Next: Interpreting the spectral axis
Up: Interpreting the spectral axis
Previous: Interpretation 1: At fixed
Contents
Interpretation 2: At fixed
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
. The
observation is naturally set up so that the source frame is also the rest
frame. This implies that
,
and
are linked through
 |
(15) |
The modeler will use the Doppler effect to interpret the observed
frequency difference
as a local velocity
difference (projected along the line of sight direction) around the
systemic velocity of the source. Let's write
(
) the velocity
associated to
(
). The velocity difference will be noted
 |
(16) |
In order to derive this velocity difference, we associate the velocity to
each observed frequency through the Doppler effect, i.e.,
 |
(17) |
It is easy to deduce that
 |
(18) |
i.e., the local velocity difference is proportional to the observed
frequency difference. We can define a linear velocity axis associated to
the set of brightnesses,
as
 |
(19) |
This velocity scale is only meaningful locally, i.e., as long as the
brightnesses can be associated to the line rest frequency under
consideration.
Next: Interpreting the spectral axis
Up: Interpreting the spectral axis
Previous: Interpretation 1: At fixed
Contents
Gildas manager
2015-03-01