Astronomers Analyze Dark Gamma-Ray Burst Environments

redOrbit Staff & Wire Reports – Your Universe Online

For the first time ever, scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered molecular gas in galaxies that had once been rocked by gamma ray bursts, according to new research published Wednesday in the journal Nature.

The new observations reveal that the molecular gas, which serves as the fuel for the star formation process, was concentrated towards the center of the galaxies, the study authors explained. The gamma ray bursts (GRBs), on the other hand, took place in atypical environments that were lacking gas but had copious amounts of dust.

Lead investigator Bunyo Hatsukade, an assistant professor at the Chile Observatory of the National Astronomical Observatory of Japan (NAOJ), and his colleagues believe that the scarce amount of molecular gas around these extremely bright explosions was due to strong ultraviolet (UV) radiation originating from massive young stars. That radiation is capable of breaking apart gas molecules, while having little impact on the dust.

According to the National Radio Astronomy Observatory (NRAO), the GRBs observed by Hatsukade’s team (GRB 020819B and GRB 051022) are located roughly 4.3 billion and 6.9 billion light-years away from Earth, respectively. The sensitivity of the ALMA telescopes made it possible for the researchers to both detect carbon monoxide gas in a GRB host galaxy for the first time.

By mapping out the molecular gas and dust in the host galaxies of the GRBs, the study authors hope to gain new insights into these objects. Recently, astronomers have been working to learn more about how these explosions form by probing their host galaxies, expecting to find that GRB progenitors would be found in active star-forming regions surrounded by large amounts of molecular gas.

However, as officials with the European Southern Observatory (ESO) explained, there had been no observational results to back up this hypothesis. Now, however, the study authors have found that the gas-to-dust ratio made the two observed GRBs to appear as “dark bursts,” meaning that they do not possess the slowly-fading emissions that typically accompany these explosions.

Those emissions, which are known as the afterglow, are believed to be caused by collisions between ejected material and the surrounding gas. One possible explanation for this phenomenon, according to the ESO, is that the afterglow radiation is absorbed by clouds of dust. During their study, Hatsukade and his associates were able to use ALMA to detect radio emissions from the molecular gas in the host galaxies – a never-before-accomplished feat.

The researchers also note that GRBs are currently classified as either short- or long-duration events. Long-duration GRBs last at least two seconds and are believed to be the result of a supernova explosion of a star at least 40 times the mass of our sun. On the other hand, short-duration GRBs are less than two seconds long and have been linked to the collision and merger of neutron stars. GRB 020819B and GRB 051022 are both dark, long-duration GRBs.

“We have been searching for molecular gas in GRB host galaxies for over ten years using various telescopes around the world,” said research team member and University of Tokyo professor Kotaro Kohno. “As a result of our hard work, we finally achieved a remarkable breakthrough using the power of ALMA. We are very excited with what we have achieved.”

“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,” added Hatsukade, adding that the findings have led to speculation that massive stars which die as GRBs cause changes in the star-forming region of their galaxies before they actually explode.