Background power subtraction in Lyman-alpha forest
  When measuring the one-dimensional power spectrum of the Ly$\alpha$ forest,
it is common to measure the power spectrum in flux fluctuations red-ward of the
Ly$\alpha$ emission of quasars and subtract this power from the measurements of
the Ly$\alpha$ flux power spectrum. This removes excess power present in the
Ly$\alpha$ forest which is believed to be dominated by metal absorption by the
low-redshift metals uncorrelated with the neutral hydrogen aborbing in
Ly$\alpha$. In this brief report we note that, assuming the contaminants are
additive in optical depth, the correction contains a second order term. We
estimate the magnitude of this term for two currently published measurements of
the 1D Ly$\alpha$ flux power spectrum and show that it is negligible for the
current generation of measurements. However, future measurements will have to
take this into account when errorbars improve by a factor of two or more.
