More Observations of GRB 090423, the Most Distant Known Object in the Universe

This image shows the afterglow of GRB 090423 (red source in the centre) and was created from images taken in the z, Y and J filters at Gemini-South and VLT (credit: A. J. Levan).

On April 23, 2009 the Swift satellite detected a gamma ray burst and as we reported back in April, scientists soon realized that it was more than 13 billion light-years from Earth. GRB 090423 occurred 630 million years after the Big Bang, when the Universe was only four percent of its current age of 13.7 billion years. Now, continued observations of the GRB by astronomers around the world have yielded more information about this dramatic and ancient event: the GRB didn’t come from a monster star, but it produced a fairly sizable explosion.

Several of the world’s largest telescopes turned to the region of the sky within the next minutes and hours after Swift’s announcement of the GRB’s detection, and were able to locate the faint, fading afterglow of the GRB. Detailed analysis revealed that the afterglow was seen only in infrared light and not in the normal optical. This was the clue that the burst came from very great distance.

The Very Large Array radio telescope first looked for the object the day after the discovery, detected the first radio waves from the blast a week later, then recorded changes in the object until it faded from view more than two months later.

Images of the afterglow of GRB 090423 taken (left to right) with the Y, J, H and K filters. The absence of any flux in the Y filter is a strong indication that the GRB is very high redshift (Credit: A. J. Levan & N. R. Tanvir)
Images of the afterglow of GRB 090423 taken (left to right) with the Y, J, H and K filters. The absence of any flux in the Y filter is a strong indication that the GRB is very high redshift (Credit: A. J. Levan & N. R. Tanvir)

Astronomers have thought that the very first stars in the Universe might be very different — brighter, hotter, and more massive — from those that formed later.

“This explosion provides an unprecedented look at an era when the Universe was very young and also was undergoing drastic changes. The primal cosmic darkness was being pierced by the light of the first stars and the first galaxies were beginning to form. The star that exploded in this event was a member of one of these earliest generations of stars,” said Dale Frail of the National Radio Astronomy Observatory.

Universe Today spoke with Edo Berger with the Gemini Telescope shortly after the GRB was detected, and he said the burst itself was not all that unusual. But even that can convey a lot of information. “That might mean that even these early generations of stars are very similar to stars in the local universe, that when they die they seem to produce similar types of gamma ray bursts, but it might be a little early to speculate.”

“This happened a little more than 13 billion years ago,” said Berger. “We’ve essentially been able to find gamma ray bursts throughout the Universe. The nearest ones are only about 100 million light years away, and this most distant one is 13 billion light years away, so it seems that they populate the entire universe. This most distant one demonstrates for the first time that massive stars exist at those very high red shifts. This is something people have suspected for a long time, but there was no direct observational proof. So that is one of the cool results from this observation.”

The scientists concluded the explosion was more energetic than most GRBs, but was certainly not the most energetic ever detected. The blast was nearly spherical that expanded into a tenuous and relatively uniform gaseous medium surrounding the star.

Antennas of the Very Large Array CREDIT: NRAO/AUI/NSF
Antennas of the Very Large Array CREDIT: NRAO/AUI/NSF

“It’s important to study these explosions with many kinds of telescopes. Our research team combined data from the VLA with data from X-ray and infrared telescopes to piece together some of the physical conditions of the blast,” said Derek Fox of Pennsylvania State University. “The result is a unique look into the very early Universe that we couldn’t have gotten any other way,” he added.

Sources: NRAO, University of Leicester

29 Replies to “More Observations of GRB 090423, the Most Distant Known Object in the Universe”

  1. According to the released paper
    “GRB 050904 at redshift 6.3: observations of the oldest cosmic explosion after the Big Bang”


    “We present optical and near-infrared observations of the afterglow of the gamma-ray burst GRB 050904. We derive a photometric redshift z = 6.3, estimated from the presence of the Lyman break falling between the I and J filters. This is by far the most distant GRB known to date. Its isotropic-equivalent energy is 3.4 Ɨ 10^53 erg in the rest-frame 110-1100 keV energy band.”

    Why doesn’t the article here mention the actual redshift?

    Clearly the central part of the story is the staggering measurement of a redshift that is 6.3!!

    (Knowing this, the means we can easily calculate the distance and age of the object – depending on the selected value of the Hubble Constant required. Just stating the age one has to assume some unstated parameter, and not the figure need to do the calculation in the first place!

  2. Congrats to Nancy for another thought provoking article on GRB 050904. Again, well confirmed spectroscopic redshifts are generally more precise than photometric redshifts, due to the more precise determinations of redshift available to astrophysicists.

    Congratulations to all involved making such sensitive and rigorously counterchecked data determination before all the facts are in.

  3. “”It’s important to study these explosions with many kinds of telescopes. Our research team combined data from the VLA with data from X-ray and infrared telescopes to piece together some of the physical conditions of the blast,” said Derek Fox of Pennsylvania State University. ”

    And what did the spectra say about what elements were present? Any metals?

  4. The value for z is

    z = a(T)/a(t) ā€“ 1,

    for a(t) the radius of a region at time T and a(t) the radius much earlier. The Hubble relationship v = Hd indicates an approximate linear relationship with that radius and time. So for a(t) = 13.7 a(t) = .7 this would give z = 18.6, which is rather astounding. Of course out to the CMB with t = 3e5 and T = 1.37e10 one gets a z ~ 10^5. So an optical ~ 1 micron photon gets expanded into a .1cm photon, which is about right.

    I would tend to think that a z = 8.1 would put this back to about

    z = a(T)/a(t) ā€“ 1,

    a(t) = a(T)/(z + 1) ~ 13.7e10/9.1 = 1.5e9

    or more like twice what the paper states.


  5. The fact that this GRB explosion is so uniform in shape, why wouldn’t they at least speculate on the”texture” of the space surrounding it?
    One would think being so soon after the BANG that EVERYTHING would be different at that time, wouldn’t it?

  6. ” the GRB didn’t come from a monster star”

    I’m so confused. From where did it come from?

  7. GRB’s probably come from the inspiral collision of two neutron stars. The two neutron stars are in a mutual orbit that inspirals as it loses energy by emitting gravity waves. The Hulse-Taylor observation confirmed this. Eventually the two compact bodies crash into each other with an enormous release of energy.


  8. I made a statement last night that:

    So an optical ~ 1 micron photon gets expanded into a .1cm photon, which is about right.

    where I should have written it gets expanded to .1 meter or 10 cm. So this means for 300nm to 600 nm optical wavelength photons the redshifted photons would be stretched out to 3-6 cm. That is about the measured blackbody peak of the CMB.

    My estimate on the z, where I got this to be about double the quoted value probably reflects the limits to which the low redshift approximation works. I would have to work with the more general calcuation from the FLRW cosmology spacetime metric .

    Cheers LC

  9. That would be in the words of Buzz Lightyear, to infinity and beyond! šŸ™‚


  10. I am confused as to some of the observable quantities quoted in the article While GRB 09043 may hold the distance record for GRBs, several papers recently published have found what appears to be galaxies with redshifts over 8! (check out this recent paper made with the newly refurbished HST as evinced here: ). And it’s not just galaxies in the HUDF-N but other deep fields (COSMOS, GOODS, Chandra, etc.). Farthest known GRB, possibly. Farthest known object in the universe, highly improbable.

    Btw, Thanks to Salacious B. Crumb for a link to the original paper :0

  11. I just ran a calculation using the more exact formula. A linear redshift grows faster than the general relativistic one. I got z = 7.89 for the distance claimed. That appears to be pretty close.


  12. A little confused by things: if this was entirely observed in the infrared, how do we know it is a GRB? Is it possible it could be a closer object giving off only infrared? Or is there another measure of distance taken that I am missing ?
    Apologies if my question is very basic.

  13. @BeckyWS: it is, as always, entirely possible that we have been fooled by some cosmic coincidence …

    However, the location (position on the sky; (RA, Dec)) of the IR object is well within the error circle of the Swift GRB localisation, and there’s nothing with a light curve that resembles a fading GRB nearby … ergo, the IR source IS the GRB.

    (there’s a lot more, but it’s more indirect; hopefully the above is enough to get you started …)

  14. The clincher is that atomic spectra, which we know well, are redshifted. So an .5 microns transition line from an atomic species is redshifted to the wave length L = z(5nm), which for z = 8 is 4. microns. That is in the infrared region. This is one reason infrared astronomy is so important.

    Now we infer these distances and times by observing more local objects and their redshifts. A galaxy 100 million light years away is observed to have Cepheid variables. These have a luminosity/periodicity relationship and from that the distance can be backed out. This was how Ed Hubble came up with his

    V = Hd

    relationship, for d = distance H = 71Mpc/(km/sec) and v= velocity.


  15. Thanks for the replies Nereid and Lawrence.

    I have now realised I mis-read the article and thought that the initial burst was only in infra-red, as well as the afterglow, hence my confusion about how it could be termed a GRB…. :-S
    Quick astro-blog catch up sometimes not a good idea!

  16. A quick reply to Aqua’s question of the composition of the GRB derived from its spectra. My guess (after reading the paper) is that the spectra were too low in resolution to reliably infer the presence of other possibly constituents (which I believe were gathered with the 2.1m TNG scope). Of course followup with larger ground-based and orbiting observatories are probably already being planned. Due to its extreme distance, GRB 090423 would probably exhibit a low metallicity.

  17. Some followup work on this GRB got me thinking (for once!), might this observation tie into the long-sought but currently undetected Pop III stars, presumably short-lived, massive, extremely powerful sources, already invoked to some degree by some astronomers to (partially, at least) have some role in the universe re-ionization epoch? Or is there good evidence in favor of a distant GRB popping up at this time? Clearly time to perform an extensive, deep multiwavelength study of this object. I strongly feel that GRB 0909423 originated in a galaxy (as almost all GRBs detected). I suspect the host galaxy is located in a newly forming galaxy cluster (too faint currently for our ground-based or space-based observatories). Telescopes like the JWST and the current ESA Herschel , among others will certainly help in our understanding of this GRB and in what galaxy/ galaxy cluster GRB 090423 originates. The best is yet to come!

  18. I forgot to mention thanks go out to Nancy and all of those posting intelligent, though provoking questions and answers in a most civilized manner šŸ™‚ . This degree of civility and science discourse I find intellectually stimulating. Several other thought provoking articles and posts here at UT also make my day. That’s why I navigate to UT several times a day with legitimate questions and lucid, cogent and well thought out answers by other posters. Unfortunately, some articles are hijacked in the comments section w-individuals off topic for their own agenda or just plain trolling. Anyway, keep up the good work as I look forward to honest astronomical news

  19. I imagine this was likely a PopIII star. There was only H, D and He around then, with maybe traces of Li. I don’t know much about these, but from what I understand they pretty much implode, ignite S-D fusion rapidly and then explode.


  20. LBC, thanks for your reply to my post mentioning a possible Pop III interpretation. I was hoping that someone from the theoretical side would weigh in about this idea (i.e. any major objections, glaring problems, etc.). Of course, if this GRB had a Pop III progenitor, this would be big news in the astrophysics community. Obviously, many followup observations and close scrutiny of the available dataset are called for, to either refute or confirm the GRBs progenitor. And given the GRBs transience, this may take some time… šŸ™‚

  21. Okay, I think I need to bring a bit of order into this chaos…

    Concerning the last point, I’m pretty sure both papers point out that there is NO evidence for this being a Pop III star. Excepting this extreme distance, it looks just like a normal GRB – with the one difference that it is very short temporally. Check out by Zhang et al. (I’m a co-author) for more on that. This also should answer HeadAroundU – “a monster star” meant a supermassive Pop III, whereas this was due to a more normal (probably still 50 solar masses, but nothing extraordinary) star.

    @Aqua: As already more or less stated, the spectra do not allow any analysis beyond the redshift detection. They were taken many hours after the GRB, when the afterglow had already faded by several magnitudes. It just took some time to realize what a special event this GRB is, and by that time it was too late to observe from Hawaii. The TNG (@Jon Hanford: it is 3.6m and proud of it šŸ˜› ) was the first to be able to target it, and then came ISAAC at the VLT (already 17 hours after the GRB).

    @rudeyd: 600 million years after the Big Bang is a loooong time after the Big Bang. With the exception of the fact that reionization was still going on and metallicity was still low, the Universe was just like today.

    @Jon Hanford: We are talking spectroscopically confirmed redshifts here. The WFC3 HUDF observations are fascinating but much less conclusive in terms of distance. Therefore, GRB 090423 rules.

    also @Jon Hanford: Spitzer observed about 45 days after the burst at 3.6 microns (warm mission test phase), detecting a faint source in an ultra-deep integration. A follow-up is planned for early 2010. With high probability, this was still the afterglow, and the host will be too faint for – again probably – ANY existing technology. But for sure, JWST and possibly ALMA will target this source.

    Hope I wasn’t trolling. šŸ˜‰

  22. This does of course say there is no evidence of there being a supermassive star as the source. GRBs are, as I understand, thought to be due to the collision of two neutron stars. I was speculating that the neutron stars came from PopIII stars. There was an eariler report about PopIII stars being formed in pairs. So maybe the collapsed cores of PopIII stars ended up colliding subsequently. This observed GRB occurred a few hundred My after reionization.


  23. Don Alexander, thank you for your informative post (from a researcher involved in these studies, to boot).

    Excuse me for my previous error about the size of the TNG scope. Many thanks for your post clarifying and explaining the current observations (and thanks for the link to that 2nd paper). I do realize the difference in spectroscopically derived redshifts and less accurate photometric redshifts used to estimate distances. All these z>8 candidates described in in deep fields (using the photometric dropout method) gives us a rough estimate of the distance to the galaxy, but I have read that photometric redshifts of some extremely distant galaxies/ galaxy clusters may be off up to 20%.

    Also thanks for the clarification as to the properties and epochal age of these distant GRBs.

    Also, what is the current status regarding the use of GRBs as standard candles. IIRC, there was a push in the late ’90s and early ’00s to try to establish GRB luminosities as standard candles. Are there too many variables involved that would preclude the use of GRBs as standard candles?

  24. @ Don Alexander, I’ll be anxiously awaiting the publication of the paper detailing the IR observations made by Spitzer and what clues they may hold concerning this distant beacon. Objects like this one also serve to tell us the distribution of neutral hydrogen and other intergalactic clouds between Earth (e.g. the Lyman alpha forest) and the GRB, though supersensitive and timely observations will need to be coordinated. Hopefully the coming armada of giant, land-based scopes, future sensitive orbiting observatories and balloon-borne instruments may provide interesting insights into these enigmatic objects šŸ™‚

  25. I suppose I am a bit perplexed as to what makes a GRB. I have been under the impression the energy is released in the violent collision of two neutron stars. I guess I have thought these were far more energetic than a supernova.

    Also this event occurred 600 My after the big bang during the reionization period. If this GRB was due to the explosion of a star this star was a PopIII star — by definition. It might not have been a monster star, but it was a star with low metalicity (zero metalicity).


  26. @Jon: Well, as stated, a second, even deeper stare if planned for February 2010, so maybe the first results will be presented in Kyoto in April. Concerning the Lya forest, these really distant GRB don’t help much, because the Gunn-Peterson effect turns the forest into a wall. Everything blueward of Lyman alpha is completely gone (which normally does not happen until the Lyman cutoff). This is what makes getting photo-zs rather easy though.

    @LBC: Actually, they aren’t. The amount of mass available in a neutron star merger is much less than can be achieved in the cora collapse of a massive star. While short GRBs are though to come from NS-NS (or NS-BH) mergers, long GRBs are unequivocally associated with massive stellar death.
    Point two: Wrong. The first stars ignited somewhere between z = 50 and z =20. By z = 8, reionization was mostly finished, and since Pop III stars only live a few million years, there will already have been like 100 generations of SNe and ISM metal enrichment. At z = 8, true Pop III stars are actually a dying breed! (Though very new research seems to indicate stars massive enough to produce pair-production instability SNe even exist in the nearby universe).

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