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The absorption lines due to ionized calcium will be among the strongest
("deepest") of all the lines. It's a good initial step
with any of the spectra to find these two lines.
The black lines at the bottom of the figure (Ca K and Ca H) show
the location of the rest wavelengths. These rest wavelengths are also
spelled out at the top of the figure. As you can easily see, the
measured wavelengths are going to show a sizeable shift toward redder wavelengths.
On the working spectra, you will be clicking at the bottom of each of
these strong wavelengths. For this galaxy, the measured wavelength of the Ca K
line was 3962.0 Angstroms (carry at least 5 significant digits);
and for the Ca H, 3997.2 Angstroms. The differences between measured and rest
wavelengths are (3962.0 - 3933.7) 28.3 Angstroms and (3997.2 - 3968.5) 28.7 Angstroms.
The redshifts are 28.3/3933.7 = 0.0071, and 28.7/3968.5 = 0.0072.
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As you can see in this figure, there are two strong emission lines that are
greater than 30 Angstroms away from the rest wavelength of hydrogen-alpha,
shown by the black vertical line at the bottom. Pick the strong emission line
that is to the left (blueward) of the other strong emission line, even if the
other one has more intensity. (The strong emission line on the right is usually
due to oxygen.)
We expect the wavelength shift for this hydrogen line to be slightly greater than
that of the calcium lines (for reasons that you need not worry about).
The measured wavelength was 6608.6 Angstroms, giving a shift of (6608.6 - 6562.8)
45.8 Angstroms. The redshift is 45.8/6562.8 = 0.0070. You should recognize
immediately that although the wavelength shift was greater, the redshift (z)
is nearly exactly the same. (As it should be if we are measuring the correct
line; after all, it's the same galaxy!)
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