Strong Lyman-α emission in an overdense region at z=6.8: a very large (R ∼ 3 physical Mpc) ionized bubble in COSMOS?

Our understanding of reionization has advanced considerably over the past decade, with several results now demonstrating that the intergalactic medium transitioned from substantially neutral at z = 7 to largely reionized at z = 6. However, little remains known about the sizes of ionized bubbles at z greater than or similar to 7 as well as the galaxy overdensities which drive their growth. Fortunately, rest-ultraviolet (UV) spectroscopic observations offer a pathway towards characterizing these ionized bubbles thanks to the resonant nature of Lyman-alpha photons. In a previous work, we presented Ly alpha detections from three closely separated Lyman-break galaxies at z similar or equal to 6.8, suggesting the presence of a large (R > 1 physical Mpc) ionized bubble in the 1.5 deg(2) COSMOS field. Here, we present new deep Ly alpha spectra of 10 UV-bright (M-UV = -20.4) z similar or equal to 6.6-6.9 galaxies in the surrounding area, enabling us to better characterize this potential ionized bubble. We confidently detect (S/N > 7) Ly alpha emission at z = 6.701-6.882 in nine of ten observed galaxies, revealing that the large-scale volume spanned by these sources (characteristic radius R = 3.2 physical Mpc) traces a strong galaxy overdensity (N/< N > greater than or similar to 3). Our data additionally confirm that the Ly alpha emission of UV-bright galaxies in this volume is significantly enhanced, with 40 per cent (4/10) showing strong Ly alpha emission (equivalent width >25 A) compared to the 8-9 per cent found on average at z similar to 7. The median Ly aequivalent width of our observed galaxies is also approximate to 2 times that typical at z similar to 7, consistent with expectations if a very large (R similar to 3 physical Mpc) ionized bubble is allowing the Ly aphotons to cosmologically redshift far into the damping wing before encountering H I.