Atacama Array (ALMA) detects star forming molecular gas

An artist’s conception of the environment around GRB 020819B based on ALMA observations. 

Image Credit: NAOJ

Using the Atacama Large Millimeter/submillimeter Array (ALMA), a team of researchers reports the first-ever detection of molecular gas, the fuel for star formation, in two galaxies that were previously rocked by gamma ray bursts (GRBs), the brightest explosions in the Universe.

These new observations revealed that the molecular gas was concentrated toward the centers of the galaxies, while the GRBs occurred in unusual environments that were surprisingly bereft of gas yet rich in dust.

The researchers speculate that the dearth of molecular gas around the GRBs was due to strong ultraviolet (UV) radiation from young, massive stars, which can break apart the molecules of gas while leaving the dust relatively undisturbed.

The GRBs, dubbed GRB 020819B and GRB 051022, are located approximately 4.3 billion and 6.9 billion light-years away from Earth, respectively.

Astronomer Bunyo Hatsukade, assistant professor at the Chile Observatory of the National Astronomical Observatory of Japan (NAOJ), led the research group that studied the GRB host galaxies. The results are published in the journal Nature.

ALMA's unprecedented sensitivity made it possible to make the first detection ever of carbon monoxide (CO) gas in a GRB host galaxy.

ALMA's unparalleled high resolution also revealed GRB 020819B occurred in a galaxy where the molecular gas was concentrated at the nuclear region while dust was concentrated at the site of the GRB.

The ratio of dust to molecular gas at the GRB site is ten or more times higher than in normal environments. It is the first time that the spatial distribution of molecular gas and dust in the GRB host galaxies is revealed.

Currently, GRBs are classified as either long- or short-duration.

• A long-duration GRB, which lasts two seconds or longer, is believed to be generated by the supernova explosion of a star 40 or more times the mass of our Sun. 
• Short GRBs last less than two seconds and are associated with the collision and merger of neutron stars.