2012MNRAS.420.2475M

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 using Hubble Space Telescope/Advanced Camera for Surveys V-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 radius r_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 at r > r_brk for a large sample of 78 (barred/unbarred, Sa-Sd) spiral galaxies with outer disc antitruncations (µ_brk >24mag/sec2). In the majority of cases ( ∼ 85 per cent), evidence indicates that excess light at r > r_brk is related to an outer shallow disc (Type III-d). Here, the contribution of bulge light at r > 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 at r > r_brk is dominated by the bulge (Type III-s). Here, for most cases the bulge profile dominates at all radii and only occasionally (three galaxies, ∼ 5 per cent) extends beyond that of a dominant disc and explains the excess light at r > 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.