Science and Reason: Most Distant Known Object In The Universe

On April 23 of this year the Swift Gamma-Ray Burst Telescope detected, just as it was designed to do, a gamma-ray burst (GRB). Within less than a day two of the most powerful Earth-based telescopes had begun studying the quickly fading light of the object, known as GRB 090423. (There were other observatories investigating it as well.)

Because the light from GRBs fades so rapidly, most such objects are detected from space-based instruments that are especially designed for the purpose, like Swift. On average about 1 GRB is detected every 2 or 3 days. But GRB 090423 turned out to be, perhaps, the most interesting one yet observed.

I've written about GRBs a couple of times before, such as here and here.

You can refer to those articles for more details, but the most widely accepted hypothesis concerning the nature of GRBs is that they result from either the core collapse supernova explosion of a massive star or the merger of neutron stars in a binary system. Both processes most likely occur. In the former case, the emission of light lasts somewhat longer than in the latter, so the corresponding types of GRBs are called either "long" or "short".

The long type lasts for more than 2 seconds and tends to be brighter than the short type (whose emissions last less than 2 seconds). Since short GRBs typically appear in locations where new star formation is not occurring, they must result from some cause other than the explosion of a very massive star, which is necessarily quite young. In both cases, there is a prolonged, but much dimmer "afterglow", which is the only part of the event that can be observed except from satellites such as Swift.

Not just any supernova or neutron star collision will result in a detectable GRB. Most of the energy from the event must also be directed in two very narrow and oppositely directed beams, and the Earth must be in the path of one of those. It is because the energy is so narrowly concentrated that a GRB can appear to be far brighter than an entire galaxy or quasar. For such narrowly focused beams to be produced, the progenitor star (or binary system) must have a great deal of angular momentum, due to rapid rotation.

The afterglow is thought to result from the interaction between the matter and energy contained in the beam and the interstellar medium at the location of the GRB. Although it's a secondary effect, there is a wealth of information that can be deduced from this afterglow.

However, the information concerns more than just details about the nature of GRBs – events that occur much closer to us are much more useful for that. There are two other things of far greater interest about which we can learn from very distant GRBs, such as 090423 and subsequent examples we are likely to observe.

Since this event had a redshift of z ≈ 8.2 it actually occurred very long ago, a comparatively short time after the big bang. About 630 million years after, to be more exact. GRB 090423 is farther and earlier than any other object or event we've actually observed, except for the cosmic microwave background (CMB). It is more distant in time and space than even any galaxy or quasar we've ever seen. (See here if you want to review how redshift works.)

GRBs, directly or indirectly, tell us about the nature of the largest stars at that time, which is one thing that's presently very difficult to study with other observations. There's simply no way to observe individual stars 13 billion light-years away. The other thing astronomers are very curious about regarding that time period is the nature of the intergalactic medium (IGM) then.

Let's start with the second of these things. 630 million years after the big bang is somewhere in the middle of what astronomers call the "cosmic dark ages". Precisely where it falls is what we don't know. I wrote about that subject here, last March.

We do know pretty closely when the "dark ages" began – about 380 thousand years after the big bang. That's when the CMB appeared. The time is equivalent to a redshift of z ≈ 1100. The CMB was, and still is, "blackbody" radiation, so wavelengths are distributed around a peak. At the peak of the distribution now, CMB photons have a wavelength of 1.9 mm, in the short microwave part of the spectrum. (That's a lot shorter, so more energetic, than in your microwave oven, where microwaves are about 12 cm.) But when the CMB actually appeared, its photons had a wavelength 1100 times shorter, around 2200 nm – in the infrared part of the spectrum. That's still much cooler and "darker" than human eyes can see.

Effectively, then, there wasn't much light in the universe, certainly not human-visible light, until the first stars showed up. So the time after the CMB appeared but before the first stars is known as the "dark ages". However, we don't know very closely when those first stars appeared – they're far too faint to observe, and even the first galaxies and quasars are too dim to be detected by our present instruments. GRBs seem to be our best observational hope, and GRB 090423 is the earliest we've seen yet.

Since existing GRB models presuppose an origin that involves massive stars, at least indirectly, we now know there were such stars 630 million years after the big bang. There is evidence in the GRB 090423 observations that the progenitor of this object was not one of the first population of stars to form. (Astronomers call such stars "Population III".) Also, GRB 090423 doesn't seem to have been one of the most powerful GRBs ever, so astronomers now figure that with current technology we may be able to find GRBs back to z ≈ 20. That would correspond to 250 million years after the big bang.

Theoretical models suggest that the first stars may have appeared even earlier than that, at perhaps 150 million years after the big bang. When they formed, these first stars did not contain elements heavier than helium, as such elements were first created (in other than trace quantities) only in the earliest generation of stars. (See here for a discussion of the earliest stars.)

The observations of GRB 090423 give some evidence that the galaxy in which it occurred did have small amounts of elements heavier than helium. Further, the afterglow that occurs following the peak brightness of any GRB is generally similar between both GRB 090423 and much closer GRBs. These facts are hints that the progenitor of GRB wasn't one of the Population III stars.

How does a GRB give us information about the intergalactic medium at z ≈ 8.2? That turns out to be closely related to the way that the redshift was estimated in the first place. Normally this is done by identifying known emission or absorption lines in a spectrum and simply calculating the amount of shift directly. However, GRB 090423 is so faint, and declined in brightness so quickly, that it wasn't possible in the time available to obtain a detailed spectrum.

Instead, an important feature of the IGM at that time comes to the rescue. The feature is that there is at that time a substantial, though not precisely known, quantity of un-ionized hydrogen atoms in the IGM. This hydrogen is left over from the period of "recombination" in which the CMB appeared. CMB photons are far too weak to "scatter" from hydrogen atoms, because quantum mechanics requires that a photon must have a certain minimal amount of energy to raise an electron bound in a hydrogen atom out of its "ground state" of lowest energy.

The minimum amount of energy required is that possessed by a 10.2 eV photon of wavelength 121.6 nm, which is in the ultraviolet part of the spectrum. This spectral point is known as "Lyman-α". (We covered this concept in detail here.) Ordinary stars like the Sun in the universe at present emit little light at this wavelength, but much larger, hotter, and brighter stars emit quite a bit of this ultraviolet light.

Consequently, as soon as such hot stars began to shine, the atomic hydrogen in the IGM began to reionize. There was a price to be paid for this, of course: all the photons with enough energy lost some of their energy in the process of reionizing the hydrogen. And so, most of the light whose photons had enough energy was absorbed or "scattered" in this reionization period.

Eventually most of the hydrogen in the IGM of the early universe would become fully ionized, so that very energetic photons could again pass unimpeded. That stage marks the end of the reionization period. The whole period may have lasted from 150 million years to a 900 million years after the big bang, but we are quite unsure of the actual endpoints.

GRBs may be able to tell us. To begin with, it's easy to derive the redshift of a GRB that flared during the reionization period. If the redshift is z, then in the "rest frame" of the GRB, the light will start to be absorbed strongly at 121.6 nm. But we will observe this drop-off to occur at a wavelength of (z+1)×121.6 nm. For example, if z = 8.2, we see the drop-off at 1120 nm, which is in the infrared. Reasoning backwards, we can compute z from the observed drop off.

If we have enough GRB observations with sufficiently good spectra available, we can infer the amount of reionization that has occurred from the rate of drop-off. Further, if we find GRBs where there is no drop-off, we can infer that reionization is complete. Unfortunately, GRB 090423 tells us little by itself, and only two other GRBs have been observed with z > 6, corresponding to somewhat less than 950 million years after the big bang.

Why do we care about the rate of reionization? Because it tells us indirectly about the rate of formation of very large, hot stars in the early universe. So it turns out that the two things that astronomers want to know about the early universe – the nature of the IGM and the rates at which stars were forming – are pretty closely related. On top of that, the distribution in time of early GRBs gives us an independent estimate of the rate of star formation, since the progenitors of GRBs are exactly the sort of large, hot stars that cause reionization.

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