Z number

[ST47]

What is the Z number that everyone's talking about? I understand that it is related to the redshift, and has to do with the distance, but what sort of scale is it? is 0.001 closer than 0.010? What is a 'benchmark' for these numbers?

[Sebastian]

Yes 0.000 is closer then 0.010 and 0.000 is in the Milky Way and 0.01 is another galaxy, but what's about z = 0.001 ?

[Pharun]

As you have said z is the redshift of an object. Redshift is the relative difference between the observed and emitted wavelengths (which are shifting to a lower energy as the object moves away, resulting in longer wavelengths) of an object. As to the scale, it is simply a ratio between the difference between observed and emitted wave lengths and the emitted wave length.

So the higher the number the farther away the object is. I hope that answered your question, and if anyone can give a better answer, since I'm not an astronomer, I wish to see it so I can learn more about it myself.

[zookeeper_Kate]

Hi,
'z' is used to represent 'redshift'. This is the amount that the wavelength of light has changed between being emitted from a source and being received by us - ie. it measures the stretching of light. And this can be due to the object moving away from us when the light was emitted, or due to the expansion of space. Hubble's detection of the redshift of light from galaxies led him to discover that the Universe was in fact expanding.

So - we can use the 'z' to give an idea of how far away an object is. A redshift of 0 means it is very close to us (because the light has not stretched at all). A redshift of 1 is very far from us (about 10,000,000,000 light years away!) and this is at the very edges of this telescope. Most galaxies in the SDSS are between 0 and 0.3, but galaxies exist out to redshift of z~7.

If an object is at exactly z=0.000 in the SDSS then it is probably a star (or something in our own galaxy).

(To be more specific, if R is the size of the Universe now, then the size of the universe at redshift z is R/(1+z). So at a redshift of 1 the Universe was half of its size.)

[ST47]

I see! That's very interesting, and also quite helpful, however:
How can we measure the 'emitted' wavelength? Do we need to know the makeup of the star/galaxy and use that element's spectrum (We learned this in earth science, so this may be completely unrelated) or is there another method?

[zookeeper_Kate]

That is a very good question! Indeed - we use the spectrum. And you can recognise familar patterns of absorption and emission lines in the spectrum, from chemical elements known to be in the sources. And you know at which wavelength these elements absorb and emit at - and so you can basically see how much the lines have shifted to get the redshift.

[suprtrkr]

Most galaxies in the SDSS are between 0 and 0.3, but galaxies exist out to redshift of z~7.

GalRef# 587729227690279111

A nice little blue blobby one I'm gonna call a spiral clock. However the SSDS page calls it TARGET_GALAXY and offers a "z" of 15.94 which is more than double the 7 in the quote (Log scale? That'd be a whopper). Also, this isn't anywhere near the farthest object I've seen-- a bit blurry, but clear enough to classify.  Nor is it the biggest "z" I've found: didn't save the number, but I saw one of 18.something. Or should I be reading the "err z" entry? That's 0.01, although I've been assuming this entry is an estimate of the error in the measurement.

Have I failed to understand, or am I doing something wrong?

[zookeeperKevin]

Hey, that's a different z, the z-band apparent magnitude (or brightness).

The redshift z of this galaxy is 0.062, see slightly lower in the page.

[mecurtin]

I like to use a Map of the Universe
http://www.astro.princeton.edu/~mjuric/universe/
to see approximately what distance a particular Z-value corresponds to. For back-of-the-envelope calculations, I use:

distance in parsecs=z*c/HubbleConstant, with HubbleConstant = 71 and c=speed of light. I don't know how well this works for Z >.1, though.

[Mercure]

Is it Possible to see a lets say 5.23 Redshift number... meaning the galaxy would be Getting Closer To us?

[zookeeper_Kate]

A redshift of 5.23 means that it is most definitely moving away from us! At a huge speed.
With this SDSS survey you cannot see galaxies that far away.
(For a galaxy to be getting closer to us the redshift would have to be negative, ie. a blueshift)

[Halibut]

Is it not that our galaxy is 0.0 red shift or did I remember that wrong from somewhere?  If we are .0 then surely a negative number means that the item is already on top of us?  I used to be really good at maths and algebra but not the brains wheels are moving quite slowly

[Kurosora]

On Edward L. Wright's homepage, there is a great calculator to convert redshift to lightyears:

http://www.astro.ucla.edu/~wright/CosmoCalc.html

Also, there is a great cosmology tutorial on that page.

[Mercure]

Wow Great Website.
Thank you Kate for info.

[adsp]

[snip]
Most galaxies in the SDSS are between 0 and 0.3, but galaxies exist out to redshift of z~7.
[snip]
(To be more specific, if R is the size of the Universe now, then the size of the universe at redshift z is R/(1+z). So at a redshift of 1 the Universe was half of its size.)

Is there a specific reason (today's technology?) for SDSS telescope being z=1?
Do we have better telescopes (able to see z>1)? If not, how do we assume there are galaxies further away?

Many thanks,
Adrian