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The space between superclusters is not absolutely void. Invisible clouds of hydrogen have been detected by their effect on the spectra of distant quasars. As shown in Figure 03-07, when light from the quasars travels through the hydrogen clouds, each cloud imprints an absorption line of Lyman-alpha onto the continuum spectrum. As the clouds are expanding with different rate respect to the quasar, a series of red-shifted lines called Lyman-alpha forest is formed. These features encode information about the distribution and density of cold gas along the line of sight to the quasars. |
Figure 03-07 Lyman-alpha Forest [view large image] |
Incidentally, such pattern constitutes another evidence for cosmic expansion. |
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Figure 03-08a shows two quasar spectra. One is the relatively nearby quasar 3C273 at redshift z = 0.158 while Q1422 is a high redshift object at z = 3.62. The intervening neutral hydrogen clouds would absorb that part of the continuum spectrum from the light source at  = 0/(1+z'), where 0 = 1216Å for the Lyman alpha spectral line and 0 z'z is the relative red shift between the source and the clouds. Since there are more clouds in between the high redshift object and us, the Q1422 spectrum displays much more numerous absorption lines earning the name of a "forest". The data also show that there are small number of very big clumps of hydrogen in the distant Universe: the galaxies, but small chunks related to the dwarf galaxies are very much more common. |
Figure 03-08a Lyman-alpha Forest Examples [view large image] |
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Huge blob of Lyman alpha cloud (hundred times larger than the galaxies) has been found in the 2000's. They are seen in an era about 11 billion years ago when galaxies collided and merged with bursts of star formation. The Lyman alpha emission could be either from cooling gas or a central source such as a black hole. Detection of polarized emission suggests that photons from a central source are able to escape because the medium is highly ionized or its wavelength is shifted by random motions of the atoms, otherwise it would be repeatedly absorbed trapping it inside the cloud. Such kind of scattering would form concentric ring patterns as observed and shown in Figure 03-08b. |
Figure 03-08b Lyman-alpha Blob Polarization [view large image] |