Doppler & Absorption

Point_Particle

The Doppler effect corresponds to a percieved change in the wavelength of impinging light, correct? Does this mean that the Absorption spectra we would see for bodies in relative motion to light sources should be the same independent of the type of motion between the body/source (assuming that there is no relative motion between the observer and the light source)?

turin

Correct, though I would not emphasise the fact that it is a perceived change.




How would we see the absorption spectrum?

Stingray

If two bodies are moving with the same velocity, then emission and absorbtion between the two is the same as if they're not moving. If they are not at the same speed, then the lines appear shifted.

There is a very neat thing called the Mossbauer effect where the line width is so small that even motions of a few mm/s give enough of a doppler effect to move things out of resonance. This is used in many high precision experiments.

Point_Particle

What I mean is this: Say we have an observer and a light source separated by some distance, with a cool, diffuse gas spanning that distance. If the diffuse gas cloud and the source were in relative motion, would the absorption spectra seen by the stationary observer be different from that seen by the observer if it were moving with the same velocity as the relative velocity of the source/gas cloud (the "moving frame spectrum" if you will)?

turin

I still don't understand how one would see absorption spectra. I don't know all that much about it. Can you (or anyone) please explain it to us (me)? I'm assuming white light is emitted by the source. Then, the gas only absorbs certain spectral lines of the gas. Whatever it doesn't absorb it can either reflect or pass. I'll assume that everything is passed. So, the observer would see the light that passes? And if the observer considered the spectrum, assuming that the source emitted white light, would see missing (black) lines? Is that what you mean by seeing an absorption spectrum? And why are there more than one spectra?

steve stillman

The light after the absorbing coud is the light you have to see and experiment with. If the sodium line is now visible (cloud or no cloud inbetween) due to doppler shift it cannot be used in your (what ever) experiment. This is the Radial Doppler Effect.

Can we say that there is no doppler shift if the cloud is moving with the observer. The source lines shift but the absorption lines dont.

There is also something called the Transverse Doppler Effect for perfectly normal light passing by the observer on a moving object. Mc Graw Hill Dictionary of Science says it varies by SQRT(1-(v/c)^2). Dosnt work for sound.

I also think we are talking about the passed absorption spectrum and not the stopped absorption spectrum as Turin explained.

Nereid

Real situation(s): a star, moving towards the solar system barycentre (or away) emits copious quantities of photons towards the barycentre. The star has a thin gas layer just above the photosphere, which contains (for the sake of this discussion) neutral Na atoms. Many thousands of years before the photons set off towards Earth, the star expelled a vast amount of gas, including Na atoms; these are now moving away from the star at several hundred km/s. In fact, the star did this six times, and the gas clouds are now moving away from the star at different speeds (the reasons are not important). Further, between the star and our solar system, there are ten clouds of 'cold' gas, containing Na atoms. Each cloud is moving at a different speed wrt the star, the solar system, each other, ...

Point_Particle, steve, turin and Nereid have observed this star with a high dispersion spectroscope, attached to a big telescope. In particular, we have taken a good hard look at the Na D lines.

What do we see?

Point_Particle

Your question really gets to the crux of my original inquiry. Since all of the Na atoms are moving at different speeds, they would see the light red shifted by different amounts, correct? Because of resonant absorption and energy quantization, they would each absorb the light that they see as being at specific wavelengths (all of the Na atoms would have the same rest absorption spectrum). We are stationary with respect to the star, so assuming that the star is not moving relative to us, the continuous emission spectrum we see from the star would not be shifted. However, from the intervening cold Na clouds, we would see dark absorption lines where the wavelengths the clouds absorbed would normally be. We would not see the same thing as if there were a cold cloud of Na gas in between two objects, with no relative velocity involved. Is this correct?

turin

I think I agree with Point Particle.

Nereid

Yep*

In fact, studying the 'line profiles' and multiple absorbsion lines is something that many astronomers do an awful lot of. Two examples (try googling and follow the good links): Lyman-forest (re quasars, or QSOs) and P-Cygni (or P Cyg) stars.

Has this helped your understanding, Point_Particle?

*there are a number of (small) details that should really be pointed out; maybe later.

TeV

Nereid,how preceisely today astrophyicists could distinguish photon red shift of a massive quasar due to it's fast moving from the gravitational red shift of the photon emitted from the same quasar?

Nereid

Excellent question TeV, really first-rate!

I'm more than a little rusty on the details of this story, so I may need to revise or extend my post here once I've done some digging. But, here goes (imagine my usual caveats about (over-)simplification and general summary only and (some) details really do matter have all been said).

If all you could 'see' of a quasar were a point source, and its spectrum had just a few absorbsion lines, with one or two Ly[tex]\alpha[/tex] (or H[tex]\alpha[/tex]) lines at lower red-shift, it would be 'impossible'. And in the early days, the sum total of high quality data wasn't much more than that (indeed, for quite a long time some astronomers held to the non-Hubble redshift view as to the cause).

In a nutshell, the question was resolved by huge numbers of observations, as many questions in astronomy are. Some of the main (classes), in no particular order (and certainly not historical order!):
- quasars reside in galaxies, some of which seem perfectly 'normal' otherwise
- the redshifts of the underlying galaxies is the same as that of the quasars at their heart
- some of the inter-galactic clouds of hydrogen, inferred from the Lyman forest, were detected independently
- quasars, AGN, and Seyfert galaxies (and more) came to be seen as essentially the same 'beast', seen from different perspectives, and at different ages
- ... and AGN and Seyferts showed no sign of the sorts of supermassive body needed to create the observed redshift
- if the observed redshift were due to gravity, the amount of mass would be far, far greater than was otherwise observed ('in the neighbourhood', so to speak)
- ... or the mechanism for generating the intensity of observed photons ('light') beyond any conceivable physics
- (included in the two above - limits on the physical size of the quasar 'engine' implied by shortest time-scale of observed, significant variability; and other lines of evidence)

Which parts cry out for further details or explanation?

TeV

Thanks for the fine reply.I was just wondering..
To summarize:Redshifts of a typical quasars are mostly Doppler redshifts?
regards

Nereid

You're welcome.
"mostly" ~>99% (IIRC). The gas which gives rise to the absorbsion lines is not in the accretion disk around the supermassive black hole (SMBH) at the heart of a quasar (usual caveats), but further out where the gravitational redshift is negligible.

Gravitational redshifts *have* been observed near SMBHs at galaxy nuclei, but not quasars AFAIK. One interesting PR from HST (they were perhaps a tad premature in claiming the relationship; subsequent observations have shown it's not so strong), and a direct HST spectroscopic observation.

TeV

This info was unknown to me and explains clearly why the Doppler redshift dominates ( better than your first post´).Thanks again!Being a complete amateur in this field I thought " the heart of a quasar" contributed significantly to the rise of the absorbsion lines. Hence my wondering before.I know almost nothing about theories of quasars,what is a status of a current theory of their evolution and "the engine modus operandi" how they burn so much energy etc.
I'm tied to the earth lab probing mainly microcosmos matter behaviour,dealing from time to time with computer models that have nothing to do with astrophysics of a galaxies.
But theoretically I know something else as well to throw in the mix.That there is the difference of energy for an atom corresponding to its change of size of the Bohr radius-observed as a red shift of its spectroscopic lines due to the change of the gravitational potential.The change of mass can be applied generally to any particle/subatomic particle in physics placed in the G fields.To mention this additionally it can also be applied to astronomical bodies like galaxies since it relies on the principle of mass-energy conversion always valid.

Nereid

With a name like TeV, you must at least know of the existance of CANGAROO: "We observe ~1 TeV gamma rays with an imaging Cerenkov telescope".

If you're interested in really high energy physics, you might enjoy this paper by Stecker.

TeV

Seen that one (and a few very good others).
On other hand,my nickname isn't dedicated to high energy cosmic rays Cangaroo deals with (big lab of universe can create them with an ease,that's for sure), but to capabilities of mankind to create particle collisions in excess of 10 TeV.My nick is sort of commeroative one. You must have heard of a big crime in Texas...

regards,
TeV