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View Full Version : How does the opacity of space affect our observations of distant objects?



mkline55
2012-Sep-24, 04:21 PM
To start with, I am just a newbie as my title implies, so please excuse the terms I use and my simple-minded observations. I did do some background research, and quickly realized I need help. Also, I didn't put this in the question section because it seems to go a little farther than just a question/answer.

Just to help make this simple enough for me to explain, assume for a moment that there is only one element, HI, in the entire universe. Also assume that the stars and galaxies still exist, as well as all the interstellar medium (now made up entirely of HI).

Also, everything is in motion. That's important to this line of thought. Would the interstallar dust have motion relatively similar to that of other nearby objects? I think it would, except for localized exceptions, and the effects of larger events such as supernovas or colliding galaxies.

Now, light from the stars is generated in certain predictable wavelengths. Nearby interstallar dust, because it is moving relatively slowly compared to the star, would tend to absorb a certain amount of that light and scatter it in random directions, wouldn't it? Most of the light would pass through the nearby dust, though.

What about as that light continues onward? As it encounters random free HI, wouldn't more and more of it be absorbed as long as the "frequencies matched up"? Eventually, provided it passed through enough random particles, wouldn't nearly all the light be absorbed? Yes, I understand that space is pretty empty, but we're also talking about vast distances of over a billion light years.

All that seems to make sense (to me), but as I said, things are in motion. What about light from a star or galaxy that is moving unusually fast? If the light is sufficiently redshifted, it would be less likely to match up with that interstallar dust, wouldn't it? The dust nearest that object would still absorb some of the light, because it is also moving right along with the object, but dust farther away may tend to be more transparent.

So, if much of the interstallar dust is more transparent to redshifted light, then wouldn't the red-shifted light be more apparent at a significant distance than 'unshifted' light? (okay, there's probably a term for that, but 'newbie', remember?)

Suppose we started with two distant galaxies transmitting light at approximately the same brightness from perhaps a billion light years distance. (We also assume this little scenario started a billion years ago). Galaxy X is moving at a speed which is fairly common in this imaginary universe. Galaxy Z is moving away from us at an uncommonly high speed. The light from both galaxies passes through very nearly the same space to get here. But, because galaxy X is moving at a speed common to much of the intervening particles, most of its light never gets here. On the other hand, the light from galaxy Z is out of phase with most of the intervening particles, and a significant portion of it arrives safely at our telescopes. We would observe galaxy Z and be able to calculate that it is moving rapidly away from us. What would we conclude about galaxy X?

antoniseb
2012-Sep-24, 05:54 PM
You might take a look at this paper: http://arxiv.org/abs/1209.4636 to get a snapshot of current research on this topic. The short summary answer is that space became a lot less opaque to visible and UV light during the epoch of reionization (roughly z=15 through z=6). Aside from that there are places inside galaxies (dust clouds) that have small but intense local opacity even today.

ngc3314
2012-Sep-24, 06:09 PM
Dust grains differ in their absorption from gas atoms or ions - their absorption doesn't come in narrow discrete wavelengths but varies only gradually with wavelength. Therefore the changes in dust effects are mild with redshift (the major effect is that when we see optical radiation from the distant Universe, we see what started out in the UV, so dust near the object has had a greater effect than would local dust where the radiation arrives at longer, less extinguished wavelengths). The line-absorption change with redshift applies to certain limited conditions, such as looking at neutral hydrogen 21 cm radiation from nearby galaxies free of absorption or confusion from the Milky Way's own gas. (And Lyman alpha radiation from recombination of ionized hydrogen is a major mess for this reason. Don't get me started. If it's not the interplanetary medium it's the interstellar medium, and it makes a big difference which components are moving which way).

In the very early Universe, when the intergalactic gas was both dense and ionized, scattering by electrons was a major factor. In between, up to about 800 Myrs into the cosmic expansion, the deep UV was almost fully absorbed by neutral intergalactc hydrogen (this shows up in spectra of the most distant quasars, and is one of the major drivers to find such objects).

mkline55
2012-Sep-24, 06:26 PM
You might take a look at this paper: http://arxiv.org/abs/1209.4636 to get a snapshot of current research on this topic. The short summary answer is that space became a lot less opaque to visible and UV light during the epoch of reionization (roughly z=15 through z=6). Aside from that there are places inside galaxies (dust clouds) that have small but intense local opacity even today.

Thanks, Antoniseb. That's quite a paper, but surprisingly easy to interpret. It seems, however, to directly sidestep my question with this statement, "Furthermore, only objects with J input/125 < 30 were included - preventing excess noise being supplied by extra ultra-faint sources". It appears to be concerned only with objects with redshifts in the 6.5 to 7.5 range, which is precisely my contention that if other objects at those distances existed, would we even be able to see them?