The Edge of Darkness

I just picked up an item from the BBC Website that refers to news announced in this week’s edition of Nature of the discovery of a gamma-ray burst detected by NASA’s Swift satellite.  The burst itself was detected in April this year and I had a sneak preview that something exciting was going to be announced earlier this month at the Royal Astronomical Society meeting on October 9th. However, today’s press releases still managed to catch me on the hop owing to the fact that a rather different story had distracted my attention…

In fact, detections of gamma-ray bursts are not all that rare. Swift observes one every few days on average. Once such a source is found through its gamma-ray emission, a signal is sent to astronomers around the world who then work like crazy to detect an optical counterpart. If and when they find one, they try to measure the spectrum of light emitted in order to determine the source’s redshift. This is very difficult for the distant ones, and is not  always successful.

However, what happened in this case – called GRB 090423 – was that a spectrum was that not one but two independent teams obtained optical spectra of the  object in which the gamma-ray burst must have happened. What each time found was that their spectrum showed a sharp cut-off at wavelengths shorter than a given limiting value.

Hydrogen is very effective at absorbing radiation with wavelengths shorter than 91.2 nm (the so-called Lyman limit, which is in the ultraviolet part of the spectrum), and all galaxies contain large amounts of hydrogen; hence galaxies are virtually dark at wavelengths shorter than 91.2 nm in their rest-frame. The position of the break in an observed frame will be at a different wavelength owing to the effect of the cosmological redshift.

The Lyman break for the host of  GRB 090423 appears not in the ultraviolet but in the infrared, indicating a very large redshift. In fact, it’s a truly spectacular  8.2.

Together with the direct observations of galaxies at high redshifts I blogged about a month or so ago, this discovery helps push back the frontiers of our knowledge of the Universe not just in space but also in time. A quick calculation reveals that in the standard cosmological model, light from a source at redshift 8.2 has taken about 13.1 billion light years to reach us. The gamma-ray burst therefore exploded about 600 million years after the Big Bang.

Another interesting thing about this source is its duration. The optical afterglow of a gamma-ray burst  decays with time. Gamma-ray bursts are usually classified as either short or long, depending on the decay time with the dividing line between the two classes being around 2 seconds. The optical afterglow of GRB 090423 lasted about ten seconds. But that doesn’t make it a long burst. We actually see the afterglow stretched out in time by the same redshift factor as an individual photon’s wavelength. So in the rest frame of the source the optical glow was only a bit over a second in duration, i.e. it was a short burst.

Long gamma-ray bursts are thought to be associated with core-collapse supernovae which arise from the self-destruction of very massive stars with very short lifetimes. The fact that such things die young means that they are only found where star formation has happened very recently. One might expect the earliest gamma-ray bursts to therefore be of this type.

I don’t think anyone is really sure what the shorter ones really are, but they  seem to happen in regions without active star formation in which the stellar populations are quite old, such as in elliptical galaxies. The fact that the most distant GRB yet discovered happens to be a short burst is very interesting. How can there be an old stellar population at a time when the  Universe itself was so young?

If the Big Bang theory is correct, astronomers  should eventually be able to reach back so far in time that the Universe was so young that no stars had had time to form. There would be no sources of light to detect so we would have reached the edge of darkness. We’re not there yet, but we’re getting closer.

4 Responses to “The Edge of Darkness”

  1. […] instead! (For more details on the distant galaxy see this post over at the always excellent In the Dark. There’s a lot of effort being put into this field at the moment, as already discussed here […]

  2. Hi Peter,

    So, I think (as do some of the GRB 090423 co-authors!) that this might not necessarily be a *short* burst, but a “short long” burst so tae speak.

    To quote from the paper:
    “…collapse. However, in the case of GRB 090423, a more careful extra- polation of the observed c-ray and X-ray light curves to lower redshifts shows that its duration would have appeared significantly longer than suggested by naive time-dilation considerations14.”

    Which I don’t *fully* understand, but will go look at ref. 14 now.

    However, the BIG news is Nature this week I think is the discovery of over 200 galaxies only a few parsecs away, p. 1177, Vol. 461, 29th Oct 2009… 🙂

    Cheers,
    Nic

  3. telescoper Says:

    You didn’t expect me to actually read the paper did you?

    Perhaps it’s a brand new type called “intermediate”…

  4. Another year, another star,
    Most are near, this one’s far.

    It first was found, by the looking round,
    Of an eye compound,
    With a mighty thirst,
    For a gamma burst.

    None yet have asked,
    If this cosmic torch,
    Might have been tasked,
    To discouragingly scorch,
    One whose gaze trespassed,
    On his neighbor’s porch.

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