Variations in the fine structure constant

[enotstrebor]

In http://arxiv.org/PS_cache/astro-ph/pdf/9803/9803165v4.pdf" the authors use (second paragraph) "Spectroscopic observations of gas clouds seen in absorption against background quasars" to search for variations in alpha.

Is my understanding correct that
A) The quasars are background and they are looking at absorption peaks in a "gas cloud" between the quasar and us.

If so
B) Are they using absorption lines rather then the quasar's emission lines because the emission lines are two broad to be able to measure electron transitions used to measure alpha.

If so;
C) Since the gas cloud is somewhere between us and the quasar, how does this give alpha at the period of the quasar's redshift distance rather than at the clouds distance.

 

[PAllen]

In http://arxiv.org/PS_cache/astro-ph/pdf/9803/9803165v4.pdf" the authors use (second paragraph) "Spectroscopic observations of gas clouds seen in absorption against background quasars" to search for variations in alpha.

Is my understanding correct that
A) The quasars are background and they are looking at absorption peaks in a "gas cloud" between the quasar and us.

If so
B) Are they using absorption lines rather then the quasar's emission lines because the emission lines are two broad to be able to measure electron transitions used to measure alpha.

If so;
C) Since the gas cloud is somewhere between us and the quasar, how does this give alpha at the period of the quasar's redshift distance rather than at the clouds distance.

I took a glance through the paper. I seems to me that they are using the cloud's red shift. They measure the clouds red shift by fitting laboratory absorption lines against the clouds. The redshift applies equally to all absorption lines. However, they then compare precisely different lines that are affected differently by a change in alpha. The quasars appear to be used simply as a bright, distant, broad spectrum source.

 

[enotstrebor]

I took a glance through the paper. I seems to me that they are using the cloud's red shift. They measure the clouds red shift by fitting laboratory absorption lines against the clouds. The redshift applies equally to all absorption lines. However, they then compare precisely different lines that are affected differently by a change in alpha. The quasars appear to be used simply as a bright, distant, broad spectrum source.

Thanks, Thats basically my understanding.

Just to verify one specific with respect to quasars,

a) "Everyone knows" (except me) that all of the quasars spectral lines are too broad to be able to directly observe the peak splitting's that can be used to measure alpha?