To determine the metallicity of an HVC we must first
the velocity of the given cloud. As a result of the Doppler shift, an
ion in an HVC absorbs photons at a slightly different wavelength than
would be measured in a laboratory from a given orbital transition.
Consequently, the shift of the absorption can be directly used to find
the velocity of the ion. So we look at the
absorption range of a given detected metal and by comparing this with
the neutral hydrogen 21-cm absorption spectra we can find the velocity of
obtaining a range for the velocity of the high velocity cloud we are
ready to measure the absorption. We do this by first fitting a
continuum to the data and then by measuring the column density and
equivalent width of the absorption line.
we find the combination of column density (N) and intrinsic line width
(b) that best predicts the measured equivalent widths. Normally, Iron
or Oxygen will give the best constraint on b. Consequently, the b value
that we find
for that ion is then used to find the column densities for the rest
of the ions. In the end this method should give us the most accurate
column densities for each ion.
combine the derived column density with the neutral hydrogen column
density to determine the abundance of the ion as compared
to the known abundance of the ion in the Sun. This gives us a ratio
that allows us to more readily determine the origins of the high