Expected average surface brightness profiles for exponential nebulae SBLyα(r) = Σ0 × exp( − r/r0), measured within circular apertures and compared to the our surface brightness upper limits (dashed lines, indicating the range of our limits shown in Fig. 6). Coloured solid lines show the expectations including the effect of our sky subtraction procedure, while the dashed lines ignore it (blue: Σ0 ≈ 10-16 erg s-1 cm-2 arcsec-2 and r0 = 2′′; green: Σ0 = 5 × 10-17 erg s-1 cm-2 arcsec-2 and r0 = 4′′).
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