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 the stellar populations inhabiting stellar halos .
Measurements of this type are however plagued by large uncertainties which calls for follow-up studies .
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 .
A total of 1510 nearly edge-on , bulgeless 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 _ { r } \sim 31 mag arcsec ^ { -2 } .
After a careful assessment of instrumental light scattering effects in the stacked images , 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 \pm 0.15 and r - i = 0.80 \pm 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 .