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Interpretation 1: At fixed
and fixed
Combining Eq. 22 to 24, we yield
 |
(25) |
i.e., the local velocity difference in the source/rest frame
is proportional to the observed frequency difference in the
observatory frame. We can define a linear velocity axis associated
to the set of brightnesses,
as
 |
(26) |
This can be interpreted as follows. Let's assume that the measured spectrum
is made of a line centered around
in the observatory
frame. The associated rest frame frequency, i.e., the frequency of the
line at rest, will be
. If the tuned frequency at channel
in the observatory frame is set to the redshifted frequency
 |
(27) |
the velocity axis defined in Eq. 26 can be interpreted
as the local variation of the gas velocity in the rest/source frame for the
line whose rest frequency is
.
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2015-03-01