Lyman-alpha forest
By Yuan-Sen Ting (Harvard University). May, 2013.
Studying the intergalactic medium
Previously, when we discussed the stellar line formation (link), we talked about studying spectra due to stars and the gas between stars (uninventively termed as the interstellar medium). However, this analysis is not only restricted to these gases. If the light source is outside our own Galaxy (the Milky Way), we can even study gas outside the Milky Way such as other galaxies, and also the gas between the galaxies -- the intergalactic medium. Of course, we can only see the light from stars that are relatively nearby -- those in the Milky Way and the nearest galaxies. However, the Universe is much (much!!) bigger than this, and so we need a more powerful light source to probe the intergalactic medium. It turns out that exotic objects called -- quasars fit the bill. Quasars are extremely massive black holes (billions of times more massive than our Sun!) located at the centre of external galaxies. They accrete gas from their own host galaxies and produce powerful light beams in this process. In the course of the light travelling to us from the quasar, if there are intervening galaxies or neutral gas, the light from the quasar will be absorbed by these gases.
Furthermore, the Universe is expanding. Therefore, the spectrum of the light source is continuously being redshifted; in other words, the wavelength increases as the farther the light travels from the source. You can think of the expansion of space being like an elastic string that is continuously being stretched. This expansion of space also stretches the light in term of its wavelength. Hence, the wavelength at which the light is absorbed depends on when the light is absorbed and therefore depends on the distance of the intervening gas or galaxy. By looking at a spetrum, and studying at which wavelength the light is absorbed, we can then tell there must be galaxies or gases at specific distances corresponding to these wavelengths, even though we might not even see the gases themselves! By studying the spectral lines at these wavelengths will then inform us on the properties of gas from objects that are both near and far from us. Note that since light travels with a finite speed and takes time to reach us, by studying objects farther away from us, we are essentially looking back in time and therefore we are studying the evolution of gas in the cosmic history.
Since hydrogen is the most prevalent element in the Universe, most of the light outside the Milky Way is absorbed by hydrogen. The physicist -- Theodore Lyman contributed significantly in the study of this absorption -- in the 19th century. Therefore, the primary absorption line due to hydrogen is called the Lyman-alpha line. The effect of seeing many such lines at different wavelengths superimposed on the quasars spectrum due to hydrogen in the intergalactic medium is called the Lyman-alpha forest. In the interactive module, one can study the distribution of hydrogen gas in term of its distance from us by changing the redshift index in the applet. By increasing this value, we assume that there are more hydrogen gas located farther away from us. One can also change the distribution as a function of density by changing the column density index. By increasing this value, we assume that there are more low density gas clouds than high density gas clouds. Given the same total column density, since the absorption might get saturated, doubling the density in a single gas cloud does not always double the absorption. In turn, many low density gas will create more absorption than a few high density gas, even their total densities are the same. One can also include/take off other absorption lines due to hydrogen, such as the Lyman-beta and Lyman-gamma absorptions in the applet.
Gunn-Peterson trough
Most of the intergalactic medium is made up of plasma with temperatures of a few ten thousands of degrees Fahrenheit/Celcius. In such a hot environment, the electrons become separated from their host atoms. Therefore, instead of considering hydrogen as a combination of proton and electron as well as being neutral, the proton and electron behave like free particles. Hydrogen in these conditions is known as being ionized.
Attentive and persistent readers might wonder how do we know the intergalactic medium is made up of plasma in the first place? In 1965, astrophysicists James E. Gunn and Bruce A. Peterson calculated the expected level of Lyman-alpha absorption they should see in the quasar spectra available at that time. It was not yet known that the intergalactic medium is very hot, so they assumed that all atoms between the galaxies were neutral. They predicted that all of the light from the quasar should be absorbed by the neutral gas that fills the Universe (to see this in the interactive module, increase the column density as much as possible).
But this is not at all what was observed; to their surprise, not all of the light was absorbed in the observed quasar spectra. To explain this observation, they conclude that this is possible only if the hydrogen is in fact not primarily neutral, but mostly ionized. The Lyman-alpha absorption process occurs when an electron in an atom moves from one energy level to another, and cannot occur in the case where electrons can move freely. Therefore, if most of the hydrogen is ionized, the quasar light can pass through the intergalactic medium without being severely absorbed, exactly like what Gunn and Peterson observed. Gunn and Peterson's study thus shows that the intergalactic medium is indeed filled mostly with plasma.
Modern research shows that the early Universe was once neutral. The intergalactic medium only became ionized after more and more stars formed. The emission of radiation (mostly at ultraviolet wavelengths) from these stars heated the gas and ionized the medium between the galaxies. It is still an active research area to understand when the Universe started to become dominated by ions instead of neutral atoms. This is a very important question for scientists as it traces the star formation history of the Universe.
But how could we possibly study that? One of the clever ways proposed is illustrated in Figure 3. As calculated by Gunn & Peterson, decent amount of neutral hydrogen gas will cause complete absoprtion in a quasar spectrum. By studying at what distance onward there is total absorption from the Lyman-alpha forest, we can estimate the distance at which most of the atoms in the intervening gas are neutral. Since looking at objects far away is the same as to looking back in time, we can then estimate when the Universe started to become dominated by ions (i.e. when there began to have some transmission in the Lyman-alpha forest). This region of total absorption in the Lyman-alpha forest, which corresponds to the time when the Universe was mostly neutral, is known as the Gunn-Peterson trough named to remember the contributions from James E. Gunn and Bruce A. Peterson.
