When I saw this paper’s title I was intrigued. When I realised what this paper was actually about, I was genuinely enthralled! A couple of months ago, I was wondering whether it would be possible to take a gamma ray burst spectrum and look for interstellar chemicals. Well, now someone has! And they’ve got the first concrete observations of molecules in a redshifted GRB host galaxy!
To fill you in on the background, gamma ray bursts (usually shortened to simply “GRBs”, because people seem to like abbreviations) give out more than just gamma rays. The initial burst peaks at gamma ray energies, but all GRBs fade slowly over time in what’s known as an “afterglow”. The afterglow of a GRB falls through x-rays, ultraviolet… right the way down (eventually) to submillimetre frequencies. Prochaska et al have, in this work, recorded the spectrum of the optical part of a GRB, compared it with a model spectrum and assigned some spectral lines. The concept is exactly the same as the more normal stellar spectroscopy that most astronomers will be familiar with.
The big difference is that this GRB (GRB 080607, in case you’d like to look it up) is at quite a high redshift. z = 3.0363, to be precise. The most distant GRB recorded so far was GRB 080913 with a redshift of 6.7, to give you some perspective.
From a spectroscopic point of view, this means that most of the transitions seen in the visible here would normally be seen in the ultraviolet. Indeed, the spectrum shows a wealth of atomic lines from various ionised metals, all around 5500Å. It also shows some H2 absorptions, as well as band heads from electronic transitions in CO molecules. Extragalactic molecules!
But the part that excites me is that the spectrum shown on page two of this paper is a complex forest full of all manner of lines. Better yet, the vast majority of them look like real absorption lines and not just noise. If, as I suspect is the case, most of the people looking over this spectrum were physicists and not chemists, there may be a few prominent chemical signatures there which were overlooked. As I’ve written about before, PAH molecules are potent absorbers in the UV, meaning that there may even be some PAH absorptions in this mess of spectral lines! Even with more basic molecules though — I can say with some certainty that some other molecules (such as H2O and C2) should be present on closer inspection.
As a field of study in its own right, astrochemistry is still fairly young, even if it can stand proudly on the shoulders of the more mature fields of chemistry and astronomy. I have a feeling that GRB spectra may be paramount in extragalactic astrochemistry. They’re such bright light sources (much better than quasars) that it would be foolish to miss an opportunity to use them. Personally though, I’d like to see infrared or submillimetre spectra of such high redshift GRBs. That’s where the majority of the molecular lines will be evident, and it would be fascinating to get even a glimpse of the chemistry of the early Universe. Perhaps even more fascinating if we were to find that the chemistry back then was much the same as it is here and now.
I also have my own suspicions that intergalactic space may not be as empty of molecules as many astronomers believe. Unfortunately, the only way to study the intergalactic medium is by using light from distant quasars and intermittent GRBs. Perhaps there might even be diffuse intergalactic bands out there…
Image yoinked from the NRAO.
J. X. Prochaska, Y. Sheffer, D.A. Perley, J. S. Bloom, L. A. Lopez, M. Dessauges-Zavadsky, H.-W. Chen, A. V. Filippenko, M. Ganeshalingam, W. Li, A. A. Miller, D. Starr (2009). The First Positive Detection of Molecular Gas in a GRB Host Galaxy Astrophysical Journal Letters (accepted) DOI: arXiv:0901.0556v1