Today I am zeroing in on the answer to a GO (General Observer) question about why
wavelengths from data taken in March 94 are off by 2 angstroms After looking at his data
and seeing how little signal there was, I wondered how he could tell anything about the
wavelengths associated with the individual spectra. I found that the signal improves
(i.e. you could start to see features as opposed to noise) when the individual spectra
are combined or co-added, i.e. summed together.
Still I was confused because I thought he was trying to compare the same feature (an
absorption (usually) or emission line) in wavelength space but the data weren't taken at
the same central wavelengths at all. There was hardly any overlap. It turned out he was
not comparing the same feature but different features in redshift space, not wavelength
space. Redshift is the amount a feature moves in wavelength or velocity, etc. due to the
fact that it is moving away from us. This relates to something you may have heard of
called the Doppler effect? It is expressed in terms of "delta lambda over lambda" which
is equal to "v over c". Lambda (the Greek letter lambda) stands for the rest wavelength,
where you expect to find the feature if it weren't moving away from you. Delta lambda is
the difference in wavelength between where you found the object and the rest wavelength.
"v" is velocity, the speed at which it is moving away. And "c" is, of course, the speed
of light.
The General Observer) was plotting his lines of different wavelength on a redshift
(or velocity) scale, then trying to fit them with theoretical profiles. What he found
was that the absorption lines in the March 94 data line up at the same redshift, but the
line in the October 95 data appear to be at a slightly different redshift. And that is
the shift he didn't understand. But I figured out that the wavelength calibration has
changed since the first data were taken. It is quite possible, probable, and hopeful
that if he recalibrates his data the unexpected shift will go away.
Of course, while I am working on this problem, I also have several more that I am
trying to keep on top of...but my ability or lack of ability to do more than one chore at
a time is the topic for another journal.
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