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 & 80◦) disc galaxy which lies within the Hubble Ultra Deep Field (UDF). The unprecedented depth of the UDF data allow disc and extraplanar emission to be reliably traced to surface brightness levels of μV,i,z ∼ 29 − 30 mag arcsec −2 (corresponding to rest-frame equivalents of μg,r,i ∼ 28 − 29 mag arcsec ) in this redshift z = 0.32 system. We detect excess emission above the disc which is characterised by a moderately-flattened (b/a ∼ 0.6) power-law (I ∝ R). The structure and colour of this component are very similar to the stellar halo detected in an SDSS stacking analysis of local disc galaxies (Zibetti, White, & Brinkmann 2004) 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.