/NAOJ/NRAO)
"We didn't expect that GRBs would occur in such a dusty environment with a low ratio of molecular gas to dust. This indicates that the GRB occurred in an environment quite different from a typical star-forming region," says Hatsukade. This suggests that massive stars that die as GRBs change the environment in their star-forming region before they explode.
One possible explanation for the high proportion of dust compared to molecular gas at the GRB site is the difference in their reactions to ultraviolet radiation. The bonds between atoms which make up molecules are easily broken by ultraviolet radiation—molecular gas cannot survive in an environment exposed to the strong ultraviolet radiation produced by the hot, massive stars in its star-forming region.
These ALMA observations support the hypothesis that it is dust that absorbs the afterglow radiation, causing the dark gamma-ray bursts.
Simulation of the environment around GRB 020819B based on new observations with ALMA. Credit: NAOJ/Gemini Observatory