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Interpretation 2: At fixed
and fixed
Let's assume that the same gas cell in the source emits two lines at
different frequencies. We are at fixed
because we consider
the same gas cell. The frequency axes in the rest and observatory frames
are thus given by
 |
(28) |
This means that the frequency separation between the two lines is different
in the rest and observatory frames. The modeller has easily access to the
rest frame frequencies of the line. It is thus important to display the
spectrum frequency axis in the rest frequency axis. This can be achieved
only for one redshift (The reasoning is here done at fixed
), which is by default assumed to be the systemic redshift
of the source in the observatory frame, i.e., the mean redshift of the
source gas in the observatory frame.
As a convention, we can in addition assume that the tuned rest frequency
and its corresponding observatory frequency at the source systemic velocity
are associated to the common reference channel
. The frequency axis
in the rest frame is then defined as
 |
(29) |
The plotted spectrum thus correctly displays the line at rest frequency
positions, i.e., the brightnesses of the gas at rest in the source frame.
Next: Interpretation 3: At fixed
Up: Interpreting the spectral axis
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Gildas manager
2015-03-01