The first billion years after the big bang (out of about 13.7 billion years total since then) were among the most interesting in terms of giving birth to the kind of objects that still dominate the scene today. Mostly that means stars and galaxies, plus a few exotica such as quasars. Unfortunately, it’s very difficult for astronomers to actually see what was going on back then.
There are three reasons for this difficulty. First, astronomers can detect objects at very early times only at very large distances from us, due to the light travel time. So such objects are likely to be very dim, if detectable at all. Second, due to the expansion of the universe, light emitted by objects in the very early universe will be shifted in wavelength to much higher values. This places a large portion of the light into the infrared part of the spectrum, which is difficult or impossible to observe with ground-based telescopes.
However, the third reason that very early objects are difficult to observe is that conditions in the early universe, namely the presence of a great deal of neutral hydrogen gas in the space between galaxies, obscures light from the galaxies just like atmospheric fog. This is not only unfortunate but also ironic, since a good determination of just how much “fog” was present is one of the key pieces of information that astronomers need in order to understand what objects in the early universe were really like. Astronomers need to know how much “fog” there was in order to correct for it so the visual characteristics of early galaxies can be determined. Yet lack of understanding when and how the “fog” cleared makes this effort rather frustrating.
Research that’s just been published gives new information that helps clarify things a little. It’s based on observations of just 5 very early galaxies, which are among the earliest, most distant galaxies known. What the research is telling us is that the “fog” was clearing rapidly at the time the light we now see from these 5 galaxies was actually emitted.
An international team of astronomers used the VLT as a time machine, to look back into the early Universe and observe several of the most distant galaxies ever detected. They have been able to measure their distances accurately and find that we are seeing them as they were between 780 million and a billion years after the Big Bang.
The new observations have allowed astronomers to establish a timeline for what is known as the age of reionisation for the first time. During this phase the fog of hydrogen gas in the early Universe was clearing, allowing ultraviolet light to pass unhindered for the first time.
The new results, which will appear in the Astrophysical Journal, build on a long and systematic search for distant galaxies that the team has carried out with the VLT over the last three years.
Astronomers are sure that they know what the “fog” consisted of: ordinary atomic hydrogen gas that is not ionized. A neutral (not ionized) hydrogen atom consists of an electron and a single proton. High-energy photons interact strongly with neutral hydrogen but not with ionized hydrogen. (As will be explained below.) In the early universe about 75% of the mass was in the form of hydrogen, and the rest was helium, with only a small trace of a few other elements. It’s known that this hydrogen “fog” dissipated as most hydrogen atoms became ionized. But it’s not known just when the process of ionization began or when it ended. Even less is it known exactly what caused the ionization. The new research, however, does indicate that the process was occurring rapidly at a specific point in the universe’s history.
Let’s take a closer look at the details of the process as they are currently understood.
The first thing to consider is how galaxies in the very early universe are found, and how their distance from us and the time at which we are seeing them can be determined. Quite a lot is based on the properties of hydrogen atoms themselves. This is especially the case, since hydrogen was (and still is) the most abundant element in the universe. Almost all the elements heavier than hydrogen and helium were formed in stars over the whole history of the universe.
Quantum mechanics dictates that the electron in a hydrogen atom can have only a discrete set of energy levels. Transitions between these levels are responsible for both the emission and absorption of light by hydrogen. Hydrogen atoms absorb light when they interact with photons that are energetic enough to kick electrons from lower to higher energies. Photons lose energy in this process. However, electrons in an exited state tend to drop back to lower energies spontaneously. Photons are emitted to carry off the energy lost by electrons.
The most important transitions are those of electrons between their ground state, in which they have the lowest possible energy in a hydrogen atom, and excited states. This creates a series – called the Lyman series – of absorption lines (and corresponding emission lines) in the hydrogen spectrum. The line corresponding to the transition of an electron from the ground state to the next highest energy level is called the Lyman-α (Lyα) line. It has a wavelength of 121.6 nm – far into the ultraviolet part of the spectrum. There are infinitely many lines in the Lyman series, with shorter wavelengths, corresponding to discrete higher energy levels to which an electron can be raised yet remain bound in the atom. But the lines come much closer together, up to a limit of 92.2 nm, the “Lyman limit”
In a young galaxy that is rapidly forming stars in the early universe, many stars are very large, bright, and hot. A great deal of their energy is radiated at the Lyα wavelength, which is thus a very prominent line in the spectrum. Its presence is a reliable signal of a galaxy that’s rapidly forming stars. Very hot stars can also emit photons more energetic than the Lyman limit (due to their helium content).
Any hydrogen that lies between us and the source and isn’t fully ionized tends to absorb all the Lyman series lines strongly, as well as more energetic photons. A hydrogen atom can be completely ionized by a sufficiently high-energy photon, but the photon loses much of its energy. Consequently, even though galaxies emit a lot of light at wavelengths less than 91.2 nm, it is never observed from distant galaxies. (Photons with less energy than Lyα can also interact with neutral hydrogen if the electron is not in its ground state, but this happens less often.)
Absorption of nearly all high-energy photons gives astronomers a means to identify very distant galaxies and their approximate distance by a simple technique. The expansion of the universe also expands the wavelengths of photons during their travel from their source to our instruments. This is redshift, denoted by the letter z. The redshift of light increases steadily, though in a complicated way, with the distance the light has to travel. The exact relationship depends on assumptions about the values of certain cosmological parameters, especially the actual rates of expansion at different times.
Redshift can take any value from 0 (no shift) on up. For example, a redshift of 6 corresponds to a distance (in light travel time) of 12.72 billion years, while a redshift of 7 corresponds to a distance of 12.89 billion years. Equivalently, since the big bang occurred about 13.67 billion years ago, redshifts of 6 and 7 correspond to times after the big bang of 950 million years and 780 million years, respectively.
The actual expansion in wavelength of a photon at different redshifts is a factor of z+1 (no expansion if z=0). Thus a photon at the Lyman limit (91.2 nm) would be stretched by a factor of 8 to 729.6 nm if z=7. That’s in the near infrared part of the spectrum. Now suppose an object is observed through a series of filters that pass only a narrow band of light. If the object is detected through filters that pass light above 730 nm, but not through filters that pass only light below 730 nm, chances are good that the object lies at a distance corresponding roughly to z=7. It’s not a sure thing. There can be false positives, so further tests should be made. But chances are good. This is the basic technique used to identify objects at specific redshifts. If the object is a galaxy, it’s called a “Lyman-break galaxy“.
One test that can be performed is to look for a shifted version of the Lyα emission line. For z=7 this should be around 972.8 nm, which is further into the infrared. It’s a pretty good confirmation if the line can be detected, but since such distant objects are so dim, it’s often not possible to get a useful spectrum.
In the present research, 5 objects were identified as Lyman-break galaxies near z=7 with detectable Lyα lines. That is well below the trend of galaxies detected in the same surveys for z≤6. The most likely, though not the only, possibility is that there was a substantial decrease in the amount of neutral hydrogen – the “fog” – at z=6 compared to z=7. The decrease could be as much as 60%. That doesn’t mean all of the “fog” was gone at a time corresponding to a billion years after the big bang, only that it was substantially less than it was just 170 million years earlier.
More research will be necessary to get a better estimate of how much neutral hydrogen was left at z=6, as well as the general trend beyond z=7. And then there’s the task of identifying the cause of this reionization. It could be simply all the formation of very hot, bright stars emitting strongly in the far ultraviolet that was happening at the time. Such energetic photons ionized the hydrogen, even though the photon was absorbed in the process. Alternatively, it could be ultraviolet emissions from active galactic nuclei if supermassive black holes within galaxies were already common at the time.
|L. Pentericci, A. Fontana, E. Vanzella, M. Castellano, A. Grazian, M. Dijkstra, K. Boutsia, S. Cristiani, M. Dickinson, E. Giallongo, M. Giavalisco, R. Maiolino, A. Moorwood, & P. Santini (2011). Spectroscopic confirmation of z~7 LBGs: probing the earliest galaxies and the epoch of reionization Astrophysical Journal arXiv: 1107.1376v1|