The European Southern Observatory's Atacama Large Millimeter/sub-millimeter Array (ALMA) is making headlines again. For the first time, the molecular gas and dust shroud enveloping gamma-ray bursts (GRBs) in active galaxies has been directly mapped. Moreover, in an odd turn of events, scientists were shocked to find much more dust and far less gas than previously predicted. The additional dust gives some GRBs a darker appearance and helps illustrate ALMA's powerhouse potential.
The Birth of a Gamma-ray Burst:
Gamma-ray bursts (GRBs) are the most powerful explosive forces known in the universe, despite only lasting a couple of seconds most of the time, but not always. In that regard, GRBs come in two varieties. First, we have long-duration gamma-ray bursts (LGRBs), which last for more than two seconds and are the most commonly observed GRB. Then, we have short duration GRBs, which can come to a conclusion in the blink of an eye. The former is likely created when a massive star dies and goes out in a giant explosion, known as a supernova. Short-duration gamma-ray bursts, on the other hand, last less than two seconds and are thought to be the result of two neutron stars colliding (instead of them being associated with supernovae or hypernovae).
During their extremely short "existence, gamma-ray bursts typically release as much energy as our Sun will over it's entire lifetime. After the initial energy burst, an afterglow remains; thought to be produced by collisions between the gas surrounding the GRB and any ejected material. However, not all GRBs have an afterglow; in some cases, the surrounding clouds of dust absorb all the radiation - producing what is known as a "dark GRB." Scientists have spent many years trying to decipher what it is that makes these enigmatic bursts tick. By studying their host galaxies, astronomers expected to find massive stars producing the GRBs in active star-forming regions within galaxies (or areas rich in star-formation material ). However, there just wasn't the observational data to back any of this up. Until now.
Thanks to ALMA's high-degree of sensitivity, Bunyo Hatsukade and a team of astronomers from Japan's National Astronomical Observatory, detected radio emissions from molecular gas clouds in two different GRB sources: GRB 020819B (found 4.3 billion light-years away) and GRB 051022 (located some 6.9 billion light-years away). This marks the first time this type of radio emission has ever been detected within a GRB host galaxy.
Kotaro Kohno, a member of the team, said:
"We have been searching for molecular gas in GRB host galaxies for over ten years using various telescopes around the world. 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.”
The team was able to directly observe the distribution of molecular gas and dust in the host galaxies. ALMA, with another first, was able to reveal that GRB 020819B was harboring a galactic center full of molecular gas and a region extremely rich in dust. Astronomers were able to determine that the ratio of dust mass to molecular gas mass is at least ten times higher around GRB 020819B than in the Milky Way's interstellar medium (or in other star-burst galaxies).
Cosmic Nature vs Nurture:
So what does this mean exactly? These results tell us that when massive stars die as GRBs, they actually alter their surrounding environments. The team theorizes that the difference in dust and gas proportions can be chalked up to the material having different reactions to ultraviolet radiation.
Molecules, as we all know, are made up of atoms. The bonds that hold the atoms together are easily affected by UV radiation, so molecular gas does not fair well in the extreme environments around young, hot, massive stars - the same type of environments that would result in a GRB.
Similar dust and gas proportions have been detected in GRB 051022, but the team still needs to confirm the data. This might be difficult considering the fact that GRB 051022 is much farther away than GRB 020819B, so the image resolution is not quite as high, but the preliminary data supports the dust absorption hypothesis. The data collected from ALMA has surpassed the team's expectations, so further testing is needed on other GRB hosts to determine if these results should be considered general GRB site conditions.