Comparing PyMorph and SDSS photometry. I. Background sky and model
  fitting effects
  A number of recent estimates of the total luminosities of galaxies in the
SDSS are significantly larger than those reported by the SDSS pipeline. This is
because of a combination of three effects: one is simply a matter of defining
the scale out to which one integrates the fit when defining the total
luminosity, and amounts on average to < 0.1 mags even for the most luminous
galaxies. The other two are less trivial and tend to be larger; they are due to
differences in how the background sky is estimated and what model is fit to the
surface brightness profile. We show that PyMorph sky estimates are fainter than
those of the SDSS DR7 or DR9 pipelines, but are in excellent agreement with the
estimates of Blanton et al. (2011). Using the SDSS sky biases luminosities by
more than a few tenths of a magnitude for objects with half-light radii > 7
arcseconds. In the SDSS main galaxy sample these are typically luminous
galaxies, so they are not necessarily nearby. This bias becomes worse when
allowing the model more freedom to fit the surface brightness profile. When
PyMorph sky values are used, then two component Sersic-Exponential fits to
E+S0s return more light than single component deVaucouleurs fits (up to ~0.2
mag), but less light than single Sersic fits (0.1 mag). Finally, we show that
PyMorph fits of Meert et al. (2015) to DR7 data remain valid for DR9 images.
Our findings show that, especially at large luminosities, these PyMorph
estimates should be preferred to the SDSS pipeline values.
