The effect of spiral arms on the S\'ersic photometry of galaxies
  Context. The S\'ersic profile is a widely-used model to describe the surface
brightness distribution of galaxies. Spiral galaxies, however, are
qualitatively different from a S\'ersic model.
  Aims. The goal of this study is to assess how accurately the total flux and
half-light radius of a galaxy with spiral arms can be recovered when fitted
with a S\'ersic profile.
  Methods. I selected a sample of bulge-dominated galaxies with spiral arms.
Using photometric data from the Hyper Suprime-Cam survey, I estimated the
contribution of the spiral arms to their total flux. Then I generated simulated
images of galaxies with similar characteristics, fitted them with a S\'ersic
model, and quantified the error on the determination of the total flux and
half-light radius.
  Results. Spiral arms can introduce biases on the photometry of galaxies in a
way that depends on the underlying smooth surface brightness profile, the
location of the arms, and the depth of the photometric data. A set of spiral
arms accounting for 10% of the flux of a bulge-dominated galaxy typically
causes the total flux and the half-light radius to be overestimated by 15% and
30%, respectively. This bias, however, is much smaller if the galaxy is
disk-dominated.
  Conclusions. Galaxies with a prominent bulge and a non-zero contribution from
spiral arms are the most susceptible to biases in the total flux and half-light
radius, when fitted with a S\'ersic profile. If photometric measurements with
high accuracy are required, then measurements over finite apertures are to be
preferred over global estimates of the flux.
