Astronomy Picture of the Day

Explanation: Dubbed a Lyman-alpha blob, an enormous cloud of hydrogen gas spans several hundred thousand light-years in this remarkable image (left), a composite of x-ray, optical, and infrared data from space and ground based observatories. The gigantic, amoeba-like structure is seen as it was when the universe was a mere 2 billion years old (about 12 billion years ago). Lyman-alpha blobs are so called because they strongly emit radiation due to the Lyman-alpha emission line of hydrogen gas. Normally, Lyman-alpha emission is in the ultraviolet part of the spectrum, but Lyman-apha blobs are so distant, their light is redshifted to (longer) optical wavelengths. X-ray data (blue) indicates the presence of a supermassive black hole feeding at the center of an active galaxy embedded in the blob. Illustrated close up in the right hand panel, radiation and outflows from the active galaxy are thought to be a source for energizing and heating the blob's hydrogen gas. In fact, Lyman-alpha blobs could represent an early phase in galaxy formation where the heating is so great it begins to limit further rapid growth of active galaxies and their supermassive black holes.

Sorry, forgot to include the graphics. I'll ask the mods to delete the earlier post.

Lyman Alpha Forest

There are clouds of gas between distant quasars [QSO: quasi-stellar object] and the Earth that absorb ultraviolet light at the wavelength of the Lyman alpha line of hydrogen at a wavelength of 122 nm. Quasars also emit a strong Lyman alpha emission line. But the absorbing clouds all have smaller redshifts than the quasar since they have smaller distances. As a result the absorption lines are all on the blue or shorter wavelength side of the quasar emission line.

The cartoon above shows a quasar with its Lyman alpha emission line redshifted from the ultraviolet into the red, and the Lyman alpha absorption lines from four intervening clouds appearing as orange, yellow and green-blue.

The figure below shows two actual quasar spectra. One is the nearby quasar 3C273 while the other is a large redshift object. This figure was adapted from Bill Keel's web site.

The clouds that produce the absorption lines are much more numerous at high redshifts so a very dense array of absorption lines results: the "forest".

We know that there are a small number of very big clumps of hydrogen in the distant Universe: the galaxies. We also know that smaller galaxies, the dwarf galaxies, are very much more common. Most of the clouds in the Lyman alpha forest are much less massive than dwarf galaxies and these small clouds are much more numerous. We can only see these very low mass clouds by the absorption they produce in the strongest line of the most abundant element: Lyman alpha. Thus by studying the Lyman alpha forest we can learn about the density fluctuations in the Universe on the smallest observable scales.

Note that if Arp were correct and quasars had a redshift much larger than the redshift due to their distance, then there should be a gap on the blue side of the Lyman alpha emission line before the absorption lines began. Such gaps are not seen. So if Arp were correct the Lyman alpha forest would have to be an intrinsic property of the quasar, which would be a very unlikely situation. Distant galaxies are seen which also show the Lyman alpha forest, so we know that the intervening clouds do exist. For Arp to be correct the intrinsic absorption lines would have to act exactly like the intervening clouds would act under the standard hypothesis that the quasar redshift is entirely cosmological.

Source:
http://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html
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Also see...

Quasars as lighthouses: the Lyman-alpha forest at low and high redshift:
http://www.astr.ua.edu/keel/agn/forest.html

Many (most?) are believed to be proto-galaxies, and so are enormous in scale.

“We hypothesize that a lyman alpha cloud does not need to transport angular momentum outward as it collapses to form a galaxy with a flat rotation curve, M_interior \propto R. A cloud with uniform density and solid-body rotation naturally has the angular momentum distribution of a disk with a flat rotation curve, so the cloud should be able to collapse to a spiral galaxy while conserving the angular momentum and radial ordering of all mass elements. To test this hypothesis, we will use a simplified nbody + hydrodynamics code which can rigorously banish all transfer of angular momentum while still preserving the essential features of dissipative baryonic collapse.”
http://flux.aps.org/meetings/YR02/APR02/baps/abs/S6260005.html

Also see:
http://markelowitz.com/cosmology.htm

Lyman-alpha forest
From Wikipedia, the free encyclopedia

In astronomical spectroscopy, the Lyman alpha forest is the sum of absorption lines arising from the Lyman alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars.

These absorption lines result from intergalactic gas through which the galaxy or quasar’s light has travelled. Since the absorption and emission of light follow the laws of quantum mechanics, only photons with specific energies can be absorbed. This causes each individual absorption line. The forest is created by the fact that photons that come to us from distant light sources show Hubble redshift that depends on the distance between us and the source of light.

Since neutral hydrogen clouds at different positions between Earth and the distant light source see the photons at different wavelengths (due to the redshift), each individual cloud leaves its fingerprint as an absorption line at a different position in the spectrum as observed on Earth.

The Lyman alpha forest is an important probe of the intergalactic medium and can be used to determine the frequency and density of clouds containing neutral hydrogen, as well as their temperature. Searching for lines from other elements like helium, carbon and silicon (matching in redshift), the abundance of heavier elements in the clouds can also be studied. A cloud with a high column density of neutral hydrogen will show typical damping wings around the line and is referred to as a damped Lyman alpha system.

For quasars at higher redshift the number of lines in the forest is higher, until at a redshift of about 6, there is so much neutral hydrogen in the intergalactic medium that the forest turns into a Gunn-Peterson trough. This shows the end of the reionization of the universe.

http://en.wikipedia.org/wiki/Lyman-alpha_forest

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