Quasar Absorption Lines

SpaceTiger

Nereid brought up the subject of quasar absorption lines in the "Dark Matter" post. I worked on the subject as an undergrad, so I would be happy to answer any questions you have about it. Some examples of things it tells us are

1) The power spectrum of matter in the universe (small scales).
2) The metallicity and abundances of intergalactic gas.
3) The conditions around galaxies.
4) Deuterium abundance for nucleosynthesis measurements.
5) Kinematics of gas, such as starburst outflows.

Any discussion of a non-cosmological interpretation of redshift is unwelcome in this thread, as it has been thoroughly refuted in the scientific literature. Please take such things elsewhere.

Nereid

Thanks very much Space Tiger!

If I may go first ...

1) For how many quasars is there an apparently strong relationship between (at least one) absorption system and a 'line-of-sight' cluster which is resolvable (at least the giant ellipticals), and has a spectroscopically (not photometrically) determined redshift (and that redshift matches the QSO system)?

2) How many of the lensed quasars have absorption systems which 'match' the lensing object? What sort of 'match'?

3) What do systems at different redshifts tell us about how the metallicity of the absorbing gas has changed over time? At any given redshift, what is the observed range of metallicity in the systems?

SpaceTiger

The relationships are very strong for metal-line systems. That means, when we see absorption systems that are strong enough to contain absorption lines from heavy elements (like magnesium), then we often see galaxies nearby (and at the same redshift). There are no strong correlations, however, between the Lyman alpha forest (that is, hydrogen absorption of moderate strength) and galaxies/clusters. We believe this is because the forest is created by the large-scale distribution of matter in the universe and not extreme overdensities (like clusters).


Unfortunately, there's no easy way to simply match the two (other than to match the absorption redshift with the galaxy redshift). There are only a finite number of things you can measure about the absorption lines and most of them (temperature, density, metal content) won't be uniquely identifiable with a single galaxy. The best thing to do is to match redshifts and position on the sky, as I mentioned above.

Lensed quasars are, however, extremely useful for measuring the geometry of absorbers. Normally, absorption lines only probe the gas distribution along the line of sight and tell you nothing about the shape or clumpiness of the structure. Having two identical quasars very near each other allows you to see how the absorption changes transverse to the line of sight.


Metallicity varies a lot at any given redshift, so it's not a particularly good "cosmic clock". The reason is that you'll sometimes be probing young star-forming systems with a lot of metal enrichment, while other times you'll be probing clumps of gas that are distant from star-forming regions. Even within the Milky Way, we see at least 6 decades of variation in metallicity. I don't know if there have been any global compilations of statistics on the metallicities of all systems vs. redshift, but there are countless papers on the distribution within given subsets. Usually, they use it to constrain the environment in which the absorber is living.

Chronos

Why are the absorption lines so wide? I have a peripheral issue with that. Furthermore, Nereid knows way too much about this stuff to be a casual observer. I have read just enough papers to suspect she is holding out on us...

SpaceTiger

Which absorption lines are you referring to? Some of them are extremely narrow. A summary of the broadening mechanisms for absorption lines can be found here:

http://www.physicsforums.com/showpos...46&postcount=9

In quasar absorption lines, it's usually a mix of thermal broadening and turbulent broadening. Natural broadening does show up, but mostly in the strongest lines of hydrogen (damped lyman alpha systems).

wolram

Space Tiger.

Would it be correct to say, high metallicy, is a signature of high activity,
rather than, its a signature of age?

SpaceTiger

It is a signature of both, but much more so of star formation activity. The two are difficult to untangle.

Nereid

What are 'damped lyman alpha systems'?

turbo

The existence of damped lyman alpha systems is inferred from the spectra of some quasars, which exhibit a dip in luminosity over a broad range of frequencies. This is assumed to demonstrate the existence of tremendously deep (line of sight) clouds of neutral hydrogen. The absorption dips are generally assumed to be cosmologically redshifted artifacts of the primary absorption frequency of neutral hydrogen at 1216 angstroms. Like the lyman alpha forest artifacts in quasar spectra, most astronomers who adhere to the conventional model, assume that the redshifts of these absorption features are entirely cosmological, and are reliable indicators of the distance to the absorbing clouds, and the extent of the absorbing clouds. There are some people who believe that the redshifting of these spectral features might not be entirely due to cosmological expansion, but Space Tiger has very wisely disinvited these poor misguided individuals from contributing to his thread.

Garth

So one question is: "What does the metallicity of high z Lyman forests tell us about that early environment?
Garth

Chronos

The short answer: The universe reionized at a very early age. I like gamma bursters as an option. Huge, metal free, Pop III stars blasting away only millions of years after recombination. Something I wonder, if the IGM was sufficiently dense in the neighborhood, could the shock wave of an early SM briefly ignite the cloud?

SpaceTiger

Good question, and one that's hard to answer without a blackboard. I will try, however. As I mention in the post I linked earlier, there are several mechanisms for broadening an absorption line. Let's imagine a simple absorbing cloud with no turbulent motion. If we're looking at a light beam that has passed through this cloud, we'll see an absorption line. If it has low pressure, then there will be two mechanisms broadening the line: thermal motion and natural broadening (as required by the uncertainty principle of quantum mechanics). Now, if the cloud is small, only a small fraction of the light will be absorbed. This is called the optically thin regime and is characterized by the fact that the total light absorbed is proportional to the amount of material you're passing through. Very simple.

Problem is that, at some point, you'll run out of light. That is, once you've passed through enough material, you'll have absorbed all the light from your beam at frequencies within the width of your line (as determined by thermal motion). The line is then said to be saturated and its strength is no longer proportional to the amount of material in the cloud. In fact, adding more cloud has virtually no effect, so the line strength stays approximately flat.

I mentioned before, however, that there were two mechanisms broadening the line, one thermal and one quantum mechanical. Generally, the quantum mechanical effects are negligible because the thermal width is much larger. The two effects have different broadening profiles, however, the natural broadening not falling off quite as rapidly as you move away from the center of the line. This means that, once you have enough material in your line of sight, the absorption line will develop "wings" arising from the natural broadening mechanism. At this point, the line is called damped. Seldom is there enough material in the ISM to produce a line this strong, but when it does happen, it tends to happen in the strongest absorption line of the most prevalent atom in the universe; that is, the Lyman Alpha transition of hydrogen. Thus damped lyman alpha absorbers.

Garth

Is the dampening over a broad range of frequencies, i.e. not just on the absorption lines?

Garth

SpaceTiger

I'm not quite sure what you mean. All broadening mechanisms spread the line in frequency, by definition. The damping broadens it even further, maybe a thousand kilometers per second.

Garth

Thank you ST, I was just trying to clarify what turbo-1 said, "The existence of damped lyman alpha systems is inferred from the spectra of some quasars, which exhibit a dip in luminosity over a broad range of frequencies." And whether you agreed with his definition of a damped system.

Garth

SpaceTiger

Though a bit vague, turbo's description was not technically incorrect. I was simply trying to explain why the damping arose.

hellfire

I have also some questions...

1) Is the Gunn-Peterson effect a part of the Lyman-alpha forest and how are these absorption lines recognized?

2) Lyman-alpha clouds are assumed to be shock heated in the present universe (z < 2) forming the unbound WHIM phase of the intergalactic medium. Can this be detected as a part of the Lyman-alpha forest?

3) (related question) Why did a part of the baryons collapse in form of bounded intracluster gas in the present universe?