APOD: Looking Through Abell 68 (2013 Mar 08)

[MargaritaMc]

PS. Just looking at Chris's post immediately before, I see the figure 5.4 given in relation to red shift. So, can I deduce that z = the measurement of red shift?
M

[rstevenson]

Wikipedia's Redshift page explains all, perhaps more than you want at the moment! And yes, z is the symbol for redshift. Redshift can be attributable to the Doppler effect, but what we're talking about here is Cosmological redshift, attributable to the expansion of space.

Rob

[Chris Peterson]

I've been reading about the research relating to this image and I'd be grateful if someone could tell me what z means in the following quotation

Spectroscopic measurements are obtained for 26 lensed images, including a distant galaxy at z = 5.4.

http://m.iopscience.iop.org/0004-637X/6 ... .text.html

It is the relativistic redshift, defined as the ratio of the observed wavelength to the emitted wavelength, minus 1. In this case, the researchers were looking at the emitted lyman-alpha line, which is at 121.6 nm (deep ultraviolet). This line was observed as being at 778.2 nm (near infrared), which is how the redshift was calculated.

This redshift can be plugged into equations based on the Hubble relationship to derive additional information: the light was emitted when the Universe was 1.1 billion years old, it took 12.6 billion years for the light to reach us, the object is now 26.6 billion light years away.

[MargaritaMc]

Many thanks, Rob and Chris!
Yes, it is a leetle advanced for me at present (!) But I find the subject so fascinating that I beaver away at getting understanding. Having all the wonderful people here on Asterisk is an incredibly valuable resource.

Oddly, the concept of redshift was something I read about some years ago, and it made sense, even tho I had no background in astronomy.

Margarita

[Chris Peterson]

Oddly, the concept of redshift was something I read about some years ago, and it made sense, even tho I had no background in astronomy.

Although relativistic redshift is a completely different phenomenon than Doppler redshift, the two are nevertheless very analogous, and Doppler wavelength shift is something most people understand quite easily- in part, I'm sure, because it is so trivially observed with sound.

[MargaritaMc]

balloon_n_galaxies.jpg

I know the balloon image is really basic , but, like the sound of police sirens, it helps me begin to grasp the concept of redshift.

http://en.wikipedia.org/wiki/Redshift#section_4
In the widely accepted cosmological model based on general relativity, redshift is mainly a result of the expansion of space: this means that the farther away a galaxy is from us, the more the space has expanded in the time since the light left that galaxy, so the more the light has been stretched, the more redshifted the light is, and so the faster it appears to be moving away from us.

Margarita

[stephen63]

One more good analogy would be to take a slinkie and stretch it apart. The coils would represent what is happening to the wavelength of light.

[Beyond]

One more good analogy would be to take a slinkie and stretch it apart. The coils would represent what is happening to the wavelength of light.

Just like standing by a railroad track and there's a train comeing at you. The sound is compressed, like the slinky coils when you hold them together.
When the train goes by you, the sound is stretched out, like holding only one end of the slinky. This stretching out of the emitted light, is what causes the red shift. The faster an object goes away from you, the more the red shift, at least to a certain point. (I forget a lot of this stuff)

[MargaritaMc]

http://en.m.wikipedia.org/wiki/Redshift#section_4

Redshifts are attributable to the Doppler effect, familiar in the changes in the apparent pitches of sirens and frequency of the sound waves emitted by speeding vehicles; an observed redshift due to the Doppler effect occurs whenever a light source moves away from an observer.

Cosmological redshift is seen due to the expansion of the universe, and sufficiently distant light sources (generally more than a few million light years away) show redshift corresponding to the rate of increase of their distance from Earth.

Finally, gravitational redshifts are a relativistic effect observed in electromagnetic radiation moving out of gravitational fields.

Conversely, a decrease in wavelength is called blueshift and is generally seen when a light-emitting object moves toward an observer or when electromagnetic radiation moves into a gravitational field.

There seem to be three types of red shift - the one that specifically applies to this Apod is, I think, that which relates to the expansion of the universe, rather than any actual motion that, undoubtedly, exists.

Phew! I've spent all evening reading about redshifts! I am goggle-eyed...
Margarita

[Beyond]

And, i would suppose... a little red-eyed

[ta152h0]

I still have this question bouncing around in my head. If the lensed galaxy is 10-20 times further than the lenser galaxy and we know the Universe is considered to be 14.6 billion years old, at what distance lensed galaxies cease to be seen ? Or is this too much relativity for this biomass computer ?

[Chris Peterson]

I still have this question bouncing around in my head. If the lensed galaxy is 10-20 times further than the lenser galaxy and we know the Universe is considered to be 14.6 billion years old, at what distance lensed galaxies cease to be seen ? Or is this too much relativity for this biomass computer ?

The age of the Universe is generally taken to be 13.8 billion years. The oldest galaxies that are observed have a light travel time of over 13.2 billion years, and therefore represent galaxies that were present when the Universe was only about 500 million years old. Those must be some of the earliest galaxies that existed, and seen very early in their evolution.