Lyα SB for a narrowband with a smaller value of Δλ_obs = 3.75 Å (i.e. ∼1.19 h⁻¹ cMpc) in a simulation snapshot at z = 4.8. As in Fig. 8, the SB shown is a combination of recombination emission (of all gas in the simulation) below the mirror limit (indicated on the colour bar as HM12), and collisional excitation of gas below half the critical self-shielding density. Panel a: overview narrowband image that corresponds to region 1 in Fig. 4. This is centred on the same comoving coordinates both spatially and spectrally, but now less extended in wavelength range as the narrowband width has been decreased. This panel shows a region of 8 × 8 h⁻² cMpc² (5.2 × 5.2 arcmin²) on a pixel grid of 1024 × 1024. Panels b–d: Lyα narrowband images the size of 1 × 1 arcmin² consisting of 300 × 300 pixels (as the FOV of MUSE). The volume probed by one of these narrowband images at this redshift is 2.84 h⁻³ cMpc³. The areas covered by these maps are indicated by the white squares in the overview panel a. Halos with halo mass of M_h > 10^9.5 M_⊙ are shown as circles, their size indicating their projected virial radius (see text). The most massive halo in each panel is annotated. In the bottom left corner of each panel, two different measures of the overdensity of the region are shown (see text for more details). The baryonic overdensity is calculated taking all gas into account, even though only gas below a certain density contributes to the collisional excitation.