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Keywords:

  • galaxies: spiral;
  • galaxies: structure

ABSTRACT

We present a comparison of azimuthally averaged radial surface brightness μ(r) profiles and analytical bulge–disc decompositions (de Vaucouleurs, r1/4 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 rbrk. The excess light at large radii (r > rbrk) 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 > rbrk for a large sample of 78 (barred/unbarred, Sa-Sd) spiral galaxies with outer disc antitruncations (inline image). In the majority of cases (∼85 per cent), evidence indicates that excess light at r > rbrk is related to an outer shallow disc (Type III-d). Here, the contribution of bulge light at r > rbrk 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. μbrk, outer scalelength). In the remaining cases (∼15 per cent), light at r > rbrk is dominated by the bulge (Type III-s). Here, for most cases the bulge profile dominates at all radii and only occasionally (inline image galaxies, ∼5 per cent) extends beyond that of a dominant disc and explains the excess light at r > rbrk. 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.