Galapagos-2/Galfitm/GAMA -- multi-wavelength measurement of galaxy
  structure: separating the properties of spheroid and disk components in
  modern surveys
  We present the capabilities of Galapagos--2 and Galfitm in the context of
fitting 2-component profiles to galaxies, on the way to providing complete
multi-band, multi-component fitting of large samples of galaxies in future
surveys. We release both the code and the fit results to 234,239 objects from
the DR3 of the Gama survey, a sample significantly deeper than previous works.
We use stringent tests on both simulated and real data, as well as comparison
to public catalogues to evaluate the advantages of using multi-band over
single-band data. We show that multi-band fitting using Galfitm provides
significant advantages when trying to decompose galaxies into their individual
constituents, as more data are being used, by effectively being able to use the
colour information buried in the individual exposures to its advantage. Using
simulated data, we find that multi-band fitting significantly reduces the
deviations from real parameter values, allows component sizes and S\'ersic
indices to be recovered more accurately, and, by design, constrains the
band-to-band variations of these parameters to more physical values. On both
simulated and real data, we confirm that the SEDs of the 2 main components can
be recovered to fainter magnitudes compared to using single-band fitting, which
tends to recover disks and bulges to - on average - have identical SEDs when
the galaxies become too faint, instead of the different SEDs they truly have.
By comparing our results to those provided by other fitting codes, we confirm
that they agree in general, but measurement errors can be significantly reduced
by using the multi-band tools developed by the MegaMorph project. We conclude
that the multi-band fitting employed by Galapagos-2 and Galfitm significantly
improves the accuracy of structural galaxy parameters and enables much larger
samples to be be used in a scientific analysis.
