Observations of gamma-ray burst reveal surprising ingredients of early galaxies

According to observations of a very distant gamma-ray burst (GRB) recently reported, it appears that a couple of galaxies in the early universe, only 1.8 billion years after the big bang, contain a higher concentration of some elements heavier than hydrogen and helium than the Sun does. This is rather surprising, since the Sun is about 4.5 billion years old, and formed out of gas and dust in which heavy elements had been accumulating for about 9 billion years.

Isaac Asimov supposedly remarked, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’, but ‘That’s funny …'” That may be appropriate in this case. How did those galaxies have all those heavy elements so early?

The trigger for the observation was a GRB detected in March 2009 and designated GRB 090323. This was the type of GRB known as a “long” GRB, because the brightest phase lasts more than two seconds, and there is a diminishing afterglow that may last days. (The “short” type lasts less than two seconds and shows no afterglow.) Long GRBs are thought to result from supernova events in which a jet of relativistic particles is aimed in our direction.

Although the burst was determined to be quite distant (redshift z=3.57, corresponding to a light travel time of about 11.9 billion years), there was nothing about it at first that seemed especially unusual. However, an analysis of the spectrum of the afterglow indicated that the light from the GRB had passed through two galaxies relatively close to each other, one of which may have been the host of the GRB. What was especially odd was that the spectrum showed the presence of higher concentrations of zinc and sulfur than occur in the Sun.

Observations of gamma-ray burst reveal surprising ingredients of early galaxies

An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics has used the brief but brilliant light of a distant gamma-ray burst as a probe to study the make-up of very distant galaxies. Surprisingly the new observations revealed two galaxies in the young Universe that are richer in the heavier chemical elements than the Sun. The two galaxies may be in the process of merging. Such events in the early Universe will drive the formation of many new stars and may be the trigger for gamma-ray bursts.

Gamma-ray bursts are the brightest explosions in the Universe. They are first spotted by orbiting observatories that detect the initial short burst of gamma rays. After their positions have been pinned down, they are then immediately studied using large ground-based telescopes that can detect the visible-light and infrared afterglows that the bursts emit over the succeeding hours and days. One such burst, called GRB 090323, was first spotted by the NASA Fermi Gamma-ray Space Telescope. Very soon afterwards it was picked up by the X-ray detector on NASA’s Swift satellite and with the GROND system at the MPG/ESO 2.2-metre telescope in Chile. From the GROND observations, the astronomers estimated the minimum rate of star formation, which has to be several times higher than the one in our Galaxy. They could, however, only determine a minimum value because the detected emission could be heavily affected (i.e. absorbed) by the presence of dust in the galaxies. The real rate of star formation, once the (unknown) dust absorption has been taken into account, could easily be 50 times higher than in the Milky Way.

The authors of the research paper point out that “These are the highest metallicities ever measured in galaxies at z > 3.” (“Metallicity” refers to the relative abundance of heavy elements.) But how unusual is this, really?

From the data, it’s not clear how far apart the two galaxies were. However, the redshifts and relative velocities of the two galaxies suggest a separation on the order of 10 million light-years. That’s not especially close. (Our galaxy and M31 – Andromeda – are about 2.5 million light-years apart.) But the two galaxies could have been closer when the progenitor star of GRB 090323 formed. Or even closer somewhat earlier when large stars similar to the progenitor formed.

Anyhow, the researchers hypothesize that interactions between the two galaxies triggered a high rate of star formation in both galaxies. As the largest stars that formed during that period expired in supernova events, they could account for the high metallicity, since supernovae are generally assumed to be the source of most heavy elements.

The bottom line is that we don’t really know the explanation for the surprisingly high metallicity. But it seems to be real, and so it’s possible that prevalence of heavy elements in galaxies of the early universe is a lot more than has generally been assumed. And if so, the explanation could be high rates of star formation due to close encounters between galaxies during that era.

Further reading:

VLT observations of gamma-ray burst reveal surprising ingrediecnts of early galaxies.

Gamma ray blast zaps two distant galaxies

Super-solar Metal Abundances in Two Galaxies at z ~ 3.57 revealed by the GRB 090323 Afterglow Spectrum

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