lyman alpha systems and cosmology

Lyman alpha systems and cosmology

Lyman alpha systems are becoming a very useful source of information in physical cosmology.

The Lyman series is the series of energies required to excite an electron in hydrogen from its

lowest energy state to a higher energy state. Thecase of particular interest for cosmology is where a a

hydrogen atom with its electron in the lowest energyconfiguration gets hit by a photon (light wave) and is

boosted to the next lowest energy level. The energy

levels are given by En = -13.6 eV/n2 and the energy

difference between the lowest (n=1) and second

lowest(n=2) levels corresponds to a photon withwavelength 1216 angstroms. The reverse process can

and does occur as well, where an electron goes from the higher n=2 energy state to the groundstate, releasing a photon of the same energy.

The absorption or emission of photons with thecorrect wavelength can tell us something about the

presence of hydrogen and free electrons in space.That is, if you shine a light with wavelength 1216 at a

bunch of neutral hydrogen atoms in their groundstate, the atoms will absorb the light, using it to

boost the electron to a higher energy state. If thereare a lot of neutral hydrogen atoms in their ground

state, they will absorb more and more of the light. Soif you look at the light you receive, intensity as a

function of wavelength, you will see a dip in the intensity at 1216 angstroms, depending on theamount of neutral hydrogen present in its ground state. The amount of light absorbed ('optical

depth') is proportional to the probability that the hydrogen will absorb the photon (crosssection) times the number of hydrogen atoms along its path.

Because the universe has many highenergy photons and hydrogen atoms,

both the absorption and emission ofphotons occurs frequently. In Lyman

alpha systems, the hydrogen is found inregions in space, and the source for the

photons are quasars (also called qsos),very high energy light sources, shining at us from behind these regions.

Because the universe is expanding, one can learn more than just thenumber of neutral hydrogen atoms between us and the quasar. As

Lyman alpha systems and cosmology http://astro.berkeley.edu/~jcohn/lya.html

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these photons travel to us, the universe expands, stretching out all the light waves. This

increases the wavelengths lambda and lowers the energies of the photons (`redshifting').

Neutral hydrogen atoms in their lowest state will interact with whatever light has been

redshifted to a wavelength of 1216 angstroms when it reaches them. The rest of the light willkeep travelling to us.

The quasar shines with a certain spectrum or distribution of energies, with a certain amount ofpower in each wavelength. At right, the top picture

shows a cartoon of how a quasar spectrum (the fluxof light as a function of wavelength) might look if

there were no intervening neutral hydrogenbetween the qso and us. In reality, gas around the

quasar both emits and absorbs photons. With thepresence of neutral hydrogen, including that near

the quasar, the emitted flux is depleted for certainwavelengths, indicating the absorption by this

intervening neutral hydrogen. As the 1216wavelength is preferably absorbed, we know that

at the location the photon is absorbed, itswavelength is probably 1216 angstroms. Its

wavelength was stretched by the expansion of theuniverse from what it was initially at the quasar,

and, if it had continued to travel to us, it wouldhave been stretched some more from the 1216

angstroms wavelength it had at the absorber. Thuswe see the dip in flux at the wavelength

corresponding to that which the 1216 angstrom(when it was absorbed) photon would have had if it

had reached us. As we can calculate how theuniverse is expanding, we can tell where the

photons were absorbed in relation to us. Thus one can use the absorption map to plot thepositions of region of intervening hydrogen between us and the quasar. The middle picture at

right shows the flux for one nearby region while the bottom picture shows the case forseveral intervening regions.

It is common to see a series of absorption lines, called the Lyman alpha forest. Systems whichare slightly more dense, Lyman limit systems, are thick enough that radiation doesn't get into

their interior. Inside these regions there is some neutral hydrogen remaining, screened by theouter region layers. If the regions are very thick, there is instead a wide trough in the

absorption, and one has a damped Lyman alpha system. Absorption lines generally aren't justat one fixed wavelength, but over a range of wavelengths, with a width and intensity (line

shape) determined in part by the lifetime of the excited n=2 hydrogen atom state. Thesedamped Lyman alpha systems have enough absorption to show details of the line shape such as


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that determined by the lifetime of the excited state. These dense clumps are thought to have

something to do with galaxies that are forming.

One can use the ionization of neutral hydrogen to find galaxies as well, for example the Lyman

break systems described in these technical conference proceedings.

There is a very cool visualization of how different systems absorb Lyman-alpha here by

Andrew Pontzen.

Some numbers:

How dense are the Lyman alpha forest systems (column densities)?

1014 atoms per square centimeter. Lyman limit systems are 103 times denser and damped

Lyman alpha systems another factor of 103 times more.

How far away are the quasar sources?To be observed from the ground, the Lyman alpha wavelength needs to be stretched by

factors of 3 to 5. The amount of "stretching" is related to redshift z by stretch = (z +1).So ground observed sources have redshifts z of 2-4. A qso with redshift of z=2 emitted

its light when the universe was less than about 1/4 of its current age (that is, about 1/4of 13 billion years or so), while at redshift z=4 we are seeing objects from when the

universe was less than 1/10 of its current age.How many systems are expected in upcoming surveys?

Large statistical samples which have been publicly relased include the 2dF-SDSS quasarsurvey (2SLAQ) SDSS and the SDSS Quasar catalogue (latest releast spring 2010

abstract is here with 105783 quasars). Currently, BOSS is underway to produceLyman-alpha forest spectra of 160,000 quasars at redshifts 2.2

Uses: We learn several things from these systems, including the following.

Neutral hydrogen: because we see any light at all, we can limit how much neutral

hydrogen is out there between us and the quasar and what its distribution is. Itused to be thought that there was a smooth intergalactic medium (IGM) with

regions embedded in it, and the smooth background would provide an absorption atall positions between us and the quasar (Gunn-Peterson effect). But observers only

see evidence of lumpy regions. There isn't evidence for a spatially smoothcomponent of neutral hydrogen between us and the quasar sources. It is a question

of active research what is making the amount of neutral hydrogen so small (that is,what is ionizing the rest of the hydrogen).

Structure formation: the regions in the Lyman alphasystems are not very massive compared to objects like

galaxies. As a result, reliable computer simulations(numerical experiments) of their gravitational collapse

(formation) from primoridal fluctuations are possible.


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Until the 90's, it was thought that gravity alone

could not form all the structure, in particular thefilaments, walls, and voids which we observe.

However, the Lyman alpha simulations did producethis structure by starting with small fluctuations in

matter density and then letting gravity and other known forces act. It was notnecessary to add other mechanisms to get the observed structure in these systems.

The detailed properties of the structure and distribution of matter are frontiers ofcurrent research. At right is a picture of the distribution of neutral hydrogen found

in simulations.

Hot dark matter: The numerical simulations have also shown, via their successful

reproduction of properties of the regions, that one cannot have too much hot darkmatter if one wants to agree with observations. (Too much hot dark matter erases

structure on small scales.)

Distribution of matter: The Lyman alpha regions are formed by gas falling into

gravitational potential wells of all the matter, not just the luminous matter. So theyprovide another tracer of dark matter.

Nucleosynthesis: deuterium is produced in 'the first three minutes' in the earlyuniverse, and afterwards is believed to only be destroyed. The Lyman alpha systems

have deuterium in them too, and as these systems also have low amounts of metals(heavier elements), one might hope that they are measuring unprocessed or

primordial deuterium. The deuterium also absorbs light from quasars, and thus itsabundance can be measured in a similar way. Searches in many of these systems

have provided the current strongest constraint on the amount of primoridaldeuterium and thus the baryon density in the universe. See this link or this poster

on big bang nucleosynthesis.

Cosmological constant: the path back to a given redshift depends on how the

universe has expanded since that time. The angular extent of an object at anyredshift also depends on the expansion of the universe since the light was emitted,

but in a different way. Thus one can compare angular and radial (more preciselyredshift) lengths for objects. If one knows the expected ratio of these lengths for

an object for other reasons, one can constrain the expansion history of theuniverse, in particular the cosmological constant. A variant of this Alcock Pacynzski

test involves tracing lines of sight through the Lyman alpha regions for neighboringquasars.

Further reading:

Lya notes by M. White (~2005) for reading group at UC Berkeley

"The Lyman-alpha Forest as a Cosmological Tool", David H. Weinberg, Romeel Dav'e,Neal Katz, Juna A. Kollmeier, AIP Conf.Proc. 666 (2003) 157-169


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"Shadows of Creation: Quasar Absorption Lines and the Genesis of Galaxies," Jill

Bechtold, Sky and Telescope, Sept. 1997Perspectives in Astrophysical Cosmology, by Martin Rees, Cambridge University

Press, pp. 88-94.Lecture notes by D. Weinberg on QSO absorption lines and the IGM

Lecture notes by D. Weinberg on Cosmology with the Ly-alpha forest

Back to theoretical cosmology

Please send suggestions to [email protected],thanks to M. White and L. Hernquist for suggestions and figures.


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