Redshift

   As the Universe expands the space between the galaxy groups opens up stretching out the light waves that travel between them.    With a longer wavelength the light has been shifted to the red end of the spectrum.     If there is a way of knowing the original wavelength of the light we can work out how much it has been shifted.   The method you are using for spotting distant galaxies is great for finding candidates but does not get a measurement of how redshifted the galaxy is.   For that we need to spread the light out into a spectrum and scan it for intensity.   For light from nearby galaxies we can look at the characteristic absorption lines of elements in the stars.    We can also do this for nearby galaxies.   But as we look for galaxies further and further away they get dimmer and dimmer.   The few photons we get from the furthest galaxies in the HUDF do not give us enough information to see absorption lines.   Instead we look for a gap in the galaxies light output called the Lyman break.   This step in the spectrum is much easier to see but there are other breaks that we could mistake it for.    

Redshift is worked out using this formula:   

So if the Calcium II K line which is usually at 393.4 nm  is observed at double that at 786.8 the  z = 786.8-393.4/393.4 = 1   

the resulting redshift scale is logarithmic so that the difference between redshift 1 and 2 is billions of years whilst between 7 and eight only a couple of hundreds of thousands of years .    Whilst redshift 7 puts a galaxy at 800 thousand years after the big bang,  redshift 12 puts it at 0nly 400 thousand year.

If a galaxy's redshift was measured as redshift 6, how long ago would we be seeing it?