A faint red stellar halo around an edge-on disc galaxy in the Hubble Ultra Deep Field

We analyse the detailed structure of a highly-inclined (i greater than or similar to 80degrees) disc galaxy that lies within the Hubble Ultra Deep Field (UDF). The unprecedented depth of the UDF data allow disc and extraplanar emission to be traced reliably to surface brightness levels of mu(V,i,z) similar to 29-30 mag arcsec(-2) (corresponding to rest-frame equivalents of mu(g,r,i) similar to 28-29 mag arcsec(-2)) in this redshift z = 0.32 system. We detect excess emission above the disc, which is characterized by a moderately flattened (b/a similar to 0.6) power law (I proportional to R-2.6). The structure and colour of this component are very similar to the stellar halo detected in an SDSS stacking analysis of local disc galaxies and lend support to the idea that we have detected a stellar halo in this distant system. Although the peculiar colours of the halo are difficult to understand in terms of normal stellar populations, the consistency found between the UDF and SDSS analyses suggests that they cannot be easily discounted.