Cartoon illustrating the Lyα transmission profile of galaxies in a plane of the sky around an ionized bubble. Left: Galaxies in and around a spherical bubble with the observer at the top. Right: The integrated Lyα transmissions, 𝒯 as a function of transverse distance (x) measured using galaxies in the green box in the left panel. Transmission profiles 𝒯(x) viewed in different windows (A, B, and C) are different. Window A: Galaxies are all outside the bubble and all have 𝒯 ≈ 0 , resulting in a flat transmission profile in the window. Window B: Galaxies are located around the center of the bubble, therefore 𝒯 in the window are all high and the transmission profile is symmetric around the window center. Window C: Half of the galaxies are outside the bubble and have 𝒯 ≈ 0 . The other half of the galaxies are inside the bubble, having rising transmission toward the bubble center. The transmission profile around the center of window C is therefore asymmetric. The level of asymmetry can therefore be a probe of bubble edges.