| As well as our
estimate of the distance to the galaxies we also need to find the redshift
to each galaxy, this is done by taking the spectrum of the galaxy and
working out how much it has been moved towards the red end. |
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Lots of professional astrophysicists want to know the redshift to lots of
objects, so they have built telescopes to measure the redshift of thousands
of objects and catalogue the measurements. These devices are very
sophisticated, they place a little fibre optic sensor on every object the
telescope sees, and then get a spectrum for each object. |
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| Just like the
SSS, there is a catalogue on the ROE website which contains all the redshift
data collected by the Six degree field galaxy redshift survey (6dfGs) - as
you might imagine this is a survey of the redshift of galaxies covering six
degrees of the sky. You can find the homepage for this catalogue at
http://www-wfau.roe.ac.uk/6dFGS/.
Since this catalogue has much more information than the last one you used,
it looks a bit more complicated. |
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| There are a lot
of options to help you configure the search you are going to do, but
everything we need is found by just clicking on 'menu', underneath 'database
access'. This will load up the page where you can enter the coordinates of
your galaxies. |
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| This search
form is pretty complicated but the only forms we need to use are the three
in the middle - the search size, 'RA' and 'DEC', simply enter the
coordinates of your galaxy, and in the field 'return objects within ...
arcmin of' put in 1 arcmin. This should be small enough so that only the
galaxy that you want is returned by the database. When you click on 'query',
the database system looks for any objects which are within the distance you
specified around the coordinates you have specified. Once this has finished
you will be shown a table containing the data for your galaxy (hopefully -
if not then make sure you have entered the coordinates exactly as given to
you). |
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| All you need to
do with this table is make sure that the values for 'obsra' and 'obsdec'
match the right ascension and declination of the galaxy, and then take a
note of the value in the column 'z_helio'. This is the redshift of the
galaxy as measured by the telescope. (The helio part of the name is because
the motion of the Earth has been subtracted, and 'heliocentric' means
relative to the Sun.). It is a good idea to check that you are looking at
the right galaxy; click on 'show' next to the table, and a page containing
images of the galaxy and the measured spectrum will appear. Check that the
image on this page looks the same as the picture you took from SSS of the
galaxy. Whilst you are here, take a look at the spectrum; by identifying
absorption (and possibly emission) lines in this spectrum, and knowing the
wavelength of the lines, it is possible to calculate the redshift. |
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Now, simply repeat this for all 5 of your galaxies to get a redshift for
each one. Once you are happy with this you have finished using the
catalogues. With the images and red shifts you can plot a Hubble diagram. |