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Distant Galaxies - observation methods

by Adrastea
Tags: distant, galaxies, methods, observation
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Adrastea
#1
Sep19-09, 10:00 AM
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Hi,

I'm a writer and I've just started researching a story about astronomy. I'm not a physicist (actually I'm a biologist by trade) but I really want to get the science right. I know a lot of work has been done lately with things like the Sloan Digital Sky Survey but I was wondering what exactly someone studying distant galaxies might be looking for, and what techniques they might use. Also what sort of techniques for observation are merely theoretical now but might be available in the future. For example would an enormous optical telescope (situated on, say, the Moon) allow researchers to see more, or is there a limit to what can be achieved in the visible spectrum?

Another question: if an observer with excellent equipment was watching an event on a distant galaxy (say, 10bn LY), and something seemed to change over a scale of a few minutes or hours (and the change isn't caused by active galactic nuclei or anything like that) then would the time taken for the change to be observed correspond at all to the time taken for the event to happen, or would it be completely different? (yes I know my character is staring 10bn years into the past, that's not my question). For example:

If a galaxy exploded 10bn years ago and we just see the explosion now, and the actual explosion lasted a year of our time (I know that isn't realistic), would the light from that explosion be seen on earth for a year, or would the event itself appear longer or shorter by the time the light reached earth?

Sorry if I am confused over basic physics or if my question doesn't make sense...

Adrastea.
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mgb_phys
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Sep19-09, 10:43 AM
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Quote Quote by Adrastea View Post
For example would an enormous optical telescope (situated on, say, the Moon) allow researchers to see more, or is there a limit to what can be achieved in the visible spectrum?
More distant means fainter so to see them you need, more sensitive detectors, larger telescopes and longer exposure times. Detectors are already pretty good so we just make the telescopes larger. Putting them in space means that fine detail isn't blurred by the atmosphere - there aren't really any advantages to them being on the moon.
You can also observe in other wavelengths - the main reason for going into space is that for very distant objects all the light from them has been redshifted into the infrared.

Another question: if an observer with excellent equipment was watching an event on a distant galaxy (say, 10bn LY), and something seemed to change over a scale of a few minutes or hours .... then would the time taken for the change to be observed correspond at all to the time taken for the event to happen, or would it be completely different?
The galaxy is observed in 'real time' however if you only see the galaxy as a single dot then you will only see events that effect the whole galaxy. For an event to effect a whole galaxy the light must cross the galaxy - typically a few 100,000 lyr across so nothing can change the brightness of an entire galaxy faster than this.
Events happening on a scale of hours could only affect a single object like a single solar system - or perhaps a black hole in the centre.
These events could still be energetic enough to be seen at a great distance - see "Gamma ray burster" for example.
ideasrule
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Sep19-09, 08:43 PM
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Quote Quote by Adrastea View Post
If a galaxy exploded 10bn years ago and we just see the explosion now, and the actual explosion lasted a year of our time (I know that isn't realistic), would the light from that explosion be seen on earth for a year, or would the event itself appear longer or shorter by the time the light reached earth?
It would appear to be exactly 1 year because there is minimal time dilation due to movement or gravity. Although distant galaxies are moving away at a great speed, some faster than the speed of light, special-relativistic time dilation doesn't apply for objects moving with the expansion of the universe.

marcus
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Sep20-09, 12:11 AM
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Distant Galaxies - observation methods

Quote Quote by Adrastea View Post
If a galaxy exploded 10bn years ago and we just see the explosion now, and the actual explosion lasted a year of our time (I know that isn't realistic), would the light from that explosion be seen on earth for a year, or would the event itself appear longer or shorter by the time the light reached earth?
...
Events would appear stretched out in time by the same factor as the wavelengths of light are stretched out----the factor is 1+z where z is the redshift.

You need to be able to calculate redshift for a given light travel time, or vice versa. Given a redshift you should be able to find the light travel time.

So google "wright calculator". You will see the case for z = 3 displayed, then you can try different z values and find out what the light travel time is for each of them.

=======
to make it easy, when I google that, I get this
http://www.astro.ucla.edu/~wright/CosmoCalc.html

and it says, without my doing anything that for z = 3 the light travel time is 11 Billion years.
Notice that 1 + z = 4.
That means that wavelengths are stretched out by a factor of 4, and also news of events like explosions. So light that starts out 1 micron will be 4 micron infrared when it arrives here. And if an explosion lasts 1 year, then the "movie" we see of it in the advanced sci-fi telescope will be "slo-mo" and will last 4 years.

Light travel time is not a good measure of actual distance, and astronomers use other types of distance, like the actual now distance (if you could freeze expansion and send a slight signal off today, how long would it take to get there) or the actual distance THEN when the light was emited or the explosion happened. This shows up as the "angular size distance" in the calculator. It means what the distance was if you could have frozen expansion and sent a signal.

Light travel time tends to be used as a distance measure when talking with children and generic teenagers. You should learn about the other measures. And the calculator will give you an introduction.

The clearest and most widely understood index is the redshift, which is what astronomers actually measure, for the most part, and use to calculate the other stuff. So be sure you understand redshift.

I don't know how fanciful you want to be in your SciFi story, about incredibly advanced telescopes. We see galaxies at redshift z = 3 as fuzzy blobs. The other poster(s) warned you about this. To see individual stars in a galaxy it has to be much closer. Do you care? Maybe you don't care about current technology. You will just pick a distance and imagine that the telescopes are good enough to pick out local events at that distance. Like seeing an individual star and what is happening to it, or in its immediate neighborhood. If you want to try for realism, let the people here know more what you have in mind.

Does the light travel time have to be 10 billion years? Or would 100 million years be OK? There's a big difference. Play around with the calculator, with different redshifts, and travel times, and actual presentday distances etc etc. Decide what you really want the telescope to do, and come back with that. Maybe someone will figure for you how big, how optically feasible. Might be interesting.


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