  Measuring the integrated stellar halo light around galaxies is very
challenging. The surface brightness of these haloes are expected to be many
magnitudes below dark sky and the central brightness of the galaxy. Here I show
that in some of the recent literature the effect of very extended Point Spread
Function (PSF) tails on the measurements of halo light has been underestimated;
especially in the case of edge-on disc galaxies. The detection of a halo along
the minor axis of an edge-on galaxy in the Hubble Ultra Deep Field can largely
be explained by scattered galaxy light. Similarly, depending on filter and the
shape one assumes for the uncertain extended PSF, 20 to 80 per cent of the halo
light found along the minor axis of scaled and stacked Sloan Digital Sky Survey
(SDSS) edge-on galaxy images can be explained by scattered galaxy light.
Scattered light also significantly contributes to the anomalous halo colours of
stacked SDSS images. The scattered light fraction decreases when looking in the
quadrants away from the minor axis. The remaining excess light is well modelled
with a Sersic profile halo with shape parameters based on star count halo
detections of nearby galaxies. Even though the contribution from PSF scattered
light does not fully remove the need for extended components around these
edge-on galaxies, it will be very challenging to make accurate halo light shape
and colour measurements from integrated light without very careful PSF
measurements and scattered light modelling.
