Antitruncated stellar light profiles in the outer regions of STAGES spiral galaxies: Bulge or disc related?

We present a comparison of azimuthally averaged radial surface brightness μ(r) profiles and analytical bulge-disc decompositions (de Vaucouleurs,r ^1/4 bulge plus exponential disc) for spiral galaxies usingHubble Space Telescope/Advanced Camera for SurveysV-band imaging from the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). In the established classification scheme, antitruncated μ(r) profiles (Type III) have a broken exponential disc with a shallower region beyond the break radiusr _brk. The excess light at large radii (r>r _brk) can either be caused by an outer exponential disc (Type III-d) or an extended spheroidal component (Type III-s). Using our comparisons, we determine the contribution of bulge light atr>r _brk for a large sample of 78 (barred/unbarred, Sa-Sd) spiral galaxies with outer disc antitruncations. In the majority of cases (∼85 per cent), evidence indicates that excess light atr>r _brk is related to an outer shallow disc (Type III-d). Here, the contribution of bulge light atr>r _brk is either negligible (∼70 per cent) or too little to explain the antitruncation (∼15 per cent). However in the latter cases, bulge light can affect the measured disc properties (e.g. μ _brk, outer scalelength). In the remaining cases (∼15 per cent), light atr>r _brk is dominated by the bulge (Type III-s). Here, for most cases the bulge profile dominates at all radii and only occasionally ( galaxies, ∼5 per cent) extends beyond that of a dominant disc and explains the excess light atr>r _brk. We thus conclude that in the vast majority of cases antitruncated outer discs cannot be explained by bulge light and thus remain a pure disc phenomenon.