Galaxies in filaments have more satellites: the influence of the cosmic
  web on the satellite luminosity function in the SDSS
  We investigate whether the satellite luminosity function (LF) of primary
galaxies identified in the Sloan Digital Sky Survey (SDSS) depends on whether
the host galaxy is in a filament or not. Isolated primary galaxies are
identified in the SDSS spectroscopic sample while potential satellites (that
are up to 4 magnitudes fainter than their hosts) are searched for in the much
deeper photometric sample. Filaments are constructed from the galaxy
distribution by the "Bisous" process. Isolated primary galaxies are divided
into two subsamples: those in filaments and those not in filaments. We examine
the stacked mean satellite LF of both the filament and non-filament sample and
find that, on average, the satellite LFs of galaxies in filaments is
significantly higher than those of galaxies not in filaments. The filamentary
environment can increases the abundance of the brightest satellites
($M_\mathrm{sat.} < M_\mathrm{prim.} + 2.0$), by a factor of $\sim 2$ compared
with non-filament isolated galaxies. This result is independent of primary
galaxy magnitude although the satellite LF of galaxies in the faintest
magnitude bin, is too noisy to determine if such a dependence exists. Since our
filaments are extracted from a spectroscopic flux-limited sample, we consider
the possibility that the difference in satellite LF is due to a redshift,
colour or environmental bias, finding these to be insufficient to explain our
result. The dependence of the satellite LF on the cosmic web suggests that the
filamentary environment may have a strong effect on the efficiency of galaxy
formation.
