# How to correct observed flux densities for redshift

Say I have a spectrum of a galaxy at a redshift $$z$$, in flux density units of erg/s/cm^2/Angstrom. I'd like to recover the spectrum (in the same flux density units) at z=0, i.e. at its rest wavelengths. How do I do this?

I saw this paper (Distance measures in cosmology, Hogg (2000)) but am still confused, because of their use of luminosity and luminosity densities, and in my class I only have fluxes.

• Since these are differential quantities, a careful application of the chain rule should be all that's necessary for a basic answer. Since I am not careful, I'll defer to someone who is ;-) – uhoh Apr 16 at 1:26