Gigantic Explosions Buried in Dust: ALMA Probes Environment around Gamma Ray Bursts

ALMA OBSERVATORY/DICYT Japanese observations from the Atacama Large Millimeter/submillimeter Array (ALMA) 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. In a complete surprise, less gas was observed than expected, and correspondingly much more dust, making some GRBs appear as “dark GRBs”. This work will appear in the journal Nature on 12 June 2014 and is the first ALMA science result on GRBs to appear. It shows ALMA’s potential to help us to better understand these objects.

Gamma-ray bursts (GRBs) are intense bursts of extremely high energy observed in distant galaxies — the brightest explosive phenomenon in the Universe. Bursts that last more than a couple of seconds are known as long-duration gamma-ray bursts (LGRBs) and are associated with supernova explosions — powerful detonations at the ends of the lives of massive stars.

In just a matter of seconds, a typical burst releases as much energy as the Sun will in its entire ten-billion-year lifetime. The explosion itself is often followed by a slowly fading emission, known as an afterglow, which is thought to be created by collisions between the ejected material and the surrounding gas.
However, some gamma-ray bursts mysteriously seem to have no afterglow — they are referred to as dark bursts. One possible explanation is that clouds of dust absorb the afterglow radiation.

In recent years, scientists have been working to better 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, leaving a long-standing mystery.

A Japanese research group led by Bunyo Hatsukade, an assistant professor at the Chile Observatory of the National Astronomical Observatory of Japan (NAOJ), conducted observations of two galaxies hosting GRB 020819B and GRB 051022 whose distance from us is about 4.3 billion and 6.9 billion light years, respectively. Detection of radio emission from GRB host galaxies, which had been a long-sought goal for astronomers, was finally made possible by ALMA with its unprecedentedly high sensitivity.

Kotaro Kohno, a professor at the University of Tokyo and a member of the research team, says, "We have been searching for molecular gas in GRB host galaxies over 10 years since 2003 using various telescopes around the world. As a result of our hard effort, we finally achieved a remarkable breakthrough using ALMA with unprecedentedly high sensitivity. We are very excited with what we have achieved." The sensitivity of ALMA in this observation was about five times better than the other telescopes ever used for the previous researches, although the observation time taken was just 47 minutes and the number of antennas used was only 27 (less than a half of the total number of antennas). The observation results demonstrated ALMA’s revolutionary capability even under limited conditions.

Another remarkable achievement made possible by the high resolution of ALMA was uncovering the distribution of molecular gas and dust in GRB host galaxies. Observations of the GRB 020819B revealed a remarkably 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.

"The results obtained this time were beyond our expectations. We need to carry out further observations with other GRB hosts to see if this could be general environmental conditions of a GRB site. We are looking forward to future researches with improved capability of ALMA" says Hatsukade.