The red halos of SDSS low surface brightness disk galaxies
  The faint stellar halos of galaxies contain key information about the oldest
stars and the process of galaxy formation. A previous study of stacked SDSS
images of disk galaxies has revealed a halo with an abnormally red r-i colour,
seemingly inconsistent with our current understanding of stellar halos. Here,
we investigate the statistical properties of the faint envelopes of low surface
brightness disk galaxies to look for further support for a red excess. 1510
edge-on low surface brightness galaxies were selected from the SDSS Data
Release 5, rescaled to the same apparent size, aligned and stacked. This
procedure allows us to reach a surface brightness of mu_g ~ 31 mag arcsec^-2.
After a careful assessment of instrumental light scattering effects, we derive
median and average radial surface brightness and colour profiles in g,r and i.
The sample is then divided into 3 subsamples according to g-r colour. All three
samples exhibit a red colour excess in r-i in the thick disk/halo region. The
halo colours of the full sample, g-r = 0.60+-0.15 and r-i = 0.80+-0.15, are
found to be incompatible with the colours of any normal type of stellar
population. The fact that no similar colour anomaly is seen at comparable
surface brightness levels along the disk rules out a sky subtraction residual
as the source of the extreme colours. A number of possible explanations for
these abnormally red halos are discussed. We find that two different scenarios
-- dust extinction of extragalactic background light and a stellar population
with a very bottom-heavy initial mass function -- appear to be broadly
consistent with our observations and with similar red excesses reported in the
halos of other types of galaxies.
